Tapeworms in Horses

Tapeworms are flat-bodied, segmented intestinal parasites that infect many types of animals including horses. Anoplocephala perfoliata is the type most commonly found in horses, although two other species are encountered infrequently. Tapeworm infestation was identified in horses well over a hundred years ago, but was not thought to cause serious problems. More recently it has been found that tapeworms play a significant part in several types of colic and other dangerous conditions of the digestive tract.

How does a horse become infected?

Tapeworms require an intermediate host, the forage mite, to complete their reproductive cycle. The mites live in the soil, crawling onto pasture vegetation especially during periods of hot, humid weather. Horses ingest these mites while grazing, thus taking tapeworm larvae into the digestive tract where the parasites mature in the next few months. It is estimated that an individual tapeworm lives in the horse for six to 18 months.

What regions of the country have the highest risk for tapeworms?

Theoretically, these parasites can be present anywhere there is topsoil. While all horses on pasture have some risk of exposure, rates of infection are highest east of the Mississippi River and in north-central states; moderate in the central part of the U.S.; and low in arid western regions.

Can a fecal check show the presence of tapeworms?

Although tapeworms do shed eggs that may be found in manure, the egg-carrying body segments are released only occasionally, so a fecal check can easily miss them. A blood test was developed in the mid-1990s to detect exposure to the parasites, but there is still no noninvasive way to determine the extent of infestation in a particular horse.

How do tapeworms affect horses?

Tapeworms attach to the intestinal wall with sucker-like mouth parts, creating irritation and inflammation. A favorite area of attachment for A. perfoliata is the ileocecal junction, the site where the small intestine joins the large intestine, and the inch-long tapeworms may be found in large numbers at this location. The two other equine tapeworm species, one smaller and one much larger, favor different sections of the digestive tract.

Paranoplocephala mamillana lives in the stomach and small intestine. Anoplocephala magna, which occasionally grows to a length of 30 inches, is also found in the stomach and small intestine. Tapeworm infestation is commonly blamed for weight loss, diarrhea, and frequent mild colic episodes, but there is no proof that the parasites are directly responsible for any of these conditions.

Impaction colic is a problem that has definitely been linked to the presence of tapeworms, with the parasites implicated in about 80% of cases. The worms also seem to play a role in spasmodic colic, where more than 20% of cases are associated with infestation.

Intussusception, a condition in which part of the intestine prolapses inside another section, is most frequently caused by tapeworms. This serious problem often requires a surgical procedure to save the horse’s life.

How can tapeworm infestation be treated?

Praziquantel, a tapeworm drug used in dogs and cats, was tested for equine application in recent years, and positive results were found. Deworming products containing praziquantel are effective in removing up to 99% of equine tapeworms. Zimecterin Gold, an oral paste dewormer produced by Merial; Farnam’s ComboCare oral gel; and Equimax Paste (Pfizer/Virbac Animal Health) are choices for tapeworm control.

The dewormers themselves have a high margin of safety, and no resistance problems have been identified. The suggested schedule for administration of anti-tapeworm products is once or twice a year, usually in the spring and fall. In horses with an extremely high parasite load, use of a dewormer could possibly lead to digestive tract problems due to the physical mass of dead worms and the toxins they release. Owners should read and follow usage instructions provided by the manufacturer and should address specific questions to a veterinarian.