Importance of Tapeworms in Colic

It is well recognized that there is a relationship between parasitic infections of horses and the clinical signs of colic. Parasites can cause colic through mechanical damage, allergic irritation, interference with nerve transmission, interference with blood flow, and alteration of intestinal motility. Recently, new information has suggested a strong association between tapeworm infection and certain causes of colic, including spasmodic colic, ileal impaction, and intussusception (telescoping of one section of intestine into another).

It should be noted that although colic due to tapeworm infection is not one of the most common forms of colic, concern over tapeworm infections has increased in recent years due to common deworming practices. Most anthelmintics currently used (including ivermectin, moxidectin, and fenbendazole) are not effective against tapeworms. This fact has led to an increase in tapeworm infections in many well-maintained horses. This article will focus on the relationship between tapeworm infections and colic.


Three species of tapeworms infect horses--Anoplocephala perfoliata, Anoplocephala mamillana, and Anoplocephala magna--but only A. perfoliata is commonly found in the United States. Although younger horses (one to five years) and those over 15 years of age can harbor a larger number of tapeworms, horses of all ages can be infected with tapeworms; severe infections might occur in old horses. Because another organism is part of the tapeworm lifecycle, infections tend to be more regionally distributed than many other equine parasites. In areas that support the tapeworm and its intermediate host, up to 81% of horses have been shown to be infected.

The lifecycle of equine tapeworms includes orbatid mites as intermediate hosts. Orbatid mites are numerous in areas with adequate humidity. Locations serving as transmission areas must have an ideal microclimate for these free-living organisms. Mites ingest tapeworm eggs, which undergo development into a larval stage. After a horse ingests the mite, the larval tapeworm is freed from the mite and attaches to the lining of the last section of the small intestine, the ileum, where it joins the cecum. After the tapeworm matures, segments are released in the intestine and digested, releasing eggs that are then passed in the feces and ingested by mites.

The Disease Process

A 1998 study demonstrated an association between the presence of tapeworms and ileocecal colic. This paper also reported that the risk of developing colic increases with the number of tapeworms present. Spasmodic and ileal impaction colic, intussusceptions, intestinal perforation, and peritonitis have all been associated with equine tapeworm infections. However, the importance of tapeworms as a cause of colic must be kept in perspective. Many things can cause horses to present signs of colic; tapeworms are only one of them. Other clinical signs (besides colic) due to tapeworm infections might include poor condition resulting in chronic unthriftiness, and diarrhea.

Even the role tapeworms play in causing intussusceptions is still unproven. It is known that anything that alters intestinal motility might cause intussusception to occur. The mechanism by which tapeworms are believed to cause intussusception involves inflammation and ulceration of the mucosa at the site of attachment. This area also might become thickened and secondarily infected and/or abscessed.

Another proposed explanation of how tapeworms alter GI motility and cause colic relates to their possible effect on the autonomic nervous system. The equine tapeworm contains large quantities of a compound that, if released, could interfere with nerve transmission, subsequently reduce peristalsis (the involuntary wave-like contractions in the bowel that move food along), and cause colic.


A variety of fecal examination techniques have been used for diagnosis of tapeworm infections in horses. Tapeworm infections are difficult to diagnose on fecal examination because the eggs do not float easily to the top of the testing device and segments often are passed intact, not allowing eggs to be present in the feces. Because a single fecal examination detects only approximately 60% of infected horses, several fecal examinations should be done before ruling out that a horse is not infected with tapeworms. Increasing the number of fecal examinations done is reported to increase the frequency of a positive diagnosis.

Therapy And Prevention

Although no drugs currently are approved for the treatment of equine tapeworms in the United States, several products have been shown to be effective. Treatment of choice for equine tapeworm infections is the use of pyrantel salts; label dosages are 87% effective while double the normal dosage is greater than 93% effective. Recently, daily administration of pyrantel tartrate was reported to effectively remove A. perfoliata. Praziquantel is 89%-100% effective in its removal.

On facilities where tapeworms are prevalent, prevention of clinical signs of tapeworm infections can be accomplished in two ways: by the routine administration of pyrantel salts on a daily basis during the grazing season, or by the administration of a single or double dose of pyrantel pamoate paste within an interval deworming program. Alternatively, one could treat horses with a double dose of pyrantel pamoate paste immediately prior to turnout for the grazing season and again at the conclusion of the grazing season. There are many questions regarding prevention and control of equine tapeworms yet to be answered. Consult your veterinarian to determine if you live in a tapeworm-endemic area and need to include a tapeworm removal program.

Douglas E. Hutchens, DVM, MS;
Joseph A. DiPietro, DVM, MS; and Allan J. Paul, DVM, MS

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