By Tom Lenz, DVM, MS, DACT

The strong association between internal equine parasites and the risk of colic has been widely known for several types of worms that commonly infect horses.  Over the past decade, there have been significant advancements in the understanding of equine tapeworms, the damage they inflict, and their role in equine colic.  While colic continues to be the single most important noninfectious cause of mortality in horses, and parasitism is one of many factors which can lead to colic; parasitism is one of the easiest factors to control through knowledge of the parasite’s life cycle and strategic use of anthelmintics (dewormers) that control infection.  This article will help shed some light on a very unique parasite that can negatively affect your horse’s health: the tapeworm.  

While there are at least three types of equine tapeworms, the most common in the United States is Anoplocephala perfoliata.  Horse tapeworms are different than the tapeworms that infect most other animals.  Rather than being composed of long chains of segments, horse tapeworms are pumpkin-seed-shaped parasites roughly 1-inch long and ½-inch wide.  They have four suckers that enable them to attach to the horse’s intestinal lining where they absorb nutrients and damage tissues.  Horse tapeworms are also unique in that they require an intermediate host, the oribatid (forage) mite, to complete their life cycle.  Tapeworm eggs are passed in the manure of infected horses onto pasture, where forage mites ingest them.  The immature tapeworm develops within the body cavity of the mite and is ingested by the grazing horse.  When the horse digests the infested forage mite, the tapeworm is released and within 6-10 weeks develops into an adult that attaches to the lining of the horse’s intestine where its lifecycle starts all over again.

Equine populations throughout North America are exposed to tapeworm infections during the grazing season; in some warmer climates, exposure and infection can occur year-round.  Survey studies conducted across the United States have found the prevalence of tapeworm infestations in horses ranges from 17.3 percent in horses along the Pacific Coast to as high as 95 percent of horses in the Midwest.  East of the Mississippi River, tapeworms were found in 60 percent of horses tested.  These studies also revealed that the highest tapeworm infection rates were found in October, following natural grazing exposure to infected forage mites.  There also appears to be an age susceptibility to tapeworm infestations.  Research has demonstrated that young horses (6 months to 2 years) have the highest level of infestation.  The level drops in mature horses (3 to 15 years) and then increases again in older horses (15 years and older).  There are probably two reasons for this pattern.  The first is that most mature horses are working horses and might not be pastured as much as young or old horses and therefore are simply not as often exposed to infected forage mites.  The second is that mature horses might develop some level of immunity against the tapeworms that wanes as horses become older. 

Equine tapeworm infection is not a benign form of parasitism.  They prefer to attach near the junction of the small intestine and the cecum.  The cecum of the horse is equivalent to our appendix but is 6 to 8 inches wide and 4 to 5 feet long.  Tapeworms damage the intestinal lining, as well as, cause nerve degeneration at the site of attachment.  Large numbers of attached tapeworms can obstruct the bowel; however, even light to moderate infections have been associated with ileal impaction, spasmodic colic, cecal intussusceptions (when the end of the small intestine collapses into the opening of the cecum) and rupture.  Recent studies suggest that as many as 22 percent of spasmodic (gas) colics, nearly 80 percent of ileocecal (the junction between the end of the small intestine and the opening to the cecum) impactions, and up to 100 percent of ileocecal intussusceptions are associated with tapeworm infections.  Tapeworm infection should be considered in herds where colic occurs on a repeated basis.

Historically, it has been difficult to determine if a horse is infested with tapeworms.  Standard fecal floatation testing methods that are routinely used to identify strongyle and roundworm detection are unreliable in identifying tapeworms, especially if the number of tapeworms is low.  Diagnostic ELISA tests of blood for anti-tapeworm antibodies have been used routinely for years but may not be completely accurate.  Because tapeworm antibodies can remain in the horse’s blood for up to six months, the ELISA test may not be accurate for individual horses but can be useful in evaluating tapeworm presence in a herd of horses.  There is a relatively new ELISA test for tapeworm-specific antibodies using the horse’s saliva that can be collected by the horse’s owner or veterinarian and submitted to a lab.  A horse infected with even a single tapeworm will test positive.  Anti-tapeworm antibodies clear from saliva within five weeks making the test much more accurate for the individual horse.

Fortunately, there are licensed anthelmintics (dewormers) that are highly effective against all three types of horse tapeworms.  They are available with broad-spectrum dewormers that also control other internal worms and should be incorporated into every horse’s deworming program, especially in the Fall.   For information on developing an effective deworming program tailored to the specific needs of your horses, contact your local equine veterinarian.

Reviewed and updated by original author in 2020.