Horse Tapeworms

Tapeworms are one of the common parasites found in horses, with studies indicating that over half of the horses in the USA and UK are infected with tapeworms. There are three types of tapeworms which can infect horses: Anoplocephala perfoliata, Anoplocephala magna, and Paranoplocephala mamillana. Of these, Anoplocephala perfoliata is the most common in the USA and UK. 

Life Cycle

The lifcycle of all three types of tapeworms involve two species: horses and mites. Their reproductive life cycle is:

  • The adult tapeworm lives in the digestive system of the horse, where it produces large number of eggs which pass out of the horse with the dung.
  • The eggs are eaten by mites (specifically the oribitad mite, which inhabits pastures). The eggs then hatch in the mite and the immature larvae develops within the body cavity of the mite, over a period of two to four months.
  • When the horse grazes, it consumes the mites present on the grass and thereby the larvae enter the horse.
  • The larvae develop to the adult stage within about two months. The adults fix themselves to the intestine wall, using suckers in their heads. They do not have a mouth to eat, instead absorbing through their skins the nutrients flowing through the intestine. The adult form varies from 8-25 centimeters (3-10 inches) in length, the majority of this length consisting of egg sacks.
  • The adults continually produce eggs, which leave the horse with the dung, completing the life cycle.

Health Issues

The tape worms can damage the intestine where they attach. This damage can cause bleeding, secondary infections (which may form abscesses) and can in some cases result in perforation of the intestine.

Tapeworms can cause colic, the severity of which varies from painful to fatal, including the following three forms of colic:

  • Spasmodic Colic - A common form of colic, where abnormal spasms of the intestines cause painful contractions. 
  • Ileal Impaction - Obstruction by tapeworms at the junction of the small and large intestine cause thickening of the bowel wall, resulting in abnormal gut movements.
  • Intussusception - Results when part of the intestine telescopes (collapses) into an adjacent piece.

In the case of heavy infestation by tapeworms, they can actually block the intestine, which is potentially fatal.

If there is a large population of tapeworms, they can absorb excessive amounts of nutrients. In moderate cases, the horse will need to consume extra feed to maintain its weight. In extreme cases, the horse will lose weight as it is unable to gain sufficient nutrition from its feed. Damage to the intestines may be a contributing factor in weight loss.

Symptoms

Examination of the horse's dung for tapeworm eggs is a common method to determine if the horse has tapeworms. However, as egg production is intermittent, it may be necessary to examine more than one dung sample and even then it is not a reliable method. Treatment of the horse with a tapeworm-effect horse wormer will result in the tapeworms being killed and exiting with the dung, where they will be visible upon examination.

Unexplained weight loss or colic are often associated with tapeworms. Although these symptoms can be caused by a variety of problems, in the event of no other cause being identified, tapeworms may be considered as a potential cause.

Treatment and Prevention

Of the various horse wormers, praziquantel is effective against all three types of horse worms, while Pyrantel and Fenbendazole are partially effective against Anoplocephala magna but not effective against Paranoplocephala mamillana. Increasing the dosage of Pyrantel or Fenbendazole (discuss with your veterinarian appropriate dosages) increases their effectiveness against Anoplocephala magna.

Use of a daily wormer (choose one which is effective against the local tapeworm species) can be used to keep tapeworms under control.

Proper herd and field maintenance can control the density of infecting eggs and larvae.