Strategic Deworming Q&A

Your questions about strategically deworming horses, answered by an expert.

How often should you deworm a horse? Can you overdo it? Do natural dewormers work? Can my dogs give my horses their worms? Members of who participated in The Horse's Strategic Deworming Webinar asked these questions and about 70 more--and they received answers by presenter Heidi A. Brady, PhD, an associate professor at Texas Tech University, during the Aug. 7, 2008 webinar. A complete transcript of these questions and answers follows.

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Q: There seem to be many opinions as to the frequency of worming, how do we decide what is appropriate? Quarterly, bi-monthly, monthly?

A: You have asked a very important question and one that is not simple to answer. A complex number of factors should be looked at. Without baseline fecal egg counts on your horses, you really do not have a good basis for making a decision. Further, without FECRT (fecal egg count reduction tests; more on these later), the possible resistance of the parasites in your group of animals is unknown. We are advocating a comprehensive approach where a screening can be done for your farm and based on what is seen, specific recommendations can be made. Ultimately, horse owners could be guided through sound decisions by veterinarians or horse extension specialists at many universities. Testing is a crucial beginning step to this procedure. One cannot always tell just by looking at horses if they have a parasite load that is of concern. Fecal exams also have their limitations, which is why consultation with a veterinarian is strongly encouraged when designing a deworming regimen.

Q: What do you think about fecal egg counts as a monitoring tool for worm burden? How often should this procedure be carried out? For example, if a horse had an EPG < 200, when should a FEC be carried out again?

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A: Unfortunately there is no one answer. Fecal egg counts (FEC) are a valuable tool if used appropriately. They refer to strongyle type eggs and can be used to identify which horses are high egg shedders and to determine which dewormers (anthelmintics) are still effective on a farm. FECs can be used to identify which of your horses on pasture are your high, moderate and low egg shedders. Parasitologists do not all agree on what "cut-off" number should be used to define high, medium, and low shedders. The high egg shedders will require more frequent deworming treatments than the low egg shedders. Genetics play an important role in determining a horse's "natural" resistance or susceptibility to strongyle type parasites. FECs can also be performed before and 10 - 14 days after administering a dewormer to determine if the drug is still effective against strongyles. In this instance an effective drug should reduce the egg count as measured in the "pre" sample by at least 90%. Some experts contend that drugs such as ivermectin or moxidectin should reduce the number of eggs in a FEC by at least 95- 98% and drugs such as pyrantel or fenbendazole should reduce the FEC by at least 90%. FEC cannot be relied upon to detect the burden of encysted small strongyles that are hibernating in your horse's intestinal wall. If you are performing a FEC to classify your horse's egg shedding status as high / medium / low, then you must be certain to wait a long enough period of time (4 weeks beyond the egg reappearance period (ERP, more on this later) for the last dewormer used) after administering the last anthelmintic before collecting a fecal for this purpose. You must wait long enough just to be certain that the FEC will give you a meaningful number. If all of your horses have FECs of "0" then you did not wait long enough! You should not have to repeat this type of fecal since horses do not tend to change their egg shedding status within a herd. If you are performing a fecal to evaluate the drugs you are using on your farm, then you should collect a fresh fecal before administering a dewormer and again 10-14 days after deworming. This type of fecal evaluation should be done periodically to continue to monitor for drug resistance. When you rotate between different classes of drugs or introduce a new drug class that has not been used for awhile, it is advisable to perform this type of fecal evaluation.

Q: Can you overdo deworming?

A: Yes, the concept that "more is better" is not a sound decision when it comes to deworming. As a matter of fact, many of the studies documenting resistance on a farm have been where there has been a history of very frequent deworming, especially with the same class of dewormer. Deworming programs, including decisions on frequency of use and types/schedules of dewormers, should be based on a complex number of factors such a geographic location, weather, density of horses, age of horses, exposure of those horses to outside untreated horses (for example, performance horses on the road), management practices (pasture rotation, etc.) and fecal egg counts of animals. It is also important to know if there is a problem of resistance on the farm, which can be determined with pre and post fecal egg count testing (Fecal Egg Count Reduction Tests, FECRT). Based on our studies at Texas Tech, we recommend a 4-way rotation for many farms and ranches. Some conditions may warrant a six way rotation only if needed. The quarterly rotation includes dewormers targeting adult parasites, larval parasites, tapeworms and bots. We believe it is important to consider the range of parasites including ascarids, and not base recommendations only on small strongyle control. Several very recent studies have documented resistance of equine ascarids to the ivermectin class of parasites. This is very alarming due to the serious consequences ascarids can pose to foals. In high numbers, blockage of the gut can occur with fatal results. Because there is no new class of dewormer on the horizon, we believe it is very important to preserve all of the current classes of dewormers, which have different applications in parasite control in the horse. Other researchers suggest even a less frequent schedule of anthelmintic administration based on a targeted approach, where only the heavy egg shedders are given the dewormers. In the targeted approach, the high shedding horses as identified by fecal egg count (FEC) testing are selectively dewormed where as the low shedding horses are not. Further scientific trials and farm testing need to be performed in the immediate future to investigate all approaches to parasite control. The bottom line is that many types of deworming regimens are being proposed. There need to be further research studies on the effectiveness of these programs throughout the U.S. as well as around the globe. In addition, remember that each farm needs to be considered individually and decisions need to be made based on testing and management information.

Q: What is the ideal FEC programme? Should you sample the same horses every time and when? Before and after deworming to check effectiveness and how long after deworming should the count be done?

A: That is a great question and one of high importance in your assessment of the efficacy and control you have in your deworming program. To get a general screening of your animals, a FECAL EGG COUNT (FEC) should be performed ideally from all horses on the premises. This FEC screening will give you an idea of the parasite load within the herd. If this is not feasible, it is very important to sample horses from different classes--several from your weanlings, yearlings, 2-year-olds, older horses, broodmares, etc. It is also important to sample horses from different living areas--for example, your pens vs. pastures. Animals that are performance horses and travel a great deal should also be sampled. To understand if there is a resistance problem to a drug on your farm, a single sample is not sufficient. To determine this, the FECAL EGG COUNT REDUCTION TEST (FECRT) must be performed. To do this test, fecal samples from horses should be taken prior to the administration of the dewormer (pre-treatment value), and at 10-14 days after the dewormer is given (post-treatment value). This test will give the percentage of egg reduction from the initial sample. If the post-treatment is zero eggs per gram, then there is a 100% reduction. There is some uncertainty by scientists as to what value of FECRT is the cut-off that indicates resistance. Many scientists use the 90% threshold, but there is not universal agreement. In addition, it has been suggested that there may be different thresholds for different classes of dewormers. Again, more research is needed in this area.

Q: Some say that rotational de-worming is an outdated practice, what is the current recommendation based on recent study?

A: I do not believe that rotation is an outdated practice. We showed at Texas Tech that rotation was very efficacious after a resistance problem was verified in a herd of production horses. After resistance to fenbendazole was documented (after experimental use of only FBZ for 18 months without rotation) by extremely poor FECRT and a larval assay, we found that fenbendazole was not effective in this herd, particularly in the young horses where the incidence of small stronglyes (cyathostomes) was very high. We implemented a quarterly rotation of pyrantel pamoate (Strongid, Pfizer, June), Ivermectin+Praziquantel (Equimax, Pfizer, September), fenbendazole larvicidal dose 50 mg/kg BW (Power-Pac, Intervet, December), and Moxidectin (Quest, Fort Dodge, March) and had all efficacies above 95.9%. We are now in the fourth year of monitoring this herd and the control of all types of parasites is excellent. I believe any parasite control program should include frequent monitoring of the herd via FEC and FECR. It is not always evident to the eye of the beholder if there is a parasite problem there. (Read more.)

Q: Are all major parasite problems in the horse colics?

A: No, there are many types of problems that can be related to internal parasites in the horse other than either acute colic or chronic colic. These can include damages to the lungs and arteries due to the larval migration process. Also, diarrhea can be seen with heavy parasite loads or in cases of cyathostomiasis, when many small strongyle larvae release from the gut at one time. We also see the general lack of energy, poor body condition, poor hair coat, pot belly in foals and other more obvious signs of parasites. However, what is not easy to see or measure but should be highly emphasized is poor feed efficiency and poor nutrient uptake by the horse. In cases of encysted cyathostomes in the intestine as well as other parasite loads, compromises in feed efficiency are most likely taking place. Because the horse is ultimately a production animal, we should strive for the highest efficiency. In addition, because we aim for ultimate performance in our horses (showing, breeding, etc.), we should strive for maximum nutrient uptake.

Q: Does it affect the efficacy of deworming paste to mix it in a small amount of feed, assuming that it is all eaten immediately?

A: Paste dewormers are not formulated to be mixed with feed. If you must do this for management purposes, we would recommend that a commercially available top dressing granule be used. This is useful in horses that are hard to work with and are difficult about their heads. Equi Bits is an alfalfa based, pelleted form of fenbendazole that most horses find palatable. Unfortunately, other classes of dewormers are not available in such a palatable form. In these cases, though, it is important that each horse is fed by weight and is fed separately, ensuring complete dosing. However, I would still recommend deworming with different classes of dewormers, and several classes are not available in top dressing granules.

Q: Are daily deworming feeds effective against all types of intestinal worms?

A: Daily deworming of horses involves low dose administration of pyrantel tartrate, which is commonly used in the equine industry. This program, however, is not labeled to treat encysted cyathostomes. There are only two treatments effective for the control of encysted cyathostomes, the larvicidal 2X - 5 day treatment of fenbendazole and moxidectin. Daily dewormers do not treat all stages of parasites, therefore it is still recommended that you include at least one annual treatment with a larvicide and boticide.

Q: What are the recommendations for how to sample horses and how to package the fecal samples?

A: In research, we sample using a fecal grab method using a sterile glove and sterile lube while the horses are in a stock. This is not recommended for the horse owner, however, due to the potential risk to the horse owner and animal. We recommend that the owner select a fresh fecal ball from the animal, collected from concrete, not the soil or field. Place it in an individual, airtight baggie and ensure all the air is removed from the bag. This anaerobic condition is very important in the transport and storage of the sample. Make sure that each bag is clearly marked with a water-resistant Sharpie pen identifying the date, animal, and farm. Ice can often ruin other methods of identification. When sending the sample to the lab for analysis, make sure that the sample does not freeze, but does remain at consistently cooled temperatures. If mailed, all samples should be overnighted by express mail. We recommend sampling horses at the same time of day for consistency.

Q: What is the maximum frequency for deworming, both with ivermectin alone and with rotational schedules?

A: The frequency of using a dewormer is based on a number of factors... the worm burden of the individual horse, the age of the horse (foal vs. mature adult vs. geriatric), the time of year, the farm management, the time spent on pasture, the specific dewormer being administered. No one class of dewormer should be used exclusively since there is a growing problem with drug-resistant parasites. Each class of drugs have their own "dosing interval" when it comes to small strongyles. No drug should be given more frequently than its egg reappearance period (ERP = the time interval from when the drug is administered until you expect to see strongyle eggs begin to reappear in the feces). The average ERP for pyrantel and fenbendazole is 5 weeks, for ivermectin it is 6-8 weeks and for moxidectin it is 10-12 wks. These values refer to the drug's efficacy against small strongyles and it assumes that the drugs are still effective on that particular farm and in that particular horse. During the hot, dry summers it is not necessary to deworm adult horses as frequently since the strongyle larvae passed in the manure are killed by the high temperatures (>85°F). During cold winters, when temperatures are consistently below 45°F, larval development is arrested and horses do not require the same frequency of deworming. During the hot summers or cold winters (i.e., times of year when the grass turns brown), it is a good time to consider giving a larvicide that is effective against encysted small strongyles. The 5-day double-dose, larvicidal treatment with fenbendazole is effective against all stages of encysted small strongyles. Moxidectin is also effective against most stages of small strongyles, but this drug should not be used in foals less than 6 months of age.

Q: When deworming draft horses (>2000 lbs), is it simply double the dose, or should it be either calculated or exponential? (Most labels advise to "give more" if it's a draft.)

A: It is always best to estimate the body weight of the horse and dose accordingly. Of all the drug classes, moxidectin should be dosed the most carefully to avoid overdosing. The safety margin is greater when administering pyrantel, ivermectin or fenbendazole.

Q: Are there tested brands that are truly more effective, or is generic just as good?

A: The well-known brand names are marketed by companies that have invested the research time and money to discover the drug and to patent and license the active ingredient. These research companies had to demonstrate drug efficacy and safety of the product in horses in order to obtain an FDA license. Once a patent expires, other companies can then produce a product containing the same active ingredient, if they can show their product is similar to the original product, which is used as the "gold standard." Companies that produce generic products do not have to invest the same time and money to license the active ingredient, nor do they have to do the same exhaustive studies to prove efficacy and safety. The amount of active drug as well as other ingredients can vary between different dewormers. Always read the entire label and consult with your veterinarian.

Q: Does the time of year as well as location of the farm dictate what type of de-wormers should be given and when?

A: Excellent question and the answer is YES. When deciding which dewormer to use and when... you need to consider:

The individual horse's own innate susceptibility to parasites (some horses are naturally more resistant than others and require less frequent deworming),

How much time a horse spends on pasture since most parasites are acquired while grazing, age of the horse (young foals are more susceptible to certain parasites such as roundworms, also known as ascarids),

Time of year (since hot, dry conditions when temperatures are above 85°F effectively kill strongyle larvae on pasture and less frequent deworming is recommended),

Whether certain drugs are still effective on your farm since drug-resistant parasites are becoming more common in different parts of the country and the world, and

Which parasites are a concern in your area (for example, are tapeworms a problem in your area?).

Q: In a perfect world, all horse owners would perform fecal tests prior to deworming. How many times a year should a fecal test be done?

A: There are at least two reasons to perform a fecal exam. One reason is to classify which of your horses are high vs. low egg shedders. This fecal should be performed after a long enough time interval following the last deworming (i.e., wait at least 4+ weeks beyond the egg reappearance period for the last dewormer administered). If performed appropriately the first time, you should not have to perform another fecal on an individual horse for this reason (i.e., egg shedding potential is believed to be a genetic trait and should not change over time unless the health or immune status of the horse changes). Another reason to perform a fecal exam is to be certain that the drugs you are using on your farm are still effective. A FECRT is required to test for drug resistance against strongyles. When using this test, a fecal sample is performed just prior to deworming and then a second fecal exam is evaluated 10 - 14 days after deworming. At least a 90% reduction in egg count is expected following deworming. If ivermectin or moxidectin was the drug used, then most experts would expect a >95 - 98% reduction in egg count following treatment. A FECRT can be performed periodically whenever new drug classes are used. Any new arrivals on your farm should have a fecal performed and should receive a larvicidal deworming treatment prior to introducing them onto your pastures.

Q: I live in North Central Florida. What is the appropriate time of year for me to administer praziquantel: Early summer, fall, or both?

A: Praziquantel is used to treat tapeworms. We do not fully understand the entire life cycle of the tapeworm both in the horse and on the pasture. The most common tapeworm in the horse, Anoplocephala perfoliata, requires an intermediate host (the oribatid mite) to complete its development outside of the horse. These mites like temperate climates with some humidity. Therefore, in temperate regions it is recommended to give a tapeworm treatment after cold weather arrives. Some experts recommend a second treatment in the spring in areas where tapeworms are prevalent. There are two drugs licensed to treat tapeworms: Praziquantel and a double dose of pyrantel. As with all parasites, it is best not to rely on just one class of drug exclusively but to use all of the drugs shown to be effective against that parasite.

Q: If you are not able to remove manure from paddocks, what is the resting time needed to kill off the worms? I have heard everything from 6 weeks to 3 months until the paddock is "safe" again. Is this time reduced by cross grazing?

A: If we focus on strongyles, we know that hot, dry temperatures above 85°F are lethal for developing strongyle larvae. So resting a pasture for 4- 6 weeks during hot, dry summers may be adequate. It is important that the parasite eggs / larvae are exposed to the sun and not hidden in manure piles or in deep grass, so mowing the pasture and / or harrowing it during hot weather will help. Cross grazing with sheep will also help keep the grass short and expose more strongyles to the sun. Planting pastures with an annual crop also helps reduce the parasite burden. Unfortunately, cold winters do not kill strongyle larvae, the cold only arrests larval development. Once the snow is gone and the temperatures warm up again, the arrested larvae on pasture begin to develop to the infective stages. Temperatures between 45 and 85°F are ideal for strongyle development and transmission. Some degree of humidity or moisture is also beneficial. Unfortunately, roundworm (ascarid) eggs are far more resistant to climate changes than strongyle eggs and ascarid eggs can persist on a pasture for as long as 10 years! Although it's time-consuming and unappealing, frequent manure removal is still a highly effective parasite control strategy.

Q: Does harrowing a pasture actually expose the worms and kill them or is it just spreading the worms across the pasture?

A: Harrowing a pasture does expose parasites hidden in deep grass or in manure piles. However, to be effective at killing parasites and not just spreading them, harrowing should only be done during hot, dry periods when sunlight exposure will help kill strongyle larvae. After harrowing, pastures must be rested for an adequate period of time to allow the "exposed" strongyle larvae to die off before reintroducing horses. Ascarid eggs will not be killed so easily.

Q: If I use Zimecterin Gold (which is supposed to cover all the bases as far as worming is concerned), which other wormer do I alternate with it to keep from always using the same thing?

A: This product contains ivermectin and praziquantel. While this product is effective against a wide range of parasites, it is not licensed to kill encysted small strongyles. Moxidectin and larvicidal doses of fenbendazole (e.g., Panacur Power Pac or Safe Guard Power Dose) are the only drugs licensed to treat encysted small strongyles. There are now published reports of roundworms (ascarids) that have become resistant to ivermectin. There are also early reports in some parts of the world of strongyles becoming less susceptible to ivermectin. So it is important not to rely on only one drug class exclusively. Work with your veterinarian to determine which of the other drug classes should be incorporated into your deworming program. It will depend on how many horses you have, the ages of your horses, time of year and region of the country. Periodic fecal exams will help identify which horses require more frequent deworming as well as the efficacy of the drugs you are using.

Q: We have 5 horses on our farm, and each one of them is on the same rotational deworming schedule. We have 2 dogs that run our pastures. Is it possible for the dogs to give worms to the horses through their feces if the horses graze where the dogs have "gone"?

A: Dogs and horses do not share the same parasites.

Q: What kind of parasite assessment program would you recommend for the horses on a facility that has 115 mostly adult horses (some at pasture and some in stalls)? We've been doing egg counts by flotation on ~20% of each herd 1 or 2 weeks after deworming the herds. Given the large number of horses, would an individual monitoring and deworming schedule be more efficient and still feasible?

A: An individual monitoring program would be feasible but it would be more labor-intensive, at least initially. One reason to consider doing fecal exams on all of the horses is to determine which horses are the high egg shedders and require the most frequent deworming and which horses are naturally more resistant to parasites and consistently shed low numbers of eggs and require less frequent deworming. Most parasitologists agree that within a herd of horses on pasture approximately 20 - 30% of the horses shed about 80% of the eggs. Those "high egg shedders" tend to remain the same horses over time. Another small percentage of the horses tend to shed low numbers of eggs due to natural or genetic resistance to parasites. In order to determine the shedding potential of horses, it is necessary to wait a long enough time interval after last deworming before collecting a fecal sample. Some experts suggest waiting 4+ weeks beyond the ERP for the last dewormer used. (Using this formula, fecals should be collected >8 - 9 wks after using pyrantel or fenbendazole, >10 - 12 wks after using ivermectin and > 14 - 16 wks after using moxidectin). This fecal exam should be done during a time of year when parasite transmission on pastures is naturally at a low level (e.g., during hot, dry summer months in the South or during cold winters up North). The goal is to optimize the use of the dewormers we have in order to slow the development of drug resistant parasites.

Q: If only part of the horses on the property follow chemical deworming, but the others are on non-traditional deworming methods, will this compromise the efficiency of deworming in either group of horses?

A: Horses receiving non-chemical methods of deworming should also be evaluated using fecal exams. If those horses that are receiving non-traditional deworming methods are shedding high numbers of parasite eggs, then they are contributing to a higher worm burden on the pasture. The goals of a sound deworming program are to kill dangerous stages of parasites within the individual horse and to reduce the number of parasites on the pasture.

Q: Is daily worming with products such as Strongid C causing/contributing to the resistance problem?

A: That is a valid concern and one that has been expressed by various parasitologists. There are published reports of strongyles developing resistance to the pyrantel class of drugs (the active ingredient in Strongid C) on farms that have been using the daily dewormer. When using any deworming program, you should work with your veterinarian and perform periodic fecal exams to see if the drugs you are using are still effective.

Q: If you are in a boarding barn that doesn't do regular worming, is it safer for your horses to be on daily worming programs?

A: Not necessarily. It depends on whether the drugs being used (including the one in the daily dewormer) are still effective against the parasites on that farm as well as within individual horses.

Q: I've had fecal samples done in the spring and fall for years. I always get a negative result from the vet. Early this summer, my horse began rubbing his tail. I checked for skin irritation, etc. and found nothing. I then decided to de-worm him with Quest and then put him on Strongid C2X for the remainder of the pasture season. Is this a good choice for parasite control? My horse's coat is good, he's got a lot of energy, and he is at bit on the pudgy side, not fat, but does have sufficient muscle and flesh. How long should I wait before I give him another kind of paste wormer?

A: Daily deworming of horses using Strongid C 2X involves low dose administration of pyrantel tartrate, which is commonly used in the equine industry. This program, however, is not labeled to treat encysted cyathostomes. There are only two treatments effective for the control of encysted cyathostomes, the larvicidal double-dose, 5-day treatment of fenbendazole and moxidectin. Daily dewormers do not treat all stages of parasites; therefore, it is still recommended that you include at least one annual treatment with a larvicide and boticide. Boticides are often given in the fall after cold weather arrives.

Q: I have yearlings and weanlings; I have problems as to when to worm these guys since they are so young. I do not want to over-medicate them and I want to stay away from bloated bellies. Can you give me a good wormer that is safe for their systems yet will help remove or lower the worms that they carry?

A: Youngsters are uniquely susceptible to roundworms (ascarids). As foals and weanlings age, if their immune system is normal, they should develop an age-related immunity to ascarids. But until that time ascarids can be difficult parasites to control. The roundworm eggs are extremely resistant to hot and cold temperatures and can survive on pastures for up to 10 years or longer. Foals can acquire ascarid infections while out on pasture or while in a stall or paddock. During their development in the foal, ascarid larvae migrate through the foal's liver and lungs before arriving back in the intestinal tract. It takes approximately 8 - 10 weeks from the time a foal ingests an infective ascarid egg until an adult female begins producing eggs within the foal's intestines. A female ascarid can lay a tremendous number of eggs (>100,000!). Large numbers of adult ascarids can cause impactions in foals and bowel rupture. Signs of ascarid infections in foals can include a host of signs that range from cough and nasal discharge to diarrhea, weight loss, unthriftiness, colic and even death. So the goal of a good deworming program is to minimize the amount of damage caused by migrating ascarid larvae and to prevent ascarids from reaching the adult egg-laying stage in order to prevent pasture contamination. Ivermectin and larvicidal fenbendazole are two dewormers that are effective against migrating and adult ascarid larvae. Pyrantel and the standard foal dose of fenbendazole (10 mg /kg) are effective against the adult ascarids. Moxidectin, although effective against migrating ascarids, should not be used in young foals < 6 months of age. There are also growing numbers of reports of roundworms becoming resistant to various drugs including ivermectin and moxidectin. Therefore, any deworming regimen should include the use of periodic fecal exams to be certain that the drugs you are using are still effective against the parasites on your farm. In general, foals should be dewormed approximately every 8 weeks during the first year of life using drugs that have been shown to be effective against ascarids on that particular farm. I recommend incorporating a larvicidal dose of fenbendazole (e.g., Panacur Power Pac - double dose of fenbendazole once a day for 5 days) into a foal's deworming regimen once before the foal is 6 months old. Foals are also susceptible to strongyles, tapeworms, pinworms, and bots.

Q: My question on the subject of wormers is how consistently effective is my liquid ivermectin worming on a 2-month schedule? I ceased to rotate wormers at least 10 years ago and am now not sure it is not promoting resistance in the parasites. Should I go back to rotating?

A: Drug resistance among equine parasites is becoming a disturbing problem not just in this country, but world-wide. One contributing factor on many farms where drug-resistant parasites are being discovered is the exclusive use of only one drug class for a prolonged period of time. I would encourage you to work with your vet to evaluate your deworming program. Fecal egg counts performed on properly collected fecals (i.e. fresh manure samples that are kept cool until analyzed and collected at the proper time in relation to the last deworming treatment given) are one way to determine if ivermectin is still effective on your farm and in your horses. There are reports of roundworms (ascarids) in young horses that have become resistant to ivermectin. There are early reports that other parasites may also be developing reduced susceptibility to ivermectin and other drugs, including moxidectin (Quest), pyrantel (Strongid), and fenbendazole. Therefore, I would encourage you to periodically rotate the chemical classes of dewormers that you use in your horses. Not all horses need to be dewormed at the same frequency. During hot, dry summers when temperatures are well above 85°F, Mother Nature is effectively killing strongyle larvae on the pastures and you can decrease the frequency of deworming in adult horses (foals need to be maintained on a regular and effective deworming schedule to prevent roundworm infections). The dosing interval between dewormers varies based on the chemical being used. The 8-week interval was designed for ivermectin. When using moxidectin, the interval is 10-12 weeks. When using pyrantel or fenbendazole, the interval is shorter (4-6 weeks). Consult with your vet about checking fecals on your horses and using more than one chemical class of dewormer on your farm.

Q: How often do you find worm aneurysms in horses? What causes them? How can they be prevented?

A: "Worm aneurysms" in horses are usually the result of blood vessel wall damage due to migrating large strongyle (bloodworm) larvae. There are three species of bloodworms. Strongylus vulgaris is the most prevalent. Horses acquire these parasites while grazing. Following ingestion, the strongyle larvae penetrate and migrate through the walls of the blood vessels that supply blood to the horse's gastrointestinal tract. Large numbers of these migrating larvae can damage and weaken the vessel wall and can result in blood vessel dilatation (e.g., aneurysm) and thrombus (clot) formation. These changes can lead to severe and potentially fatal colic due to disruption of blood flow to the intestines. The best way to reduce the risk of this worm-related lesion is to deworm your horse at least twice a year with a "larvicidal" dewormer. In this case, a larvicidal dewormer is one that kills the juvenile, migrating large strongyle larvae. Examples of drugs effective against large strongyle larvae include larvicidal dose of fenbendazole (products include the Panacur PowerPac / Safeguard Power Dose; a double dose of fenbendazole is given once a day for 5 days); ivermectin (products include Zimectrin, Eqvalan), and moxidectin (products include Quest; Quest should not be used in young horses less than 6 months of age and should be used cautiously in thin, debilitated horses; this drug must be dosed accurately). It is rare to see large strongyles in horses that receive regular and routine deworming using one of these larvicidal drugs at appropriate intervals. To ensure your deworming program is still working, it is essential to work with your veterinarian and submit periodic fecal samples to look for parasite eggs.

Q: We had a fairly serious problem with tapeworms on my farm this past year. I understand the eggs are ingested by an oribatid mite and the horses accidentally ingest the mites while grazing. Does composting the manure destroy the eggs of tapeworms and does sunlight's ultraviolet kill the eggs if the manure on the pasture is broken up and dispersed? What is my best hope of reducing the reintroduction of tapeworms into my herd?

A: The tapeworm is a difficult parasite to control for several reasons: 1) Fecal exams often do not adequately detect tapeworm eggs in horses. The blood test available in this country only tells us if your horse has been exposed to tapeworms, but not if they are still actively infected. 2) There is still some uncertainty about the entire lifecycle of the tapeworm and its intermediate host, the oribatid mite. In order to tapeworm eggs to become infective for the horse, they must spend part of their development within the mite. The infected mite is then ingested by the horse. Mites tend to thrive in temperate climates with adequate humidity. Droughts may help decrease the dissemination of tapeworms. The lifecycle of the tapeworm within the horse can last a fairly long time. Most treatment regimens focus on periodic treatment of horses with drugs known to be effective against tapeworms rather than trying to eliminate the mites, which are fairly ubiquitous in nature. The two drug classes that are approved as tapeworm treatments in the horse are praziquantel and "double dose" pyrantel pamoate. Praziquantel is available in paste preparations that also contain either moxidectin (e.g., Quest Plus) or ivermectin (e.g., Zimectrin Gold, EquiMax). In areas of the country where tapeworms are a problem, most deworming regimens recommend treating horses for tapeworms at least once a year in the fall. Some programs also include a second treatment in the spring. I would consult with your vet to decide which frequency of treatment is ideal for your farm. To combat the development of drug-resistant tapeworms, it is advisable to alternate the use of the above two drugs in your deworming program. Whenever you bring new horses onto your farm, you should quarantine them for a 2-4 week period to be certain they are not harboring infectious diseases and at the same time they should be dewormed. This is one way to be certain new arrivals do not introduce parasites onto your pastures.

Q: I was told that using Formula 11 (from circle star farms) along with the "organic" apple cider vinegar with "the mother" in it creates too acidic of an environment in the gut for worms to live. Is that true? With using these more natural ways should I still worm with regular worming methods just as often? I have only two horses on my own property but I do trail ride weekly off the property

A: I am not familiar with Formula 11 and so I cannot comment on its mode of action or efficacy. There are few if any scientific studies that examine the use of "natural" dewormers. Many reports are anecdotal. Regardless of which products you use, I strongly recommend that you work with your vet and periodically examine fecal samples from your horses to determine if they are shedding parasite eggs and to see if your deworming program is still effective. Horses are exposed to most parasites while grazing pastures that contain horse manure. Flying insects can carry other parasites. So unless your horses live in a totally protected environment, they will continue to be exposed to worms. You are at an advantage if you have only two horses and adequate pasture. Frequent manure removal is a good non-chemical means of reducing their exposure to parasites. I still recommend the use of chemical dewormers. You may be able to deworm your horses less frequently than farms that have large numbers of horses.

Q: In light of the recent knowledge of worms becoming resistant to ivermectin paste, would there be the same possibility of that happening with the daily Strongid wormer that is currently on the market?

A: Unfortunately there are already published reports of horse farms that have parasites becoming resistant to the daily dewormer pyrantel tartrate (i.e., daily Strongid). Regardless of the dewormers you use or have used in the past, it is strongly recommended that you work with your veterinarian and submit periodic fecal samples, collected at the right time, to evaluate the efficacy of your deworming regimen. This should be done even if you use the daily dewormer. Drug-resistant parasites are becoming a global problem. The parasites and the drugs involved in this resistance problem vary by region of the country, by farm, by the deworming protocol used.

Q: Could you please address the issue of daily deworming vs. paste deworming (every 8 weeks)? Which one is best for your horse?

A: There is no "perfect" deworming regimen that fits all farms and management systems. Daily deworming involves the daily administration of pyrantel tartrate. The goal of this program is to eliminate the immature stages of most parasites as they are ingested. The success of this program assumes that horses get the right dose of the dewormer on a DAILY basis. Even then there can be a problem with drug resistance developing and certain parasites may not be killed. Therefore, if your horses are on a daily dewormer you should continue to work with your vet to check fecals periodically to be certain the parasites on your farm are still susceptible to the daily dewormer and you should still include the use of a dewormer that is a larvicide and boticide (effective against bot larvae). If tapeworms are a problem in your area, you may want to use a product that contains praziquantel. At least once a year, preferably in the fall after the first frost (if you have a frost in your area), I recommend using an ivermectin / praziquantel compound. Remember that any newcomer that arrives on your property will probably have parasites that will not be killed by the low-dose daily dewormer. I recommend quarantining new arrivals, checking a fecal, and deworming them with a 5-day, double dose of fenbendazole (Panacur Power Pac / or Safe Guard Power Dose).

Q: I have heard some alarming reports recently of horses dying from the Zimectrin Gold dewormer (with praziquantel). One of them was the horse of a client of my riding instructor. Apparently the wormer can cause ulcerations of the mouth, esophagus, and who knows what else. The horses I heard about all died within a few days. I have used this wormer (or a different brand equivalent) a couple times on my teenage mare, and have had no problems, but I'm wondering if I should keep using it.

A: I cannot comment on those reports since I do not have the specific facts of the cases or autopsy reports. Sometimes an adverse drug event may be the result of more than one factor such as the concurrent ingestion of toxic plants or the presence of other underlying health issues or the presence of a high parasite burden, etc. The combination of ivermectin and praziquantel is available in several approved, licensed paste formulations. In order for these preparations to be licensed by the FDA they must meet rigid safety and efficacy standards. I still consider these combinations to be safe when purchased as the approved paste formulation (I do not approve of using compounded formulations) and administered at the appropriate dose to healthy horses. (Read more about ivermectin toxicosis.)

Q: In addition to natural management, are there any effective natural parasite control products that can reduce the frequency of administration of the pharmaceutical products? We are thinking of sensitive horses, those who have a history of reacting poorly to specific anthelmintics, or to medications in general.

A: I am not aware of any licensed, commercially available "natural drugs" that have been shown to control parasites in controlled studies. I am not sure what you include under natural management strategies, but some effective control strategies for strongyles include frequent removal of horse manure from paddocks and pastures; the use of sheep to graze horse pastures that are being rested; planting an annual crop in horse pastures; composting manure an adequate length of time to kill parasite eggs; and during hot, dry periods of the year (temperatures consistently above 85-90°F) removing horses from pastures and harrowing those pastures and leaving them open for several weeks before returning horses to them. Even with good management practices, it is hard to eliminate exposure to all parasites. Most internal parasites such as strongyles are transmitted in manure. But flying insects are responsible for introducing other parasites such as bots (Gasterophilus) and Onchocerca. Mites play a role in transmitting tapeworms. Foals and older horses are particularly susceptible to certain parasites. Periodically submitting properly collected and handled fecal samples is one way to help monitor your deworming program.

I am not sure what you mean by "sensitive" horses or what type of reaction they display when given a dewormer. Occasionally there are local reactions to the paste itself, possible irritation to the oral mucosa or stomach lining (especially if there is pre-existing irritation) or other systemic reactions such as loss of appetite, diarrhea, and colic. Some of these systemic reactions may be due to the effect of the drug on parasites your horse is harboring. Consult with your veterinarian to see if there is a pattern to these "reactions."

Q: I am looking for specific dosages to handle specific worms. Like what product to use, and how much, how many days/servings to take care of tapeworms.

A: There are two approved treatments for tapeworms in the horse: Praziquantel or a double dose of pyrantel. Most deworming protocols recommend administering a cestocide (i.e., a drug effective against tapeworms) annually in the fall of the year. If tapeworms are a major problem in your area, then you may want to include another treatment in the spring. It is always a good idea not to rely on only one drug class exclusively, so both tapeworm treatments should be incorporated into your deworming schedule (if tapeworms are a problem in your area).

Q: How often really should we be paste worming? Some deworm every other month, some every week, some say the worms re-infest every time they eat grass.

A: There is no one simple plan that fits all horses, all farms, all regions of the country and all types of parasites. To prevent the development of drug-resistant parasites, we should focus on using the right drug in the right horse at the right times. Not all horses need to be dewormed with the same drug at the same time intervals. Some horses are naturally more resistant to parasites than others. Working with your veterinarian and using properly collected fecal samples that are being evaluated for the right reasons at the appropriate times can be a valuable tool to help you and your vet design a program that is practical and works on your farm. Different classes of dewormers have different expected duration of effect. For example, if you are giving moxidectin it should not be given more frequently than every 10-12 weeks. Ivermectin has an expected duration of effect of 6-8 weeks. Pyrantel and fenbendazole have a shorter duration of effect (averaging 4- 5 weeks). No one drug should be used exclusively. A set dosing interval cannot be applied to all drugs and all horses during different times of the year. For example, during hot, dry summers the heat and lack of moisture will kill most strongyle larvae. Adult horses do not need such frequent deworming during these conditions. However, young foals are uniquely susceptible to roundworms (ascarids) and ascarid eggs are very resistant to hot temperatures and dry conditions. Therefore, young foals do need a more regular deworming schedule using drugs that are still effective against ascarids.

Q: How much is too much worming that it will be bad for the health of the horse?

A: There's no simple answer. But the goal is to be selective about which drugs we use and when. Use only what is needed to control the parasite problem. Other management strategies such as picking up and removing feces from pastures, properly composting horse manure, and grazing other species such as sheep on horse pastures are effective non-chemical means of controlling parasites.

Q: I have 1 horse and I have boarded 1 horse for 8 years. They live by themselves in my backyard with about 1 acre of rough grazing. I am careful to clean up manure in a smaller, more used area, but I don't rake the larger grazing area. Recent literature suggests that worming every 8 weeks may not be required since the only outside contact they have with other horses is an occasional organized trail ride; we don't do any showing. Could you please comment on this topic and make recommendations?

A: The best way to determine if your deworming program is adequate is to work with your veterinarian and perform periodic fecal exams to help fine-tune what drugs you are using and when you are deworming your horses. Manure removal is a very effective control strategy that should help reduce the frequency of treatments. Some horses are more resistant than others to parasites and a fecal egg count may help determine if either of your two horses is more or less susceptible to parasites.

Q: I live in San Diego county, with hot summer temps, low humidity, and zero rain for about 9 months a year. What's the best worming protocol for my desert climate?

A: During hot, dry summers you will probably be able to decrease the deworming frequency since this climate will effectively kill small strongyle larvae. If you have foals, they are very susceptible to roundworms (ascarids) and ascarid eggs are quite tolerant of heat and drought and can survive on pastures for up to 10 years. A bare minimum deworming program typically involves the administration of a larvicide at least twice a year to ensure large and small strongyle adults and larvae are adequately controlled. Manure removal is another effective, non-chemical parasite control strategy.

Q: Can you touch on cattle grubs and their effect on horses? i.e., how a horse gets them, how an owner can identify the exposure, risks to horses, what to do about them, and how to treat them.

A: Cattle grubs, also called "warbles," are due to Hypoderma infestation. Warble flies are bee-like in appearance and are most active in spring and early summer. The female flies deposit eggs on the limbs and hindquarters of cattle. Larvae hatch from the eggs and migrate in the subcutaneous tissues. Along with other signs, warbles form small nodular bumps along the back of cattle. Occasionally, horses that are pastured near cattle can become infected. Affected horses typically have only one or two nodular grub swellings that develop along the back in the region of the withers. These swellings cause local irritation, especially if they occur in tack areas. Most lesions have a small breathing pore in them which helps distinguish these bumps from other skin lesions in the horse. Larvae within the nodule can be removed CAREFULLY by your veterinarian by enlarging the breathing pore with a scalpel. The cyst and larva must be removed in their entirety and without rupturing the cyst. If the cyst ruptures, a severe systemic inflammatory reaction can occur. Ivermectin and moxidectin pour-on insecticides have been used in cattle (typically in early fall while the larvae are still in the early stages of migration) to treat this condition. If your horses are infected with grubs, you should consult with your veterinarian before pursuing any treatment. Good fly control is an important strategy to prevent initial infection with fly eggs!

Q: What really dictates the timing of using the different classes of dewormers? i.e., one product may state that it is best used in June and October anywhere in the Northern Hemisphere regardless of local conditions. How does a horse owner interpret his best schedule?

A: You are correct in assuming that it is not where you live but rather what the weather conditions are in your area. As most of us have experienced, different parts of the country can have colder or hotter, wetter or drier seasons than the "average" for that region. Hot, dry conditions with temperatures well above 85°F tend to kill strongyle larvae on pastures. During this type of weather, Mother Nature is effectively decreasing the pasture burden of strongyles for you and you do not need to deworm your horses with the same frequency. Temperatures well below 45°F will not kill strongyle larvae but will inhibit or arrest their development into infective larval stages for the horse. So during cold winter months in the North, horses do not have to be dewormed as frequently. Most deworming programs recommend using a dewormer effective against bots (a drug such as ivermectin) in the fall or after a frost, since cold temperatures will kill adult bot flies. If you administer a boticide at this time, your treatment will have a longer-lasting effect since horses will not become reinfected again until temperatures begin to warm up in the spring. If you live where there is never a real cold snap, then you will have to give a boticide twice a year. It is also recommended to administer drugs effective against tapeworms (e.g., praziquantel or a double dose of pyrantel) after the weather turns colder. Tapeworms require the intermediate host, the oribatid mite, to complete their development and mites seem to prefer warm, humid temperatures. The same thought process used to treat bots applies for the mite. If you treat for tapeworms after cold weather arrives, your treatment will remove tapeworms and your horse will not be re-exposed to tapeworm egg-carrying mites until warmer weather returns.

Treatments effective against encysted small strongyles (cyathostomes) are best given when large numbers of cyathostomes are hibernating in the walls of the horse's large intestines. This occurs when pasture conditions are not favorable for strongyle larvae survival, therefore it is best to use larvicidal drugs such as the Panacur Power Pac during the hot, dry summers or during the freezing winter temperatures. Those conditions may occur at different times of the year in different parts of the country, and may vary from year to year. A basic rule of thumb is to use a larvicide effective against encysted small strongyles whenever the grass turns brown (either due to freezing temps or drought). In general, the frequency of deworming can be decreased when temperatures rise and stay above 85°F or fall and stay well below 45°F. The exception is foals; to prevent ascarid infections, foals should be maintained on a more regular (every 2 month) schedule.

Q: I've read material that suggests picking up horse manure in pastures. How the heck can this be done in a practical manner?

A: There are commercial pasture vacuums available that can collect large amounts of feces on pastures. Otherwise, it is a labor-intensive technique that involves removing large piles of manure from the pasture on a regular basis. That may mean filling a wheelbarrow with manure from one section of the pasture once a week. The larger the pasture, the more impractical manual removal becomes. Every little bit does help reduce your horses' exposure to parasites.

Q: We harrow the manure, spreading it out to dry up. I have heard that this does not kill parasites but it does cut down on the fly numbers; is this correct?

A: Harrowing can be an effective way to expose parasite larvae and eggs to sunlight and heat. However, to be effective, harrowing should be done when climate conditions are hot (>85°F) and DRY. Then the pasture should be "rested" for at least several weeks until the exposed larvae are killed by the sunlight and heat. Only then should horses be reintroduced. This strategy applies only to strongyle type eggs; roundworm (ascarid) eggs are far more resistant and are not killed so easily. Foals and weanlings are uniquely susceptible to roundworms and are the primary source of infective ascarid eggs. These eggs can survive up to 10 years on pasture.

Q: We purchase a six times a year rotation package (3 and 3); one is advertised as an economy pack and the other a premium pack. They have the same basic idea, but one seems to have generic brands and the other name brands. Is there a strength difference between generic vs. name brands?

A: Each product differs. Read the labels to determine the amount of active ingredient as well as other chemicals and vehicles that are in the product.

Q: What are the best drugs/worming regimes to deal with encysted worms?

A: There are two products available that have a label claim to kill encysted small strongyles: Moxidectin and the larvicidal dose of fenbendazole: double dose once-a-day for 5 days (e.g., Panacur Power Pac or Safe Guard Power Dose). Encysted cyathostome larvae hibernate in the mucosa of the horse's large intestines for a variable period of time. Cyathostomes are ingested by the horse from the pasture as infective free-living third-stage larvae (L3). These L3 larvae invade and penetrate the mucosa of the horse's large intestines (i.e., the large colon and cecum) where they become "encysted" within a fibrous capsule. Within the cyst, the larvae pass through several developmental stages: Early third-stage larvae (EL3) followed by late third-stage larvae (LL3), which then molt to early fourth-stage larvae (EL4) and then complete their development to become late fourth-stage larvae (LL4). The LL4 emerges from the cyst and re-enters the lumen of intestines and develops into an adult cyathostome capable of laying eggs that then pass back onto the pasture in the manure. Approximately 75% of the encysted cyathostomes are believed to be present as EL3s. Only larvicidal fenbendazole is labeled to killed ALL THREE stages (EL3, LL3, and LL4) of the hibernating or encysted small strongyles. Moxidectin kills the later stages of encysted cyathostomes. Ivermectin is effective against the larval forms only after they leave the cyst. Drugs such as pyrantel pamoate (Strongid) are effective against the adult cyathostomes but not the encysted forms.

Encysted cyathostomes are difficult to diagnose. While the larvae are encysted, they are NOT producing eggs. So a fecal egg count may not detect how many hibernating cyathostomes are present within an individual horse. Most deworming regimens recommend administering a drug effective against encysted cyathostomes while most of the larvae are hibernating. This is because most of the damage occurs when large numbers of larvae excyst and penetrate back through the intestinal wall. Affected horses may develop colic, diarrhea, weight loss and edema. It is believed that large numbers of cyathostomes are hibernating in the horse's intestines when conditions are unfavorable for their survival on pasture. In the South this would correspond to hot, dry summers and in the North during the winters. A rule of thumb that helps remember when to use a larvicidal treatment is when the grass turns brown (either from high hot temperatures or drought conditions or after freezing temperatures). Climate and weather conditions dictate how and when to deworm rather than simply where you live in the country!

Q: I recently bought a yearling from a rescue organization. They dewormed him 2 weeks ago, and a month before that, both times with ivermectin. He still looks wormy, and I am wondering if it safe to give him another agent at this time, or should I wait given his age?

A: Ideally you should wait 6-8 wks after using ivermectin before giving another product. Have you performed a fecal egg count on this youngster to see if he is shedding ascarid eggs and / or strongyle eggs? There are drug-resistant parasites and it may be time to try a different dewormer if this youngster is still shedding high numbers of parasite eggs, particularly roundworms, after a recent deworming with ivermectin. The drug may not be as effective as expected in this youngster or he may have other problems. I encourage you to consult with your veterinarian and consider performing a fecal exam.

Q: My horses have been on the Pfizer Preventicare program for quite a number of years. They are fed Strongid C2x daily dewormer from Pfizer; and are also wormed every 6 months with an ivermectin product, also from Pfizer. This, along with annual physicals, vaccines and dental care is the Preventicare program protocol. Do I need to worry about parasite resistance? They are in a closed herd, turned out in a 1 acre dry lot in Arizona.

A: We do not always know how or why parasites develop resistance to a given dewormer. But the more frequently a population of parasites is exposed to a drug, the more likely that population of parasites will develop resistance. So even if your horses are not in frequent contact with outsiders, you should periodically check a fecal egg count just before deworming them and then 10-14 days later to see if that drug is still effective against the parasite population on your farm. Individual horses can also vary greatly in their susceptibility or resistance to parasites.

Q: Thank you for presenting this webinar on deworming strategies. We feed a daily dewormer and a friend mentioned that the compound can harm pastures. Does the daily dewormer cause any problem with pastures? Is there any additional maintenance we need to perform to keep the pastures healthy and productive?

A: I am not aware of how the daily dewormer can harm pastures.

Q: Is doing a fecal reliable to deworm or not?

A: Fecals can be helpful to determine which horses tend to shed the highest number of eggs and therefore require the most intensive treatment. Fecal egg counts can also help determine if the drugs you are using are still effective. Fecal eggs counts cannot adequately reflect the number of encysted small strongyles in a horse, and tapeworm eggs may be underestimated in most fecals. Foals may harbor large numbers of juvenile roundworm larvae and yet their fecals will not yet contain any eggs. So fecals can be very helpful in monitoring your deworming program as long as you appreciated the limitations.

Q: What potential hazards (if any) are associated with daily dewormers?

A: Daily dewormers target the early forms of parasites that the horse ingests. It is critical that horses on daily dewormer ingest the right amount on a regular daily basis. Even then some parasites may escape the effect of the daily dewormer and not all parasites will be killed. Therefore we still recommend the use of a boticide and larvicide an at least an annual basis. Resistance can also develop against the drug (pyrantel tartrate) in the daily dewormer. So it is still important to perform periodic fecal exams to be certain the daily dewormer is still effective on your farm. Foals may not voluntarily ingest a consistent amount of daily dewormer.

Q: What products and worming schedule should a horse in the Denver/Metro area of Colorado be on? Is there anything that is location specific that I should consider?

A: Rather than location, I suggest focusing on climate and weather. Strongyle larvae tend to die on pasture when temperatures rise above 85°F. Likewise, larval development on pastures is arrested when temperatures drop below 45°F. So during hot, dry seasons and cold winters, Mother Nature will help control the parasite burden on the pastures. You can decrease the frequency of deworming during those times. When the grass turns brown (either due to the cold or drought), it is a good time to administer a larvicide effective against encysted small strongyles. I recommend using a boticide in the fall after a good frost. Tapeworm treatments are best given in the fall as well.

Q: Do horses get worm aneurysms near their spine from not being wormed regularly?

A: Aneurysms due to migrating strongyle larvae occur in the walls of blood vessels that supply the intestines. These aneurysms are not near the spine. Use of a dewormer effective against large strongyle larvae should help prevent this condition.

Q: How do you address the issue of tapeworms?

A: There are now early reports from Canada that tapeworms may be beginning to show resistance. Unfortunately, fecal egg counts tend to underestimate the true incidence of tapeworms. Blood tests used in the USA tend to indicate exposure and not level of active infection. If tapeworms are a problem, there are two approved treatments: Double dose of pyrantel pamoate and praziquantel. It is recommended that one of these treatments be given in the fall. Some experts recommend a second treatment in the spring in areas where tapeworms are a real problem. Both drugs should be used rather than relying on one class of drug.

Q: We use 3 different types of dewormers - Ivermectin gold, Strongid and safeGuard. What would be a proper rotation schedule for horses in Florida? Kentucky?

A: No one size fits all. I would base my deworming schedule on fecal egg counts and weather conditions. Youngsters require a different strategy. In Florida, I would incorporate the use of a larvicidal dewormer effective against encysted small strongyles during the hottest part of the summer. In Kentucky, I would administer this treatment during the winter. I would include a boticide in the fall. Additional treatments will be based on whether individual horses are high or low egg shedders. That stratification is determined by doing a fecal egg count after waiting an appropriate time period after the last dewormer. The bare minimum that any horse should receive is two larvicidal treatments approximately six months apart.

Q: Since we deworm by weight, how do we know if we are over- or under-dosing if we don't have a scale to weigh the horse? Is it better to overestimate weight or underestimate weight?

A: A weight tape, although not as accurate as a scale, can be very helpful. Weight tapes are usually available at feed stores. I would tend to add 10% to the weight tape estimate. The one dewormer that should not be overdosed is moxidectin. Pyrantel, fenbendazole, and ivermectin have a wider safety margin and I would tend to overestimate the horse's weight when using these drugs.

Q: We are in our last month of winter right now. We almost lost a 2-year-oldpony to larval cyathostominosis (sp) recently. An unknown event triggered the encysted larvae to all burrow their way out at once, it was touch & go for the little guy. This colt grazes in an 8-acre paddock with 3 other 2-year-old colts and a 20-year-old stallion. I have 20 other horses on the property and none of them have ever suffered from this life-threatening issue. Would he have a genetic predisposition to allowing a worm burden to accumulate? I have owned him for 12 months, he is drenched regularly, they are all drenched at the same time with the dose appropriate to their weight. In your experience is the problem likely to recur? It is relatively rare in New Zealand.

A: You are very perceptive when you suggest that the pony may have had a genetic predisposition. We know that genetics plays a role in how resistant or susceptible some horses are to parasites. I would plan on deworming this individual on an annual basis with a dewormer effective against encysted small strongyles (i.e., larvicidal fenbendazole or moxidectin).

Q: I have a friend where I board whose vet told her it was ok to use ivermectin all year long as long as once a year she double-doses (give two doses at one time instead of one). She said this was the same as rotating which I don't believe. Doesn't this create resistant worms?

A: There are increasing numbers of reports of drug-resistant parasites in all parts of the country as well as from around the world. There are numerous reports of roundworms becoming resistant to ivermectin and moxidectin. Many of these reports come from farms that have used nothing but ivermectin every 60 days for many years. More recently there are reports of small strongyles becoming resistant to ivermectin. No one dewormer should be used exclusively for a prolonged period of time.

Q: Are there any species of parasites that can cross between species on a small farm with several types of livestock animals?

A: Horses can acquire lungworms form donkeys, but otherwise cross-species transmission is not common. That is one reason that grazing horse pastures with other species such as sheep can be beneficial.

Q: Where may I send a sample of my horse's manure to have it tested and how much does it cost?

A: The best place is to take it to your local vet. The fecal sample must be fresh and after collection it should be kept cool. The cost may vary a bit between clinics.

Q: What are your thoughts on daily dewormers?

A: Daily dewormers can very effective, but remember that resistance can develop to the drug in daily dewormers. Foals and weanlings may not consume enough on a regular basis. Even when using the daily dewormer, you should include at least one annual treatment with a boticide and larvicide.

Q: Is daily dewormer more effective in keeping horses healthy vs. paste wormer every 8 weeks (using healthy adult horses as an example)? Thanks!

A: There is no perfect program. Parasites can develop resistance to the drug in the daily dewormer. So regardless of the program you use, I recommend working with your vet to periodically monitor fecal egg counts. Even if your horses are on the daily dewormer, you should also consider giving an annual treatment with a boticide and larvicide. The proper use of different classes of paste dewormers at the proper time intervals can also be highly effective.

Q: My horse is rubbing his tail on trees, etc. I have wormed him for pinworms twice, but he is still rubbing his tail. Why?

A: Several explanations: Some pinworms are becoming resistant to certain dewormers. The tail rubbing may be due to something other than pinworms such as allergies and mites. I would consult with your vet. The best way to check for pinworm eggs is to use Scotch tape pressed around the horse's anus to collect the eggs that are then examined under a microscope. Not all horses that rub their tails have pinworms.

Q: Can you touch on cattle grubs and their effect on horses? I.e., how a horse gets them, how an owner can identify the exposure, risks to horses, what to do about them, and how to treat them?

A: Occasionally, horses pastured near cattle can be infected with Hypoderma sp. or warbles (also known as cattle grubs). The eggs of these parasites are spread by flies and form small nodules with a breathing pore along the back or withers of horses. Your vet can remove these larvae by enlarging the breathing pore. This must be done carefully or a systemic reaction can occur in the horse if the cyst of larvae is ruptured. Additional treatment includes ivermectin and good fly control.

Q: My yearling is constantly itching his tail. He's been doing this since I got him as a yearling. He is wormed monthly with what I consider to be a proper rotation. What chemical class or brand can you recommend to target pinworms?

A: Drugs that can be used to treat pinworms include ivermectin, moxidectin, and fenbendazole.

Q: Hello, a lot of people are now using diatomaceous earth for deworming. What are your thoughts or opinions on this?

A: I would not rely on this treatment to keep my horse's parasites under control. There is very little hard data about the efficacy of these treatments. There is no evidence that this treatment is effective against problem parasites such as encysted small strongyles, migrating ascarids, or a host of other parasites. I urge you to use standard monitoring techniques such as fecal egg counts to help monitor parasite burdens.

Q: At what age should foals be dewormed and what is the preferred product?

A: The most deadly parasite for foals is the roundworm. Roundworms migrate though the foal's liver and lungs before reaching the intestines. Foals should be dewormed beginning at 6- 8 weeks of age. Ivermectin (if there is not a resistance problem on the farm) and larvicidal fenbendazole (e.g., Panacur Power Pac) are effective against migrating ascarid larvae. Pyrantel (e.g., Strongid) is effective against adults. Foals should be dewormed on a regular basis every 60 days until they are at least 12- 15 months of age to be certain the ascarid infection is controlled.

Q: I haven't found much information about intestinal damage due to a heavy parasite load. The rescues brought to my farm are generally quite debilitated and carry a heavy parasite load. One of my concerns is the long-term damage that's possible in the intestinal tract due to worms. And can we tell without surgery whether or not damage is temporary or permanent? The deworming schedule here is every 10 weeks. Newcomers are slowly dewormed to avoid possible colic situations, so they may be given a variety of wormer paste at a lower dose over the course of several weeks, followed by ivermectin approximately 1 month after the last low dose was given. Can parasites cause scar tissue to form in the gut?

A: Past parasite damage, especially blood vessel damage due to migrating large strongyles, can be impossible to identify in horses. The result may be recurrent colic or poor condition, which are non-specific signs. Small strongyles that encyst or hibernate in the horse's large intestines can cause local inflammation and possible scarring where the cysts are formed in the mucosa. Large load of ascarid larvae that migrate through foal lungs may also cause residual damage.

Q: Is it true that some fecal exams can give false negatives when it comes to looking for eggs?

A: Yes. Fecals may not detect tapeworm eggs. Fecal egg counts may not be reflective of the true ascarid (roundworm) burden in foals or the burden of encysted small strongyles in adult horses.

Q: Are there dewormers that should be avoided by ponies or Miniature Horses?

A: No specific dewormers. Moxidectin is the one class of dewormer that must not be overdosed. So sometimes there have been problems with this class of drug when too much is accidentally given to small, underweight, or thin individuals.

Q: I was told by one vet to use only one wormer year round and to continue with that until it proves not to work anymore. Then another vet told me to continue to use the three different wormers every other month, which is what I have always done. So I am confused. I have two horses and neither one currently has worms (they were tested). I live in Florida where it's hot year-round, if that makes a difference. I always use wormers from different groups (so it's used twice a year in the rotation, ex: ivermectin, then oxy something, etc). So, given the conflicting info I have been told, what do I do?

A: In general, it is not a sound practice to rely on only one chemical class of dewormer for a prolonged period of time. There have been no new classes of dewormers developed for the horse in over 25 years so we need to preserve the efficacy of the products we have. I would prefer the program that uses more than one class of dewormer. Even so, I would encourage you to work with your vet and perform periodic fecal egg counts at the appropriate times to be certain your deworming program is still working.

Q: How were you able to identify a small strongyle problem in the Texas Tech herd? Was fecal larval culturing done? I ask because large and small strongyles produce similar eggs, correct?

A: During the early part of the study when fenbendazole resistance was intentionally induced, fecal cultures were performed. In general, the majority of strongyle-like eggs found in horse manure are from small strongyles.

Q: When you implemented your deworming program with the fast rotation, did you treat all horses regardless of their FEC or did you target only the ones shedding strongyle eggs?

A: In this study all the horses were dewormed at the same time regardless of the FEC.

Q: I have Miniature Horses. Some other owners have told me to double the size of the wormer dose in the spring. Does that advice have any merit?

A: No. I do not understand that advice.

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