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 1/2 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. Of the three, only A. perfoliata presents a problem to horse owners in Kentucky because the prevalence rate of A. magna is very low and P. mamillana is not found in Kentucky horses, although it is present in other geographical areas.
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 as shown in Figure 1. 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-types (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, but unfortunately, there is no compound currently on the market labeled as such. Until a commercial product can be developed for the removal of cestodes, concerned individuals should discuss appropriate methods of treatment with their veterinarians.
The Parasitology Section at the Gluck Equine Research Center is continuing to monitor the prevalence of tapeworms in Kentucky horses and has an ongoing program of drug-testing to find a cestocide that is efficacious and economical.
Heather Bair or Sharon Tolliver, (606) 257-4750,
Maxwell H. Gluck Equine Research Center