For a long time, parasitologists recommended that horse owners utilize the same deworming program for all of their horses on a routine and regular basis. And why not? It was an efficient and easy method of parasite control. Now, parasitologists are rethinking those protocols. We're being given a warning that if we don't take care today, we might be sorry tomorrow. In the case of dewormers, the concern revolves around parasite resistance to anthelmintics: The greater the amount of anthelmintics we throw at these parasites, the greater the likelihood of creating a genetically evolved parasite resistant to that drug.

The experts now say that while nearly every deworming product is available to the horse owner, there is no one method of deworming that benefits all horses. In fact, there is no one deworming regimen that suits all horses on one farm. This is a call to re-involve your veterinarian in planning your deworming program in order to postpone creating resistant parasites as long as possible. Sound like a threat of doom? It's scientific fact. Following we'll discuss what has happened, what could happen, and how horse owners can learn from past mistakes.

Looming Disaster?

Macrocyclic lactones, the drug class that includes ivermectin and its younger cousin moxidectin, are extremely effective anthelmintics against numerous stages and species of worms and bots. Other drug classes, such as pyrimidines (the pyrantels) and the benzimidazoles, also play important roles in parasite control.

But, history has shown that parasites can develop resistance to a drug, evolving into a sort of "super worm." These parasites can respond genetically to the pressures inherent to being treated with anthelmintics, explains Craig Reinemeyer, DVM, PhD (parasitology), president of East Tennessee Clinical Research. "In many cases, they can develop bio-chemical mechanisms that confound the activity of these drugs and make them relatively useless. Bio-chemical adaptations have a genetic basis and are passed on to future, resistant generations."

It's happened before with earlier classes of anthelmintics. Presently, small strongyles are becoming a challenge. "We know already there is quite a bit of resistance to the benzimidazole drugs (Panacur, Benzelmin, Anthelcide)," reports Anne Zajac, DVM, PhD (parasitology), parasitology teacher and researcher at Virginia-Maryland Regional College of Veterinary Medicine. "We are also getting reports of resistance to Strongid pyrantel (Strongid)."

Ivermectin has been around for nearly 20 years; as yet, there have been no documented reports of resistance--in horses. In sheep and goats, it's another matter: Ivermectin resistance is an "enormous" problem, states Zajac. She blames that, in part, to deworming protocols that call for overly frequent deworming. "We'd tell people, 'You need to deworm every month in the summer,' so people were just deworming and deworming. The more often you treat, the more rapidly you're going to select for resistance. We know there are worm populations that are capable of developing resistance to ivermectin, and I suspect that we have that to look forward to in horses, as well."

Because ivermectin and moxidectin are from the same drug class, resistance to one means resistance to both. "These modern drugs we have are really good drugs, and they do the job until you select for resistant parasites," Zajac says. "Once resistance occurs in those drugs, there won't be very many drugs left to use."

In fact, there aren't very many classes of dewormers out there anyway--only about three or four, notes Reinemeyer. "We've only had one new class of compounds in the last 15 years, the macrocyclic lactones--ivermectin in 1983 and moxidectin in 1995. We don't expect to see many new classes or compounds coming down the pike in future years. We need to treat the ones that still work with great care to keep them viable."

Responsible Deworming

Using anthelmintics responsibly involves creating a deworming program for the individual farm whereby drugs are used primarily when they're needed, and only in the horses that need them. Parasitologist Dennis D. French, DVM, Diplomate American Board of Veterinary Practitioners (equine practice), professor/clinician School of Veterinary Medicine, Louisiana State University explains, "You deworm the horses that are re-infecting the premises on an as-needed basis, and leave alone those that are not shedding eggs." Shedders are identified through fecal egg analysis.

This method of deworming is known as selective, targeted, or as-need deworming. Here's how it works:

Instead of deworming all of the horses via interval, seasonal, or continuous deworming programs, each horse receives a fecal egg analysis every two or three months during periods of maximal parasite transmission; while this does not necessarily indicate parasite load in the horse (some damaging parasites can live for many months in a horse before producing eggs), it does indicate transmission potential to other horses in the herd.

Horses which are shedding eggs are dewormed and receive follow-up monitoring of their feces to measure the effectiveness of the anthelmintics. Horses which aren't shedding eggs are dewormed periodically in order to deal with damaging stages of parasites that might not yet be producing eggs.

French believes that annual deworming is sufficient for adult horses with adequate pasture, whereas Douglas E. Hutchens, DVM, clinical assistant professor, University of Illinois, believes precautionary deworming should be done twice a year. "Large strongyles have a long prepatent period from infective stage to egg-laying adults," he says, "It can take up to 11 months."

Additionally, a responsible deworming program utilizes integrated pest management as an adjunct to deworming products in order to reduce parasite numbers in the environment. "I suggest that one of the major reasons we're getting into the problems with drug resistance now is our dependence on highly effective modern dewormers to the exclusion of any other techniques of parasite controls," Zajac states. "We've got to get back to the idea of using different modes of control as much as possible.

"One thing that works great is picking up manure. Most people just laugh when you suggest they pick up manure, and clearly if you have enormous pastures and not very much labor, picking up manure is difficult. But even if you remove manure only from a couple of paddocks near the barn where a lot of horses spend a lot of time, it will help reduce the overall transmission of the parasites. The idea here is not that horse owners should switch from using drugs to picking up manure, but that they can combine these techniques for effective parasite control. Picking up manure in a heavily populated, limited area a couple of times a week is going to help remove the number of parasites and cut down on transmission. If you can cut down on the number of worms that are out there, you can start cutting back on the number of treatments that you need. Owners should work with their veterinarians to develop integrated control programs."

Hutchens points out that while daily manure removal is best, even a couple of times a week is good "because it takes a few days before those eggs become infective." He also suggests horse owners should properly dispose of manure and not spread it on the pasture during parasite-friendly seasons. "You can harrow your pasture to help break up the fecal pat so that the larva will die, but you must only do that in the coldest or hottest, driest weather," he warns, "because when there is adequate rainfall and the weather is good, all you're going to do is spread more larvae across your pasture."

Additionally, keep fecal matter out of water and feed sources, rotate pastures if possible, and always quarantine and deworm horses new to the farm.


A targeted deworming program is created for the individual farm based on several factors: Knowledge of when parasites are a problem in the farm's geographic area, the populations and activities of the horses contained on the farm, age of the horses, and what monitoring reveals in the individual animals.

Control programs need to be specific to the geographic region and climatic pattern where the horse resides. "These parasites absolutely rely on an environmental stage for transmission," emphasizes Reinemeyer. "Many parasites cannot transmit from one infected horse to another horse or re-infect the original horse without going into the environment. All of those environmental events are controlled by climatic conditions, primarily temperature, but also to some extent humidity and sunlight."

For example, in the south, summer months are so hot and often so dry that parasites are not active in the environment. During that time and in that area, one might be able to back off on deworming. By the same token, parasites might be inactive during the harsh winter months in the north, whereas in more temperate states like Tennessee, larvae can increase in winter pastures.

The number of horses on a property, stocking rate, and populations that are either static or changing impact deworming protocols.

A closed herd of adult horses is the easiest situation with which to work. Adult horses usually are immune to roundworms (fewer parasites to be concerned about), and a closed herd means that pastures or paddocks aren't being re-infected by parasites brought in by outside horses. Once these horses are dewormed and a fecal analysis confirms a low or non-existent egg count, only minimal deworming might be necessary.

Targeted deworming becomes more challenging when there are populations of horses that come and go on a property. "Where there is a lot of animal movement onto and off of the farm, you may have incoming animals with high parasite loads," explains Hutchens. "They're going to drop these eggs all over the pasture." In addition, Zajac points out, incoming horses might introduce resistant parasites to a farm.

Even in a population of changing horses, targeted deworming still could be an option if care is taken to deworm and quarantine all new arrivals for the first few days, or if those new horses are stalled. "Horses that are housed primarily in stalls are at very reduced risk of picking up parasites and are of no risk to those horses that are housed on pasture," says French.

Stocking density also affects the risk of parasite infection. "The parasite challenge for three horses grazing on 10 acres is probably very modest, and the intensity of control for those horses could be much less than for 20 horses on five acres," notes Reinemeyer.

Because horses tend to defecate in one area and graze in another, having more grazing space per horse allows horses to keep some distance between grazing areas and fecal sites, and thus reduces the risk of infection.

Age of the horse is another factor in a targeted deworming program. Adult horses generally have acquired immunity against roundworms. In addition, many adult horses might have a varying degree of immunity against other parasites. However, horses under the age of two years have not acquired immunity against roundworms, and, warns Hutchens, "roundworms migrate through the lungs and liver before you have mature egg-laying adults. You can do a fecal exam and go for targeted treatments in young horses, and it would never work because they may have massive infections and not be shedding a single egg."

Instrumental in maintaining an effective, targeted deworming program is monitoring the herd via fecal egg counts in order to assess potential parasite transmission and presence (keeping in mind that immature stages of parasites do not lay eggs).

"Our general recommendation," Hutch-ens says, "is to deworm the horses, then check fecal samples 14 days later to make sure your anthelmintic is working and that you don't have resistant worms that are living through the treatments."

Horses with no or low egg counts will not need further deworming until egg counts rise or they receive a precautionary once or twice yearly deworming to eliminate non-egg-laying parasites that might be present; horses with higher egg count levels should be treated.

Fecals should be checked for parasite eggs every four to six weeks during periods when parasites are active in the environment. Otherwise, fecals can be checked every three months, says French.

While fecal samples indicate the presence of adult (egg-laying) parasites, samples also can hint at the individual's immune level against parasites. "Every time I do an exam on my wife's horse, it never has eggs," notes Hutchens. "Conversely, my daughter Megan's pony always has eggs. That indicates that there are differences in immunity levels where some horses seem to have greater immunities than other horses. "

Studies in England and the United States support those observations: Fecal analyses in populations of horses showed that about two-thirds of the horses had an average number of eggs eight to 10 weeks after deworming, a few had extremely high egg counts, and a few had extremely low egg counts.

"Time after time, these horses usually occupy the same ranking order," Reinemeyer says. "There are individuals that consistently have low egg counts and that can probably handle parasitism quite well on their own without any kind of chemical interference on our part. It's a genetic characteristic of the horse, and it's heritable."

Pros And Cons

Besides minimizing the risk of creating resistant parasites, targeted deworming offers other benefits.

"Deworming on an as-need basis is economically advantageous," reports French. "This is very practical for a large farm once the populations (of shedders and non-shedders) are identified." Zajac concurs, "A couple of studies on selective deworming in horse herds found it was less expensive to run fecal exams on horses and only deworm the ones that need it than it is to routinely deworm all of the horses."

Allows horses to develop immunity to parasites
"One of the dangers of the very rigorous control programs, every so many weeks all year long, is that the parasitic challenge these horses experience may be so extremely low that they do not develop adequate resistance or immunity," Reinemeyer says. "This is borne out by anecdotal reports. We are fairly certain that some exposure to these worms is necessary for the horse to maintain its immunity. Horses need to see these worms, and they probably need to see them relatively frequently or constantly in certain numbers to maintain resistance to them. As-needed deworming affords a little more exposure and immunity."

Reinemeyer adds, "Unfortunately, the immunity to these internal parasites is not absolute. But while exposure to parasites doesn't eliminate them, it probably decreases the disease-causing effects of the parasite, making the horse more tolerant to parasitism. You don't see as much illness, colic, weight loss, etc. as you might in a horse that's an absolute novice."

Like nearly everything else, though, targeted deworming programs do have a few drawbacks, as well.

More work
"Targeted deworming is more labor intensive," says Zajac. "You have to collect the fecal samples, perform the tests, and evaluate the results. For a lot of people, it has been easier to buy dewormer and go down the line and deworm everybody."

Risk parasite loads
"A disadvantage," Hutchens points out, "is you may have a very high parasite load of immature worms with a low egg count. If you're not taking care of those immature worms, you can run into disease from parasite infection and not know it because you're just checking fecals." That's why Hutchens and other experts recommend occasional dewormer.

An Ideal Program?

Given the work sometimes involved with targeted deworming, particularly with large and/or changing herds, the success of implementing targeted deworming programs on a national scale probably will come down to reality issues, such as ease and economics. "I would like to see current deworming programs change," French says, "but owners will have to become more educated as to what parasite control is about. Currently, it is very easy for owners to put a paste formulation dewormer into their horse and consider it dewormed. Changing to an as-needed basis will require effort to collect fecals and determine epidemiological cycles for parasites in various climates."

Considering these inconveniences along with the advantages, is targeted deworming the ideal program for adult horses?

"Yes, in my mind it is," affirms French, "but standards change from farm to farm as to what is ideal. I am not naive enough to think that all of my clients will support these concepts because of what they have been told for the past 30 years." Nevertheless, he says, the clients who have tried targeted deworming report they are very pleased with the program's results.

Few veterinarians or researchers expect significant changes any time soon in the way most owners deworm their horses. "This is a hard thing to sell," says Zajac. "In the near future, I expect more of the same. How much we change in the more distant future depends on how rapidly resistance becomes a serious problem. As soon as somebody describes ivermectin resistance, that will be the signal that things have to change, soon!"

Reinemeyer believes there will have to be a concerted and cooperative effort to bring targeted deworming to the forefront. "We need to get much better education into the hands of the practitioners as far as monitoring the efficacy of the drugs and the effects of the treatments. Scientists will have to develop programs for different situations--not just one recommendation that's supposed to fit a 60-horse boarding stable versus three retired horses on 10 acres." He notes, too, that changes will need to be made on the business end, where commercial pressures to sell as much compound as possible are contrary to the biology of the parasite and the long-term sustainable use of these drugs.

"It's going to boil down to good, solid, scientific information," Reinemeyer adds. "Efficient education and dissemination of that knowledge is needed by veterinarians, by extension programs, by pharmaceutical companies. We've got incredible compounds right now, but we need to use them more wisely."

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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