Equine Tapeworms
Republished with
permission from University of Kentucky
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.
CONTACT:
Heather Bair or Sharon Tolliver, (606) 257-4750,
Maxwell H. Gluck Equine Research Center
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