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Medical Index
First Posted Nov 23, 2008
Jul 30, 2010

Equine Internal Parasites

Harold C. McKenzie III DVM. MS, DACVIM
Marion duPont Scott Equine Medical Center, Leesburg, VA

The following information was given at a Seminar on Equine Nutrition and Health sponsored in part by the Virginia Horse Industry. It was held on October 17, 2008 at The Middleburg Agricultural Research and Extension (MARE) Center, Middleburg, Virginia. The following is being posted on HorseHints with the permission of the lecturer. This Seminar was beyond outstanding. My husband, Bill and I, feel so fortunate to have been able to attend.

Why do we care about parasites anyway?

  • Parasites can affect multiple organ systems
  • Can impair feed digestion and absorption
  • Can alter normal intestinal function
  • Can physically damage intestine
  • Can impair blood supply to the intestine
  • Can cause substantial blood loss and anemia
  • The liver can be affected by migrating larvae
  • The lungs are also a site for migration of some larval stages
Why worry?

For almost 30 years we have been somewhat complacent regarding parasite management. We have all felt that as long as we did our 'two-month rotations' everything was okay. Unfortunately this is not always true and this has led to a number of problems. New types of parasites are becoming a problem. Parasites that we have considered as 'controlled' are emerging as problems again. Some that we never regarded as a serious problem are turning out to be associated with disease.

How did we get there?

Three primary reasons:

  • Research in the 60's showed that deworming resulted in reduced shedding of strongyle eggs for 8 weeks. This led to the current recipe for treating with anthelmintics every eight weeks.
  • The development of easily administered paste wormers simplified anthelmintic treatment. Emphasis moved to routine treatment over monitoring. Veterinarians became less involved in parasite control.
  • Because we have Ivermectin! It was touted to be impossible for parasites to become resistant to this drug.
Historical Perspective

Forty years ago the primary parasite problem was with large strongyles. These were known as 'blood worms.' Strongylus vulgaris was the worst. It invaded the blood vessels supplying the large intestine resulting in clots which interrupted blood flow. This resulted in severe and often fatal colic. Even with surgery many horses died.

Large Strongyles

The good news was that they are not very complex parasites. They have a simple fecal-oral life cycle. Eggs are shed in manure, hatch in manure and larvae, attach to blades of grass where they are eaten by the horse, larvae migrate from intestine into bloodstream and then back into the intestine where adults suck blood from intestine wall and shed eggs. Larvae migrate from the intestine into the bloodstream and then back into the intestine where adults suck blood from the intestine wall and shed eggs. They are susceptible to a variety of anthelmintic drugs. There is not any stage in the life cycle where the organisms are 'protected' from anthelmintics.

What happened to large strongyles?

These parasites have become almost a historical footnote for most horses thanks to routine deworming. Why did treatment work so well? The simple life cycle of these parasites and susceptibility to most drugs meant that a single treatment would stop pasture egg contamination for 6 months. The larvae could only survive for 1 year in the environment. After only 18 months of routine treatment of the herd the organism is effectively eliminated.

Other Parasites Other parasites did not seem to be a major problem bach then:

Tapeworms: They were seen occasionally at post-mortem, but did not seem to be causing much damage.
Ascarids: They were only present in younger animals.
Small Strongyles: They were rarely present in large numbers and did not seem to cause problems.
Bots: Usually an incidental finding.

Anthelmintic Therapy

Historically this consisted of potentially toxic drugs that had to be administered by stomach tube: Dichlorvos, phenothiazines, piperazine. Realistically this meant that most horses were not dewormed very frequently. Some highly managed farms would monitor for parasite eggs in the manure to guide them on when to deworm. Many horses were never treated with an effective anthelmintic. Due to the poor coverage with anthelmintics colic due to large strongyles was unfortunately common.

Paste Wormers

At first only a few drugs were available, but the ease of administration meant that owners could administer the treatment themselves or it was much faster for the vet to administer The frequency with which horses received dewormers began to increase. As more drugs became available the concept of 'rotational deworming' became widely accepted.

Rotational Deworming

By using different drugs alternately we could prevent the parasites from becoming resistant to any single drug. Not a bad concept, but because of the frequency of administration resistance still developed. What we are seeing is evolution on an incredibly fast time line. Kill off the most sensitive organisms and only the resistant ones survive to reproduce. Continue selecting for the MOST resistant organisms until the population until the population doe snot include any that are sensitive to the drug anymore.

Along Came Ivermectin

Suddenly we had a new drug that supposedly had a mechanism that parasites could not become resistant to. The rotational concept was then revised for two reasons. First, if nothing can become resistant to Ivermectin why not use it all the time? Second, by using Ivermectin as the primary drug we could stop using the less effective drugs and improve our 'kill rate.'

Equine Tapeworms

While the early thought was that they did not cause many problems for te horse this opinion began to change. Veterinarians began to see more horses with obstructions of the small intestine at the ileum, where the small intestine empties into the large intestine. Large numbers of tapeworms were often present at this site (Anoplocephala perfoliata). In some cases the wall of the intestine was very thickened, perhaps secondary to the tapeworms. Research in the UK showed that there might be a link between tapeworm infestation and colic.


So why were they an increasing problem? Most of the commonly used anthelmintics did not eliminate these parasites: Ivermectin, fenbendazole, oxibendazole, pyrantel pamoate (single dose), and moxidectin. As we began to do a better job controlling many of the other intestional parasites while allowing the tapeworms to remain they were able to develop more severe infestations. Also, there is some evidence that tapeworms may alter the normal motility of the intestine, predisposing to certain types of colic.

Tapeworm Life Cycle

The life cycle is more complex than for the large strongyles. The parasite releases body segments known as proglottids. These contain numerous eggs. Eggs are released into the environment and then ingested by mites. The infected mites are then ingested when the horse eats grass. The tapeworm then matures in the intestine.

Tapeworm Treatment

Even as the appreciation of the potential importance of tapeworms increased there were few treatments available. The primary therapy was pyrantel pamoate (Strongid TM), Double-dose Strongid - 13.2 mg/kg orally once. A newer therapy is the drug praziquantel. This treatment is highly effective. Single dose of 1 mg/kg. Marketed in combination with Ivermectin, Zimectrin Gold TM, Equimax TM, Quest Plus TM (moxidectin and praziquantel).

The New Rotations

Ivermectin was used every two months except for late fall/early winter. This is thought to be the best time to eliminate tapeworms. Mites are unable to survive hard freezes in winter. Some programs include tapeworm therapy once every six months. Use of these rotations has decreased the incidence and severity of tapeworm infestations.

So, Now What?

With the use of these new regimens parasite problems should be a thing of the pst. But, along came the small strongyles! No one paid them much attention, and they probably were not a major problem when they were competing with the large strongyles for resources in the intestine. Now thy had everything pretty much to themselves. Even more importantly, these parasites have a complex life cycle which includes a stage where they are protected from most drugs which allowed them to survive despite deworming.

Small Strongyles

Cyathostomes: Adult worms produce eggs. The eggs are shed in manure. The eggs hatch and develop through three larval stages in the environment. The third stage larvae (L3) are infective and horse ingests them while grazing. L3 larvae penetrate the gut wall and encyst. The cyst wall protects them from most drugs. L4 larvae excyst, enter the intestine, develop to L5 stage and then to adulthood. The encysted L3 larvae do little harm. But they tend to all excyst (hatch) simultaneously, usually in the spring. The cumulative effect is to damage the intestinal wall and cause substantial inflammation. The inflamed gut wall can lose its normal motility pattern and this may predispose to colic.

Do They Cause a Real Problem?

The small strongyles are not typically a major pathogen in healthy horses that are well nourished and maintained at low stocking densities. Unfortunately low stocking densities are pretty rare in urban and suburban areas. Most horses probably suffer from sub-clinical disease: Poor feed efficiency, decreased performance. Horses with severe infestations show clinical disease. Cyathosomiasis: Weight loss, hypoproteinemia, anemia, poor growth, and colic.

Small Strongyle Treatment

The adult parasites are sensitive to all commonly used drugs. But the adults are not the problem. Treatment of L3 and L4 stages requires drugs that can penetrate the cyst wall. Only two currently available drugs accomplish this. Fenbendazole is only effective at higher doses and when given daily for five days. Panacur® Powerpack - 10 mg/kg once daily for 5 days and Moxidectin (Quest® - 0.4 mg/kg)

Designing Treatment Regimens

Use anthelmintics most in the horses than actually need to be treated. The horse population on any farm typically includes low, medium and high shedders. The medium and high shedders are the ones most likely to be affected adversely by parasites. By treating low shedders less frequently they are not harmed but serve to maintain a population on parasites that remain sensitive to the drugs we have available.

How Do We Determine the Risk of Shedding?

As few as 20-30% of the horses in the herd are responsible for harboring the majority of parasites; therefore, responsible for most of the pasture contamination. Use fecal egg counts to determine who they are. Perform fecal egg counts on the entire herd after a long pause in deworming (several months). High counts > 500 eggs per gram; Medium counts = 200-500 eggs per gram; Low counts < 200 eggs per gram.

When To Treat

Due to seasonality of excysting of cyathostome larval stages in areas with a true winter, you can time the treatments to best effect. Ideally once larvacidal treatment should be applied in the early spring, prior to excysting of the 4th stage larvae. A second larvacidal treatment in the fall reduces the burden of encysted larvae and aids in controlling the other intestional parasites. Animals with higher risk of shedding or farms with severe pasture contamination may benefit from one or two treatments in the summer as well.

What Now? Ascarids

Ascarids are an emerging problem in young horses typically weanling age. Horses develop natural immunity by 2-3 years of age. Infected animals are usually in good condition and are growing well. The primary sign of ascarids is typically acute colic.


Parascaris equorum has an interesting life cycle. Eggs are shed in the feces and are very hardy up to 10 years. Infective larva (L2) develop within the eggs. Eggs are ingested and hatch. Larvae migrate through the liver to the lungs. Larvae are coughed up and then swallowed. Mature ascarids reproduce and lay eggs in the intestine.

Ascarids - Why Are Ascarids a Problem Now?

Long term use of frequently dosed Ivermectin has resulted in parasite resistance. This has been reported in the U.S., Canada and Europe. More intensively managed farms are more likely to have resistant organisms develop. They often treat foals monthly with Ivermectin for the first year. The problem is hard to detect without monitoring fecal egg counts on a farm-wide basis. Typically sub-clinical; foals look and act pretty healthy unless they develop acute signs.

Ascarid Treatment

Do not deworm foals before 60 days. Treat at 8 week rather than 4 week intervals. Rotate among all effective drugs. If Ivermectin resistance is not present then continue to use it as part of the rotation. Also include: Oxibendazole (Anthelcide EQ), Fenbendazole (Panacur PowerPak), Strongid (pyrantel pamoate), and Quest (moxidectin).

Ivermectin Resistance

Only a few drugs appear to be effective. Panacur PowerPak for the full five days, Oxibendazole, Pyrantel (Strongid) - variable. Monitor the response to treatment with fecal egg counts. This will ensure that treatment is effective. It will also allow for modification of treatment if treated.

First Year of Life

Do not deworm foals before 60 days. Treat at 8 week, rather than 4 week intervals. Rotate among all effective drugs. If Ivermectin resistance is not present then continue to use it as part of the rotation. Also include: Oxibendazole (Anthelcide EQ), Fenbendazole (Panacur PowerPak), Strongid (pyrantel pamoate), and Quest (moxidectin).

Adult Horses

All horses-Larvacidal therapy in spring and fall. Quest (moxidectin), Panacur PowerPak (fenbendazole), Ivermectin (least effective). Continue a tapeworm therapy with these in the fall. (+/- spring). Double-dose Strongid, Praziquantel.

Medium Shedders

Same as for moderate shedders. Add an additional treatment in the early summer to minimize the spread of cyathostomes. Pyrantel pamoate (Strongid), Oxibendazole (Anthelcide). Ideally fecal egg counts should be monitored to ensure effective treatment. Combining these two drugs may increase effectiveness.

Heavy Shedders

Same as for moderate shedders. Add one more treatment in the late summer period. This will minimize the degree to which they contaminate the environment. Best to use a different drug than earlier summer treatment. Ivermectin (effective against adults), Moxidectin.

What About Bots?

Bots do not cause serious health issues for most horses. Control is indicated to minimize damage to the stomach. If not controlled bots can erode stomach wall and perforate. Best addressed during the winter. Eggs are laid by Gasterophilus flies. Flies do not survive in winter. Larvae migrate from the mouth to the stomach and develop on the wall. Ivermectin and moxidectin are the primary treatments.


We cannot continue to do things the way we have for the last 30 years. Evolution is constantly changing the battlefield. Effective parasite control will require assessing and monitoring the actual parasite problem on each farm and monitoring the effectiveness of treatment.

Always check with your vet.

Not part of the seminar, but of interest. I was advised not to use Quest as there has been a higher incidence of problems such as colic with its use. Therefore, I do not use it.

Also my article on Parasites will give more information and graphics.

Medical Index