Tapeworms in horses usually are not thought of as causing many, if any, problems. But as we learn more about how these internal parasites live and reproduce, we are finding that there are some problems that could be caused by these worms. The following information about tapeworms comes from the University of Kentucky College of Agriculture.
Unwelcome and unannounced they come, developing silently within the intestines of the horse. Within the cecum, the parasites usually surround the ileocecal valve. Having no mouth parts or digestive tract, they absorb nutrients through their cuticle. The equine tapeworm can grow to about 3 inches long by a half-inch wide. Its head, called a scolex, has four suckers that attach to the mucosa or lining of the intestine; below each sucker is a tiny flap called a lappet. Recent surveys in horses at necropsy in Central Kentucky revealed a prevalence rate of nearly 60%.
Tapeworms are a member of the group of parasites called flatworms, which also includes flukes. The tapeworms are referred to as cestodes. There are three species in the United States, Anoplocephala perfoliata, Anoplocephala magna and Paranoplocephala mamillana.
Tapeworm segments (proglottids) contain both male and female organs. Proglottids progress through development from immature, mature, adult and gravid. This last segment contains fertile eggs, sloughs off and passes in the manure. An intermediate host, an oribatid or free-living mite found on pastures, eats the tapeworm eggs, which undergo a period of development of two to four months inside the mite before reaching the infective or cysticercoid stage. For a horse to become infected with a tapeworm, it must, as it grazes, ingest mites containing the immature or cysticercoid stage of the parasite. The chances of a horse becoming infected are high because there are millions of oribatid mites in pasture.
Aside from the usual clinical signs of parasitism (e.g., unthriftiness, rough haircoat, lethargy, loss of appetite, diarrhea), it is very difficult to diagnose tapeworm infection. This is because the parasites do not lay eggs that can be readily detected by examining a fecal sample. Eggs present in the feces are the result of a ruptured proglottid.
Tapeworms are usually viewed as benign when compared with some of the other parasites. However, heavy infections can result in cecal hemorrhaging, blockage, ulcers, perforation and have been suspected of causing hypermotility within the intestine, leading to ileocecal intussusception.
Farm managers, owners and veterinarians who worm exclusively with the avermectin-type dewormers (ivermectin and moxidectin) are not addressing the tapeworm problem. These dewormers have no activity against tapeworms. Pyrantel has been proven to be active against equine tapeworms, and in about 2001 praziquantel, a drug used to control tapeworms in dogs and cats, was approved for use in horses.