The Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) has a rich history. Over the 50 plus years we have existed, TVMDL professionals have worked tirelessly to provide the most accurate and up to date resources to our clients. The following excerpt is part of an ongoing series of articles written by TVMDL’s first director, Dr. William Sippel, and other professionals from the earlier days of TVMDL. Some articles were written for features in The Cattleman publication where others were case studies over recent diagnoses. We hope you enjoy this look back on TVMDL’s history.
Veterinary Outlook: Parasites of Horses
By William L. Sippel, V.M.D.
Just as with cattle and other animals, the horse has its share of parasites. These include blood worms (Strongyles), roundworms (Parascaris), bots (Gasterophilus), stomach worms (Habronema), and tape worms. Probably bots and roundworms (Parascaris) are the most noticeable. Either of these can kill young animals if the roundworms are present in large enough numbers or if the bots penetrate the stomach wall during their attachment. With the exception of the strongyles the other parasites are less important.
Most horses are found to have more or less bots in the stomach at any times of the year. Generally they leave the stomach in the spring and early summer, develop into mature flies and lay their egg (nits) on hairs of horses. They gain entrance to the mouth of the horse and ultimately attach to the lining of the stomach. They do technical damage to the stomach lining and in some cases will penetrate the stomach wall and cause peritonitis and death of the horse. Death from bots is unusual however.
Parascaris, or roundworm eggs are swallowed from birth on by foals. After migration in the body, including passage through the lungs, they reach the intestinal tract and become adults. A mature parascaris female will lay as many as 200,000 eggs per day. The eggs are thick-shelled and very resistent, living for years on the ground. When large numbers of ascarids collect in one place in the intestine, they can rupture the gut and result in death from peritonitis. Heavy infestations also cause poor-doing, potbellied, long-haired foals that may have intermittent colic.
The most damaging parasites of horses are the large strongyles. One species of these parasites migrate beneath the lining of the large blood vessels of the intestines causing severe damage and death in many horses. Three different types of large strongyles affect horses in Texas, one of which requires six months to complete its life cycle; the other, nine; and the third, eleven or twelve months. Adult worms in the intestine lay eggs that fall on the pasture with the feces and develop into infective larvae in less than a week. This period of development is necessary to become infective and horsemen should not become alarmed at the habit of foals of (sic) eating fecal material as they do not acquire desirable bacterial and protozoal flora for their intestinal tracts in this manner (coprophagy).
After the larvae become infective they will migrate upward on a blade of grass and wait to be consumed by a horse. Young animals are much more susceptible as they have not acquired resistance or immunity through previous exposure.
After ingestion by a horse the parasites penetrate the intestinal wall and begin a long migration in the body. Some go to the liver (Strongyles edentates) where they wander and if the numbers are large enough can cause serious damage. Others ultimately invade the vessel walls of the intestinal tract (Strongylus vulgaris) where they burrow beneath the lining of the blood vessel. Because of this characteristic they are called “blood worms.” They cause considerable irritation and damage to the vessel wall and stimulate the formation of fibrin in the lumen of the vessel. If enough parasites are congregated in the same area, the irritation can result in blockage of the vessel or perhaps a portion of the fibrin will break off and go as far as possible until it blocks off a smaller blood vessel. The tissue served by this vessel will die unless enough collateral circulation from another source is available. It is thought that colic results from such an incident and is the reason this parasite is so damaging. Ultimately the parasites that invade the liver or the blood vessel walls migrate to the large intestine where they attach and suck blood. They reach maturity in this position and lay eggs that are passed out with the feces. As many as 5,000 eggs are produced per female per day.
In addition to the serious effects, such as colic, produced by the larval forms, the adults attached to the wall of the large gut are able to make horses quite ill. They go off feed, develop a fever, become depressed and lack pep. They also lose weight rapidly and may develop diarrhea or become constipated. Colic is also seen in this stage. Due to the blood sucking capability of the parasites, horses may become anemic. There is a serious effect on the serum proteins and horses may develop swelling on the ventral midline or “stocking” in the legs. These signs can cause confusion with equine infectious anemia or “swamp fever.” Various other blood changes are produced that can be detected in the laboratory.
Fortunately available drugs are highly effective against one or two of these parasites. A single drug that is effective against all of them is not available. Your veterinarian should be contacted for information on his choice of the drug or drugs to be used for control.
Probably of most importance is the adoption of a parasite control program. It should be apparent from the timing of the life cycle of the different parasites that a control program will need to be conducted throughout the year. It would appear that treatment for bots twice a year would offer adequate control. Roundworms are important throughout the year due to the large number of eggs on contaminated pastures and the fact that they are taken in nearly continually. The roundworm (parascaris) control program fits in very well with the strongyle control program as several drugs or combinations are effective against both of these parasites. The greater the horse population per acre, the greater the need for an intensive parasite control program.
In arid areas with very large pastures there will be less opportunity for infestation with strongyles and roundworms. However, even under these conditions horses usually use intensively, certain feeding, watering, or sheltered areas. Bot flies will seek out horses to lay their eggs (nits) so population per acre is of less importance.
In the author’s experience on Thoroughbred breeding farms it is necessary to worm horses every four weeks in order to keep the stool free of strongyle eggs. This frequency dismays some horsemen who think that a good parasite control program consists of worming their horses once or twice a year. When one considers the more or less continual intake of parasite eggs from the pasture, the inadequacy of such infrequent worming is evident.
It is theoretically possible to eliminate strongyle infestation in a band of horses. To accomplish this it is necessary to keep pastures from becoming contaminated with the parasite eggs. To achieve this, a worming program must be maintained that while keep the stools of the horses free of parasite eggs. Of course, no untreated animals can be placed in the pasture and all new animals added will need to be treated at least three times before placing in the pasture with other animals.
If all horses on a farm are given these three treatments in order to free them of strongyle eggs, and then placed in a clean pasture that has not been used for a year or more, they should not take in any more parasite larvae that can develop to egg-laying adults. By thereafter worming every four weeks for at least a year (preferably 18 months) all of the parasites that were migrating in the body should be killed and a strongyle and roundworm free animal obtained. What with the ease of worming horses in the feed, with a paste, suspension or bolus, this is not an unattainable goal. Better growth of foals, fewer cases of colic and animals that feel and look better will result.
To be effective, therefore, the worming program should start when the foals are eight weeks of age and treatment for strongyles (and ascarids) should then continue every four weeks thereafter.
Ranchers and horsemen are encouraged to discuss an intensive parasite control program with their veterinarian.