While at least three types of tapeworms (Anoplocephala perfoliata, Anaplocephala magna and Paranoplocephala mamillana) are commonly found in horses, until recently no commercial dewormer was approved or labeled by the Food & Drug Administration for their removal. While we’ve always known that horses can be infested with tapeworms, traditionally we haven’t done much about it.
First of all, they are very difficult to diagnose. Fecal flotation tests that are commonly used to diagnose other horse internal parasites, such as roundworms and bloodworms, are not effective in diagnosing tapeworms. Secondly, although tapeworms were commonly seen during abdominal surgery, they were thought to be innocent bystanders that caused no real problems for the horse. Only recently have researchers determined that as high as 22 percent of spasmodic (gas) colic and nearly 80 percent of ileocecal (the junction between the end of the small intestine and the opening to the cecum) impactions are associated with tapeworms. Thirdly, no FDA-approved treatment has been commercially available to rid horses of tapeworms.
Horse tapeworms are different than the tapeworms found in most animals, including man. Rather than being composed of long chains of segments, horse tapeworms are pumpkin-seed-shaped parasites roughly 1-inch long and ½-inch wide. They have four suckers that enable them to attach to the horse’s intestinal lining where they absorb nutrients. Horse tapeworms are also unique in that they require an intermediate host, the forage mite, to complete their life cycles.
Immature stages of the tapeworm are passed with manure onto the pasture, where the forage mites ingest them. The immature tapeworms develop within the body cavity of the mite and are ingested by the grazing horse. When the horse digests the infested forage mite, the tapeworm is released and develops into an adult that attaches to the horse’s intestine. Tapeworm eggs are passed in the horse’s feces when body segments of the adult tapeworm break off and the cycle starts all over again. Because the segments are either absorbed or disintegrated before being excreted from the horse, tapeworm eggs are never seen in the horse’s manure.
The common of the three horse tapeworms, Anoplocephala perfoliata, prefers to attach near the junction of the small intestine and the cecum. The cecum of the horse is equivalent to our appendix but is 6-8 inches wide and 4-5 feet long.
Large numbers of tapeworms clustered around the ileocecal junction can prevent passage of feed, resulting in colic. The fecal flotation test commonly used to diagnose internal parasites is only 3 percent effective in detecting the presence of tapeworms, but a new serology test that screen’s the horse’s blood for a specific tapeworm protein antigen is much more effective in detecting their presence.
Survey studies conducted across the United States using this new technology have found the prevalence of tapeworm infestations in horses ranging from a low 12 percent of the horses along the Pacific Coast to as high as 95 percent of the horses in the Midwest.
East of the Mississippi, tapeworms are found in 60 percent of the horses tested. There also appears to be an age susceptibility to tapeworm infestations. Research has shown that young horses (6 months to 2 years) have a higher level of infestation. The level drops in mature horses (3-15 years) and then increases again in older horses (15 years and older).
There are probably two reasons for this pattern. The first is that mature horses are working horses and might not be pastured as much as young or old horses and therefore are simply not exposed to the forage mite. The second is that mature horses might develop an immunity against the tapeworms that wanes as horses become older.
Fortunately, FDA recently licensed a new anthelmentic (dewormer) that controls all three types of horse tapeworms. The tapeworm dewormer is combined with a broad-spectrum dewormer that also controls other internal worms and should be incorporated into every horse’s deworming program. For more information on this new dewormer program for your horses, contact your local equine veterinarian.