No More Hitchhikers

Internal parasites are insidious culprits. They steal, damage, and even kill. The worst part of the whole scenario is that they can't be totally eliminated, only controlled. In the theft department, internal parasites rob in two ways: First, they steal nutrients from the horse's body; second, they take money from the owner's pocket in the form of lost food value. Adding to the problem is the fact that there are many types of parasites, and each seems to have its own unique way of attacking its host--your horse.

It is estimated that more than 150 internal parasites afflict horses, according to an American Association of Equine Practitioners (AAEP) informational brochure that was funded by Bayer. Some parasite species, the report stated, can lay more than 200,000 eggs per day, so it is readily apparent that parasite loads can escalate quickly.

The good news is that scientific research has been successful in developing dewormers that are effective if used properly. There also has been improvement in administration of the dewormers. In days gone by, there were two basic ways to get the dewormer into the horse's body--via tube deworming or ingestion with feed.

Obviously, administering the dewormer with a nasogastric tube was the most effective because then one could be certain that the entire dose went into the horse's body. However, this method required the services of a veterinarian. Dewormers in powdered form were also effective, but sometimes horses found them unpalatable and simply wouldn't eat them.

Enter paste and gel dewormers, which made deworming less invasive to the horse and a chore that could be handled effectively by the average owner. However, these dewormers are only effective if the appropriate dosage is administered and if the dewormer is the proper one for that season, that area, and the parasites the horse has.

As we shall see shortly, the types of problems encountered with internal parasites vary by geographic area. The approach taken in Minnesota, for example, might not be appropriate for Florida. Horse owners must consult with a veterinarian so that an appropriate deworming program can be established.

No area in the United States is devoid of internal parasites. The United State Department of Agriculture's National Health Monitoring System (NAHMS) collected data on equine health and management practices from a stratified random sample of equine operations in 28 states as part of the Equine '98 study. More than 2,904 horse operations were involved in the overall study, with 985 of those operations participating in the parasite portion.

Fecal samples were collected from 8,516 horses, with the samples being analyzed at the National Veterinary Services Laboratories (NVSL) in Ames, Iowa.

Overall, 51% of the horses shed parasite eggs in their feces, and 79% of the operations had at least one horse shedding parasite eggs. Most of the eggs detected, said the NAHMS report, were from strongyles.

As we shall soon learn from experts in the field, internal parasites are most deadly in young horses. As the horse ages, he develops some immunity to internal parasites.

The ability of internal parasites to kill young horses was demonstrated emphatically in a presentation at the 1996 AAEP convention in Denver, Colo. Louise Southwood, BVSc, of Colorado State University (CSU), reported on a joint study by CSU, Washington State University, and the University of California, Davis. Its purpose was to determine the success rate of surgery on foals and horses with ascarid (roundworm) impactions. Most of the surgical cases involved foals under one year of age, with the range running from four to 24 months. The median age was five months.

The study results were grim. The mortality rate for horses treated surgically to remove the impactions was about 90%. Often, the impacted horses had been dewormed shortly before impaction, indicating that large numbers of the parasites were killed and that the sheer volume of dead worms moving through the intestine caused the impaction and resulting colic.

Having made the case that frequent deworming to avoid heavy parasite loads, especially among young horses, is vital, we'll now look at some of the specific parasites. This information comes from several sources, including the AAEP brochure, information from companies such as Fort Dodge Animal Health, Bayer Corporation, Pfizer Animal Health, and Farnam Companies, that market deworming products, as well as researchers who have authored papers and chapters in textbooks.

Parasites Cause Problems

First, it should be noted that different parasites harm the horse in different ways. Some can damage tissues and vital organs, including the major blood vessels of the intestines, lungs, liver, and stomach, as the parasites migrate through the horse's system during their life cycles. Others can cause obstructions and even ulcerations within a horse's digestive tract.

While there are some key signs that indicate that a horse is carrying a heavy parasite burden, looks can be deceiving. Sleek, fat horses can also be heavily afflicted.

That being said, some of the key signs of internal parasite infestation include:

  • Dull, rough hair coat;
  • Lethargy or decreased stamina;
  • Loss of weight;
  • Coughing and nasal discharge;
  • Tail rubbing;
  • Colic;
  • Depression;
  • Summer sores; and
  • Diarrhea.

Here is a brief rundown on the various internal parasites that afflict horses.

Ascarids (Roundworms)

The roundworm begins its life cycle when the horse swallows eggs, which can be found anywhere in the environment. Roundworms most often afflict young horses. As the horse matures, he develops some immunity. However, by that time damage might already have been done by the migrating parasite.

Eggs hatch after they are ingested by the horse. Internal damage can occur in the wake of hatching as the larvae burrow into the walls of the small intestine and, in the next stage, migrate into the veins. At this point, the larvae have their own highway for travel as the bloodstream carries them through the liver and heart. Eventually, the larvae are deposited in the lungs.

While in the lungs, the larvae find their way into the air sacs. Irritation to the air sac causes the horse to cough. When he coughs, the larvae wind up in the mouth. They then are swallowed and once again take up residence in the small intestine where they grow to maturity. Roundworms might attain a length of one foot. If left unchecked, they can accumulate by the hundreds and grow in size to the point where intestinal blockage is the outcome.

But that's not the end of the story. While roundworms line the small intestine, growing ever larger, the female roundworm is busy laying eggs that are passed out in the feces, and the whole cycle starts all over again. One female roundworm is capable of laying 200,000 eggs per day.

The roundworm is difficult to combat for several reasons. The eggs are very hardy and resistant to a variety of adverse weather conditions. Infective eggs can remain viable for years in contaminated soil. This means that undisturbed eggs might lay in a pasture or paddock for long periods of time and remain capable of infecting the equine which ingests them.

Virtually all dewormers are effective in combatting roundworms. Horse owners should consult their veterinarians to learn what is best to use in their specific areas.

Small Strongyles

As the NAHMS report stated, many of the eggs detected in equine feces during its 1998 study were from strongyles, with the most insidious and most prevalent being those of the small strongyle.

Strongyle eggs hatch when the temperature exceeds 45°F. They survive much longer in cold conditions, but no more than two weeks in hot weather. Moisture helps their survival; desiccation (drying out) is rapidly fatal. After being ingested, the larvae burrow into the wall of the large intestine, where they develop into fourth-stage larvae or, as they are referred to by parasitologists and veterinarians, "encysted cyathostomes."

When the larvae are encysted within the tissue (where they can remain for several weeks up to 2 1/2 years), they aren't causing much inflammation in the horse. However, when they emerge, especially if the emergence is in large numbers, the mucosal lining of the gut becomes inflamed. The result can include mild to severe colic, anemia, severe diarrhea, and weight loss.

After emerging, encysted cyathostomes mature into adults and produce eggs, which are excreted in the feces. After the eggs hatch, larvae molt twice and become the third-stage larvae, which can be present on the grass. They are ingested by grazing horses, and the cycle starts all over.

There are a number of dewormers that attack small strongyles. These include fenbendazole, oxfendazole, oxibendazole, moxidectin, ivermectin, piperazine, pyrantel tartrate, and pyrantel pamoate.

Large Strongyles

While large strongyles are still around and are capable of doing damage, they are no longer public enemy number one in the internal parasite world. That designation has been taken over by the previously described small strongyles.

The prime weapons involved in the demise of a great many large strongyles, says Craig Reinemeyer, DVM, PhD, of East Tennessee Clinical Research, are larvicidal dewormers. If a horse herd is well-managed and larvicides are used, he says, large strongyles can be eradicated. The key to eradication is to use a larvicide at least every five months and to maintain this program for at least 1 1/2 years. It's also essential to deworm any new arrivals with a larvicide and keep them confined for three to four days before turning out to pasture. Although the large strongyle has been beaten down on well-managed farms, it has not been eradicated and can still be found in populations that are not dewormed regularly.

Large strongyles cause damage as they migrate in the walls of intestinal arteries and other organs, triggering a chain reaction that starts with inflammation and ends with blocked blood flow to part of the horse's intestinal tract.


The tapeworm is a different breed of internal parasite because it needs an intermediate host--the oribatid mite--which exists free-living on pastures. The infection process begins when tapeworm eggs pass in the feces of the horse and are ingested by a pasture mite. Within two to four months, the mite will harbor a stage (cysticercoid) that is infective to horses. Six to eight weeks after an infective mite is eaten by a grazing horse, the tapeworm matures and begins laying eggs. Large numbers of tapeworms can cause ulceration in the large intestine and cecum, colic, and intestinal blockage.

One dewormer used to fight tapeworms has been Strongid (pyrantel pamoate) although technically, it has no label claims for efficacy. However, say officials of Pfizer Animal Health, manufacturer of Strongid, there has been lots of recent research on equine tapeworms. Some findings are that tapeworms afflict far more horses than previously thought. Pfizer is in the final stages of developing a new product for combating tapeworms.

Other Parasites

There are many other internal parasites, including bots (a product of fly eggs that are laid on the horse's body in late summer), pinworms, threadworms, and lung worms that can afflict the horse, but the ones listed previously appear to produce the most common problems nationwide.

Geographic, Climatic Differences

Approaches to deworming vary by area and climate. The important thing is to establish a deworming program in consultation with a veterinarian, then stick to it. One of the most important tools available to the veterinarian is fecal sampling. By examining a horse's feces, the veterinarian can determine what parasites are attacking the horse, which dewormers are still effective against a farm's unique populations of parasites, and how long various dewormers can minimize egg contamination. Armed with that knowledge, the veterinarian can advise the owner on how to combat them. Veterinarians also understand the limitations of fecal egg counts and can adjust a deworming program accordingly.

In the south, says Dana Zimmel, DVM, Dipl. ACVIM, Dipl. ABVP, of the University of Florida, the prime internal parasite is the small strongyle, and it's at its worst during the damp winter months. During the summer, she says, intense heat is a destructive force on the eggs and larvae of internal parasites, but in the winter environmental conditions favor strongyle survival.

Many veterinarians recommend using the egg reappearance period (ERP) to determine when deworming should occur again. ERP is the time between treatment and resumption of significant egg counts.

Says Heather Kaese, DVM, of the University of Minnesota, many horse owners cut back on deworming programs in the winter because of the intense cold, but this past winter was abnormally warm. As a result, she says, it was recommended that deworming continue as in summer--every six to eight weeks, and more often for young horses. It should be remembered that strongyles exist in large numbers even on frozen pastures. The only real respite from infection in northern climates is stabling horses to avoid the source of infection.

In the west, says Nicola Pusterla, DVM, University of California, Davis, the hot, dry summers help curtail parasite eggs and larvae when horses are on non-irrigated pasture. Horses on irrigated pastures will need a different deworming program. Management practices and types of dewormer used previously will determine how often horses are dewormed and with what product(s).

The care of our horses is our responsibility. They are in our control, and must be managed wisely. Consult with your veterinarian to use the products necessary to control internal parasites in your horses, and avoid overuse of unnecessary products.


The Food and Drug Administration (FDA) recently approved a new broad-spectrum parasite control product from Merial that will treat and control 61 species and stages of equine parasites. It is the first equine dewormer to be licensed for action against tapeworms. For more information, see page 18.


While little research has been done on anthelmintic (dewormer) resistance in horses, one study has shown that the problem might be worse than previously thought.

Ray M. Kaplan, DVM, PhD, Assistant Professor, Department of Medical Microbiology & Parasitology, College of Veterinary Medicine, University of Georgia, and colleagues investigated this problem in the southern United States. Parasite eggs in feces were tested in 39 farms with 24 or more horses in Georgia, South Carolina, Florida, Louisiana, and Kentucky. No management or treatment history was used to select the farms. The only exclusions, said Kaplan, were farms where fecal egg counts were too low to measure drug efficacy.

Horses were assigned to one of four treatment groups--fenbendazole (5.0 mg/kg), oxibendazole (10 mg/kg), pyrantel pamoate (6.6 mg/kg), or ivermectin (200 ug/kg). Fecal egg count reduction was calculated 10-14 days after treatment to determine efficacy of the dewormers. For the purposes of the study, the researchers assumed that all eggs were small strongyles (cyathostomes) since many studies over the past 20 years have demonstrated that in managed horses, greater than 99% of all eggs are small strongyles.

Efficacy was evaluated using the following criteria: Less than 80% reduction was considered ineffective/worms resistant; 80-90% reduction was considered equivocal/worms suspected resistant; and greater than 90% reduction was considered effective/worms sensitive.

The study noted, "Using a conservative measure for declaring resistance, the percent of farms with anthelmintic-resistant cyathostomes was 100%, 66%, 45%, and 0% for fenbendazole, oxibendazole, pyrantel pamoate, and ivermectin, respectively, with mean percent reductions (of fecal egg counts) of 32.4%, 69.2%, 74.7%, and 99.8%, respectively."

Kaplan said, "The prevalence of resistance found in this study is higher than that reported in previously published work, suggesting that the problem of drug resistance (in horses) is rapidly worsening. However, just because our study demonstrated that on 100% of farms we tested fenbendazole was ineffective doesn't mean it is ineffective everywhere. We only have a relatively small sample (although it's the largest study of this type done in the U.S.).

"Resistance to oxibendazole and pyrantel pamoate is highly prevalent" on these farms, he added. "Ivermectin remains highly effective. Furthermore, data from these five southern states--which are geographically and physiographically distinct--was remarkably similar," he added. "This suggests that drug resistance in cyathostomes (small strongyles) is highly prevalent throughout the entire southern United States and probably nationwide."

It should be noted that small strongyles go through an encysted stage when they don't lay eggs and during which they might not be susceptible to dewormers.--Kimberly S. Herbert

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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