Tapeworms can cause serious disruption to a horse’s intestines. Anoplocephala perfoliata is the most common tapeworm of the horse. Each worm, the size of a pumpkin seed, burrows into the cecum and ileocecal junction (where the small intestines join the cecum), but also can be present in other intestinal areas.
The incidence of tapeworms varies throughout the United States. Reports identify Pacific Coast horses at about 17% incidence and horses east of the Mississippi at 60% incidence, whereas as many as 95% of horses in the Midwest are affected. Because this worm requires an oribatid mite as a vehicle for infection, horses on pasture are most at risk.
Studies have demonstrated that 22% of gas or spasmodic colic cases, 80% of ileocecal junction impactions, and all intussusceptions (the small intestines telescope into the cecum) are related to tapeworm infestation. Prevention can go a long way to minimizing these intestinal disturbances.
At this time, there is no evidence of any resistance by tapeworms to the two existing drugs that target tapeworms—praziquantel and a double dose of pyrantel pamoate.
Testing for Tapeworms
Historically, it has been difficult to identify tapeworm infestation in a horse. Standard fecal floatation testing methods that are useful for strongyle and roundworm detection have been unreliable in identifying tapeworms, particularly if numbers of worms in the horse are low, i.e., are less than 20 tapeworms. Up until now, diagnosis has relied on ELISA testing of blood for anti-tapeworm antibodies. Because antibodies against tapeworms can remain in the blood for up to six months, serological blood testing might not provide accurate information for individual horses. However, it can be useful for evaluating tapeworm presence in a herd of horses.
There is an ELISA test for tapeworm-specific antibodies using saliva to identify tapeworm burdens in the horse. To validate this test, saliva-testing results were compared to postmortem counts of tapeworms. A horse infected with even only a single tapeworm is confirmed positive through the saliva ELISA test, yet no false positives were reported. Anti-tapeworm antibodies clear from the saliva within five weeks, making it a more appropriate test for individual horses.
It is significant that there is a useful test for tapeworms, especially the saliva test that can identify even a single tapeworm. Curtailing the use of unnecessary deworming administration has the potential to mitigate the development of resistance to the limited number of drugs that target tapeworms.