Equine Parasites

Controlling parasites that feed on and damage the equine digestive system and various body organs must be an ongoing effort on the part of horse owners if these relentless attackers are to be prevented from compromising the animal's health and ability to perform. Despite scientific advances, it is a war that is difficult to win. We can win battles, but the war goes on. Relax for a moment and the parasites will attack with renewed vigor and quickly multiply.


Today, effective dewormers are available in paste and gel form that can be administered easily and effectively.

A discussion on the parasites themselves and of some management procedures that can be employed to control them is relatively basic and straightforward. However, when the discussion turns to treatment programs involving dewormers, it moves to controversial ground. The reason for controversy also is basic. The equine dewormer market is a large one, and a number of major companies are vying for their share.

Competition is healthy, and, in the case of dewormers, the horse has been the beneficiary as each company has sought to place before the consumer a product that answers the horse's needs. In this article, we will attempt to negotiate a path through this informational maze in such a way that we will inform the horse owner so that he or she can make proper decisions on how to treat a particular horse in a particular situation.

It is a must that parasites be controlled. If they are not, the horse they infest becomes unthrifty, will be prone to colic, and can suffer permanent damage to its intestinal structures. Parasites also can bring with them an economic burden. The horse which is carrying a heavy load of internal parasites is getting only partial value from the food it ingests.

Regular deworming, while highly important, is only part of the control picture. Management also plays a significant role. A horse which spends most of its time in a stall or paddock, for example, will be more apt to be infested with parasites than one which roams over hundreds of acres of open range.

Where one lives also can have a bearing. Horses in areas of the country that have long seasons of warm, moist weather will be more at risk from parasites than those horses living in hot, dry areas.

This means that a parasite control program must be geared to specific needs in given situations and areas. The best approach is to consult with a veterinarian before deciding on a parasite control program. He or she will know the needs of the area and will be able to discuss the merits of the various dewormers on the market.

While there is no deworming schedule that fits all horses, there are some basic guidelines that do exist. Most veterinarians will recommend that horses be dewormed at least four to six times a year. Many practitioners recommend deworming foals and weanlings every 30 to 60 days for the first year of life.

The time when one administers a dewormer during the year is significant, says Craig R. Reinemeyer, DVM, a researcher and instructor at the University of Tennessee. Research, he says, indicates that deworming programs work best if treatments are concentrated during the times when climatic conditions are favorable for the hatching of eggs and development of larvae.

Many horse owners who live in cold climates, says Reinemeyer, mistakenly believe that cold weather breaks a parasite's life cycle. Cold has little effect on survival of parasite eggs, he says. By contrast, heat can be a destructive force.

The parasite eggs, he says, are similar to plant seeds in that they flourish at temperatures that range between 45° and 85° Fahrenheit. When temperatures get higher than 85°, he says, the eggs often are destroyed. What this means, in Reinemeyer's opinion, is that the emphasis on controlling parasites in many areas should begin in early autumn and continue through February or March.

Basically, he contends, there is no need to deworm during the hot summer months because little transmission occurs at that time. The horses themselves tend to assist in keeping themselves parasite-free when on summer pasture. They will, whenever possible, graze on the "greens" of a pasture rather than on the "roughs," where they have left piles of manure--avoiding the ground around their fecal deposits. However, as the summer season wanes and the greens disappear, the horses will graze closer and closer to the roughs and in the process will end up consuming grass that might be harboring large quantities of parasite eggs.

The magnitude of the parasite problem is underscored by the fact that there are more than 150 types of internal parasites known to infect horses.

Rather than attempt to discuss each and every one of them, we will concentrate on the most significant parasites--ascarids, strongyles, bots, threadworms, and pinworms.

Much of the information that follows comes from Reinemeyer and two of his colleagues at the University of Tennessee, Doyle G. Meadows, PhD, and John E. Henton, DVM.

Ascarids (Roundworms)

Ascarids also are known as roundworms and primarily are a problem for foals and growing horses. As the horse matures, it often develops an immunity to ascarids and, as a result, older horses normally don't have a problem with this parasite. However, until that immunity develops, ascarids can wreak havoc with a young horse.

The life cycle of this parasite begins when the horse swallows eggs that are located on the food it is eating or in the water it is drinking. As part of this parasite's insidious nature, infective eggs can remain viable for years in contaminated soil. The principal source of contamination in a given year generally derives from eggs that have been deposited in pasture, stall, or paddock the previous year.

The first stage of the life cycle after ingestion is the hatching of the eggs. The resultant larvae burrow into the wall of the small intestine and from there migrate into the veins. The flow of blood through the veins allows the larvae to travel to the liver and heart and eventually to the lungs.

While in the lungs, the larvae make their way from lung tissue into the air sacs. They are then coughed up by the horse and reswallowed, putting them back to where they started as eggs--in the small intestine. Here the larvae grow and mature, reaching lengths that range from five to 22 inches. While in the small intestine, the worms, if allowed to grow unchecked, can reach such proportions that they cause intestinal blockage that can result in death. At best, the adult worms will compromise the ability of the young horse to digest food.

When mature, the roundworms lay eggs that are passed out with the manure. In about two weeks, larvae form within the eggs. At this point they are infective and ready to start the whole cycle all over again. One female roundworm is capable of laying 200,000 eggs per day.

While the adult roundworms pose a threat to the young horse's well-being, serious damage also can be caused by the migrating larvae. The larvae can cause scarring of liver and lung tissue as they travel along their migratory route.

All told, it takes about three months for a roundworm to complete its life cycle.

Because the roundworm eggs are so resistant to most adverse weather conditions, they can remain infective for long periods of time. It is up to the horse owner to break this deadly cycle so that one year's foal crop doesn't set the stage for a roundworm assault on the next year's newcomers. This involves deworming regularly and frequently during the foal's first year of life and adopting management practices that involve removal of manure from stalls, paddocks, and even pastures.

Obvious signs of ascarid or roundworm infection in foals include a rough coat, an unthrifty appearance, pot belly, depression, and, in a number of cases, a cough and nasal discharge that don't respond to antibiotics.

Strongyles (Bloodworms)

Perhaps the most common internal parasites to afflict horses are strongyles or bloodworms, also known as palisade worms, sclerostomes, and red worms. These dangerous parasites can infect horses of all ages, except newborn foals, and are found in all parts of the world. While roundworms take up residence in the small intestine, strongyles set up shop in the large intestine. The strongyles are divided into two types--large and small.

They also differ from roundworms, whose eggs can be found wherever there is manure, in that strongyle eggs generally are located on vegetation and little transmission occurs indoors. Like the roundworm eggs, strongyle eggs and larvae are resistant to adverse weather conditions, other than heat. The strongyle larvae are encased in a sheath that makes it possible for them to survive freezing winter weather. However, they are undone by hot, dry, summer conditions.

There are three major species of large strongyles. They are Strongylus vulgaris, which are up to 25 millimeters in length; S. edentatus, up to 40 millimeters long, and S. equinus, up to 50 millimeters in length.

Under favorable conditions, the larvae develop into the infective stage within one to two weeks after the eggs are passed. The horse ingests the eggs by eating vegetation that harbors them. Once in the large intestine, the larvae shed their protective sheath and begin a migratory route. The Strongylus vulgaris larvae are the most destructive. They burrow into small arteries in the gut wall and use them as pathways to the anterior mesenteric artery, which is the main blood supply to the digestive tract. Their migration causes disruption in blood flow by formation of blood clots in the artery. The larvae remain in the anterior mesenteric artery for approximately 120 days. During this period, they continue to grow and develop.

Upon reaching an appropriate level of maturity, the larvae start a return trip along the arteries, winding up once again in the large intestine, where they grow to maturity. The mature female is capable of laying several thousand eggs each day. The eggs are passed out with the manure to set the stage for a new cycle of infestation. The life cycle is six to seven months.

The other two families of large strongyles have similar life cycles, but they normally do not cause as much damage during their migration. They migrate within the liver and are capable of causing damage to that organ in the process, then return to the large intestine. They also have a longer life cycle than Strongylus vulgaris--eight to 11 months.

The availability of effective dewormers has curtailed the large strongyle population in horses. However, those same deworming agents were not quite as effective against small strongyles. Today, that has changed with the advent of a compound that appears to be effective in combatting small strongyles even before they shed their protective sheath.

The small strongyles normally are found in the cecum and colon. While they can cause serious health problems, small strongyles do not migrate. Most of the small strongyles feed superficially on the intestinal mucosa. The larvae are capable of lying dormant in the intestinal wall for prolonged periods. Toward spring in temperate climates, there can be a mass emergence from the intestinal wall, causing the afflicted horse to suffer diarrhea that sometimes reaches the severe stage.

Stomach Bots

Stomach bots bring us into contact with an internal parasite that varies a good bit from those already discussed. The stomach bot is actually the larva of the bot fly. The adult bot fly, which resembles a honey bee, can be extremely bothersome to horses during the summer months as it buzzes incessantly around the animal's legs and body.

The goal of the female bot fly is to lay eggs on the horse's hair as an integral part of the insect's life cycle. Once this has been accomplished and the little yellow eggs are in place, friction and moisture speed the hatching process. The horse obliges by licking at the eggs with its tongue. As it does so, small larvae emerge from the eggs and attach to the tongue and burrow into the tissue of the mouth. While friction and moisture assist the hatching process, the eggs located on the neck and face of the horse that can't be reached by the animal's tongue will hatch anyway and migrate to the mouth and lips on their own.

After about two weeks, second stage larvae emerge. They are swallowed and attach to the lining of the stomach. Once firmly ensconced in the stomach lining, the bot larvae, if undisturbed, will remain there for about nine months. They will pass out of the horse's system in the spring and the larvae will hatch into bot flies to begin the cycle anew.

Because adult flies are active from late spring until there is a killing frost in the fall, treatment is normally carried out in mid- to-late summer and after a killing frost in the fall.

Fortunately, the bot larvae normally do not cause serious damage to the stomach lining, although there have been rare reports of rupturing of the stomach wall. However, the larvae do cause damage to the tongue and gums.


Tapeworms have no age bias and can appear in horses young and old. They are parasites that need an intermediate host. That host is the oribatid mite, which exists as a free-living form on pastures. The infection process begins after the horse ingests the mites. About two to four months after ingestion, the tapeworm reaches maturity.

Large numbers of tapeworms pose a threat to a horse's well-being by causing ulceration in the large intestine and cecum, colic, and a severe form of intestinal blockage.

Strongyloides (Threadworms)

The scientific name for this parasite is Strongyloides westeri. It mainly infects young foals. The foals become infected by ingesting larvae in the dam's milk or by larvae present in the bedding. The larvae are capable of penetrating the youngster's skin. Once the skin has been penetrated, the larvae migrate through the lungs and the small intestine, causing injury while passing.

There are two bits of good news with this parasite--the life cycle is only about two weeks in length and the foal quickly will develop an immunity against recurring infestation.

While the life cycle may be short, the bad news is that the foal can be seriously ill during that two-week span. As early as 10 days of age, the foal can suffer from a severe form of diarrhea that resists treatment efforts.


Perhaps the least harmful of all internal equine parasites is the pinworm. Compared to the parasites already discussed, pinworms have a relatively simple life cycle. The adults are found primarily in the colon and rectum. They lay their eggs around the anus. The eggs then drop off and contaminate pastures, water, bedding, and feeding areas. The horse consumes the eggs with its feed or water, and the life cycle is renewed as the eggs hatch in the rectum and colon.

About the most serious damage caused by pinworms is irritation around the tail. This often causes the horse to rub his tail, sometimes until it is chafed and inflamed.

As mentioned earlier, a key ingredient in parasite control is good management of barns, paddocks, and pastures. The goal is to interrupt the parasite's life cycle. Obviously, based on the fact that manure is the primary means through which parasites are spread, it is imperative that proper manure disposal practices be employed.

If manure is to be spread on a pasture, it first should be composted. Putting uncomposted manure on a pasture that is being grazed guarantees that the horses using that pasture will be exposed to a greater population of parasite eggs.

Because the parasite eggs can't tolerate high temperatures, dragging or harrowing pastures during hot, dry summer days can be effective in reducing the quantity of eggs. However, if this is done during cool, damp conditions, one might only succeed in spreading the eggs over a wider area.

A key element involved in pasture management is population. It stands to reason that if a pasture is overcrowded with horses, it will be almost impossible for horses to avoid parasite eggs when grazing.

In enclosed areas, regular cleaning of stalls, pens, and paddocks can be effective in reducing the parasite population. It is important in these situations not to feed hay on the ground. Placing hay on the ground almost certainly guarantees that parasite eggs will be ingested. Raised feeders reduce the exposure.

In the case of bots, one can reduce the number of larvae that enter the stomach by removing the eggs before they have a chance to hatch. The bot eggs can be removed with a scraper, a warm moist sponge, or by clipping the hair.


Now, we come to the deworming agents. There are many products on the market, with the manufacturers of some laying claim to controlling all families of parasites that infest the horse, while other manufacturers say they are aiming at a more specific parasitic target.

Some dewormers, says Reinemeyer, are designed to remove only the adult parasites from the animal's intestinal tract. That, he says, has been a somewhat flawed approach because much of the damage is done by the migrating larvae.

There now are products on the market that can effectively destroy the strongyle larvae before they can even emerge from their protective sheaths. In the past, horse owners had two basic approaches they could take in administering dewormers. They could either place a powder in the horse's feed, something the animal often refused to eat, or they could have the horse tubed by a veterinarian.

Today, effective dewormers are available in paste and gel form that can be administered easily and effectively by the horse owner.

There also is a dewormer on the market that is in a palatable pellet form. If fed on a continuous basis, it keeps parasites from ever setting up shop within the horse's digestive system.

This type of dewormer, says Tennessee's Henton, enables a horse to utilize better the food that it ingests. Its only potential drawback, he says, is that it is so effective that it might prevent a horse from developing its own immunity to parasites. This could mean that if the horse were taken off the daily dewormer regimen, it might, for a time at least, be more vulnerable to parasitic attacks.

One of the newest and most effective classes of dewormers, say the researchers, is avermectin, which has been successful in combatting the four major internal parasites that afflict horses. Since avermectin has become available, they say, the incidence of large strongyle-related colic has decreased dramatically.

Named by the trio as the largest class of dewormers are the benzimidazoles. This class of dewormers is listed as being very safe. However, bots are resistant to benzimidazoles.

Phenylguanidines turn into benzimidazoles within the horse's body and provide similar parasite control. They should be rotated as though they were in the benzimidazole class.

Pyrimidines are a separate class of dewormers. They are effective against ascarids, strongyles, and pinworms, and if given in a higher dosage, can be effective in controlling the tapeworm.

Listed as the least safe of the dewormers are the organophosphates, which are used primarily to kill bots. They normally are used in conjunction with other drugs.

The newest product on the market utilizes the new drug compound moxidectin. Its supporters maintain that it can break through the encysted barriers of even small strongyles when they are in the encysted cyathostome stage. At this stage of the parasite's development, it has burrowed into the bowel wall and is encased in a resistant barrier or cyst. The dewormer making use of moxidectin is touted as being effective against large and small strongyles, roundworms, pinworms, bots, stomach worms, and hair worms.

The ongoing battle against internal parasites is complicated by the fact that these parasites often develop immunity to the drugs being used against them. There are dewormers on the market against which parasites have been unable to develop an immune response, according to their manufacturers. Whether that will continue remains to be seen. Internal parasites, just like the organisms that have developed resistance to many antibiotics, are adept at circumventing man's best efforts.

The only way to keep parasites under control is to combine good management practices with a deworming program that fits your specific area and stable or pasture situation. An indispensable ally of the horse owner in this battle is a veterinarian who understands the various compounds used in the manufacture of dewormers and also knows what is needed in a given geographical area. The veterinarian also is capable of doing fecal egg counts to help determine an appropriate deworming approach and to monitor the success, or lack of it, of a parasite control program.

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 www.exclusivelyequine.com or by calling 800/582-5604.

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