Three tapeworm species infect horses: Anoplocephaloides mamillana (formerly Paranoplocephala mamillana), Anoplocephala magna, and A perfoliata. In recent years, the two former species have become rare in managed horses. The only species with documented clinical impact is A perfoliata. This parasite commonly occurs in grazing horses throughout the world but tends to be rare or absent in arid climates.
Free-living and commonly occurring oribatid mites act as intermediate hosts, and horses accidentally ingest these while grazing. The mites carry the infective stage of the parasite, the cysticercoid, which is digested free of the mite’s tissue in the intestinal tract of the horse and forms a scolex. The scolices attach to the cecal wall and tend to cluster around the ileocecal valve. In cases of large burdens, worms can be found in the ileum as well. As the worms mature, they form proglottids (tapeworm segments), which are released into the intestinal content as they mature and fill with eggs. The proglottids subsequently disintegrate, releasing the eggs into the intestinal lumen.
Tapeworm-associated colic occurs when a large number of A perfoliata attach to the cecal and ileal walls. This can lead to a mechanical obstruction as well as mucosal damage, ranging from hyperemia to areas with multiple confluent necrotic ulcers at the attachment sites. The latter is speculated to cause intestinal hyperperistalsis, leading to intestinal spasms, ileal impactions, and ileocecal intussusceptions.
In countries with distinct seasonality and grazing seasons, the presence of mature tapeworms in the intestines is often lowest during summer and highest over winter and early spring, reflecting fluctuations in pasture infectivity.
Cases of tapeworm-associated colic occur at any age and can present with a variety of signs, depending on the pathology in the affected organs. In cases of ileal impactions or ileocecal intussusceptions, horses can present with the following clinical signs:
moderate to high heart rate
decreased or increased intestinal peristalsis
firm intestinal mass that can be palpated rectally
moderate to severe pain
positive gastric reflux
Clinical laboratory findings are generally unremarkable, but can include:
increased white blood cell counts
A tentative diagnosis of ileal impaction or intussusception is made based on the clinical findings, and this can be supported by ultrasonography.
Although diagnosing the parasitic etiology has limited implications for managing the patient in the acute phases, it does have value when subsequently advising the client. The reliability of currently available equine tapeworm diagnostic methods are well investigated, and standard egg-counting techniques such as the McMaster have a diagnostic sensitivity of <10%, making them practically useless for this purpose. However, modified egg-counting techniques exist. These have a sensitivity of ~60% for detecting any tapeworm burden and 90% for detecting the more clinically relevant burden of >20 worms. These techniques involve processing larger amounts of fecal matter and centrifugation to concentrate and float the eggs.
Repeated episodes of spasmodic colic are sometimes assumed to be caused by tapeworm infection. Although this cannot be ruled out, other etiologies need to be considered as well. A positive fecal test can help support the tentative tapeworm diagnosis. Commercially available ELISAs exist to measure anti-tapeworm antibodies in serum or saliva, but these have limited utility on an individual case basis because they reflect recent parasite exposure and not necessarily current infection.
Cases of ileal impaction or ileocecal intussusception require surgery, which should be combined with general supporting measures:
correction of acidosis/alkalosis
The current recommendation is to only consider anticestodal treatment once the colic has been resolved and the patient has stabilized. A double dose of pyrantel pamoate/embonate (13.2 mg/kg) or products containing praziquantel are effective options.
Current parasite control recommendations are designed to reduce the risk of parasitic disease. Serum or saliva antibody testing can help determine the tapeworm infection pressure within an equine operation. This, in turn, can guide decisions about including anticestodal treatments as part of the control strategy. On most farms, current recommendations are to treat for tapeworms at least once a year in the autumn. Evidence of high infection pressure or cases of tapeworm-associated colic on the farm can justify treating more frequently. Overall, the aim is to reduce parasite contamination of pastures and thereby reduce the infection pressure.
A detailed presentation of current recommendations for equine parasite control can be found in the American Association for Equine Practitioners (AAEP) Guidelines (see below).
Anoplocephala perfoliata can cause colic in the ileocecal region.
Lesions can include ileal impaction and ileocecal intussusception.
The parasite is also implicated as a cause of spasmodic colic.
Modified egg-counting techniques reliably detect larger tapeworm burdens.
Current recommendations for equine parasite control are designed to reduce the risk of parasitic disease, including tapeworm-associated colic.