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Oxyuris equi is a species of parasitic roundworms that infects horses and other equids (e.g. donkeys, mules, etc). They are found worldwide but prevalence varies from region to region. They are usually less frequent than Strongylid worms.

Oxyuris equi, adult worm. Image taken from www.techniquesdelevage.frThe disease caused by Oxyuris equi worms is called oxyuriasis.

These worms do not affect cattle, sheep, goats, pigs, dogs or cats.

Are horses infected with Oxyuris equi contagious for humans?

  • NO. The Horse Pinworm does not infect humans. Human pinworms belong to the species Enterobius vermicularis.

You can find additional information in this site on the general biology of parasitic worms and/or roundworms.

Final location of Oxyuris equi

Predilection site of adult Oxyuris equi is the large intestine (mainly colon).

Anatomy of Oxyuris equi

Adult Oxyuris female worms are a to 150 mm long with a very long pin-like tail of variable length. Males are much smaller, only 9 to 12 mm long. Typical for this species is the long pin-like tail. The worm's body has a whitish color and is covered with a cuticle, which is flexible but rather tough. The worms have no external signs of segmentation. They have a tubular digestive system with two openings, the mouth and the anus. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. The female ovaries are large and the uteri end in an opening called the vulva. Males have only one rather long chitinous spicule for attaching to the female during copulation.

The eggs measure about 40x90 mcm, are elongated, slightly flattened on one side, have a thick envelope and an operculum at one of the poles. Infective eggs contain mostly a single larva often coiled.

Life cycle of Oxyuris equi

Egg of Oxyuris equi. Image taken from http://www3.vetagro-sup.frOxyuris equi has a direct life cycle. Female worms in the large intestine move towards the rectum. Their posterior end crawls outside the anus where they lay eggs around the perianal region. These eggs are surrounded by a sticky fluid of a whitish, yellowish, or greenish color. This sticky fluid with the eggs remains attached to the host's skin for several days. Infective larvae develop inside the eggs within 5 to 7 days. Such sticky egg masses can be rather itching for the host that reacts rubbing and licking the affected parts. This way, infective eggs are directly ingested by the host. Or the egg masses dry out, fall to the ground, contaminate the environment and are ingested by the host with food or water. Infective eggs may survive for 8 to 10 weeks.

Ingested eggs hatch in the small intestine releasing the L3 larvae that penetrate into the mucosa. About a week later they molt to L4-larvae that attach to the mucosa of the ventral colon. About 2 months later they complete development to adult worms. Sexual maturity is reached 3-4 months later.

The time between infection and fist eggs shed (prepatent period) is 4-6 months, although there are reports suggesting that it may be shorter based on the observation of foals not older than 3.5 months that already carried Oxyuris egg masses in the perianal region.

Harm caused by Oxyuris equi, symptoms and diagnosis

As a general rule, young horses seem to suffer more of Oxyuris infections than adult or old animals. This is probably due to acquired immunity in exposed horses. However, Oxyuris infections are not very pathogenic to horses. Larvae attached to the intestinal mucosa may cause damage that can result in colic. The main harm seems to come from the intense itching caused by the egg masses in the perianal region. Affected horses rub vigorously against objects, often during the night. This often results in dull haircoat and hair loss (rat-tail). Skin injuries may occur that are subject to bacterial infections and myiasis.

Diagnosis is based on the detection of characteristic egg masses in the perianal region. Eggs can be sampled in the perianal region with adhesive tape and examined under the microscope for identification. Such samples are often positive even if no macroscopic egg masses are visible. Fecal egg counts are usually negative due to the particular egg-laying behavior of this species. Dead worms may be recovered occasionally in the feces. Living worms may also be seen protruding from the anus during egg laying.

Prevention and control of Oxyuris equi infections

Oxyuris equi egg masses around the anus of a horse. Image taken from http://www.1cheval.comThoroughly washing of the perianal region (water with mild, non-irritating detergents) to remove the eggs can reduce infections. Hygienic measures indoors such as removing manure and potentially contaminated bedding and/or food are highly recommended in stud farms or wherever numerous foals are hold together, particular if they have a history of Oxyuris infections. Steam vapor disinfection of stables and premises may be considered. However, they are unlikely to completely eliminate the infections. 

Other preventative measures for gastrointestinal roundworms are explained in a specific article in this site (click here). 

So far no true vaccine is available against Oxyuris equi. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of Oxyuris equi (i.e. using its natural enemies) is so far not feasible. Learn more about biological control of worms.

You may be interested in an article in this site on medicinal plants against external and internal parasites.

Most broad-spectrum benzimidazoles (febantel, fenbendazole, mebendazole, oxibendazole, etc) are effective against adult worms and larvae in the gut, e.g. several benzimidazoles (febantel, fenbendazole, mebendazole, oxibendazole, etc). Macrocyclic lactones (mainly ivermectin and moxidectin) are also effective as well. But not all of them are effective against immature larvae. Most of them control other roundworms that may affect horses together with Oxyuris equi. Tetrahydropyrimidines (e.g. morantel, pyrantel, mainly pamoate salts) are also effective against adult worms and L4-larvae. 

Several commercial products contain mixtures of two or even more active ingredients of different chemical classes. This is sometimes done to extend the spectrum of activity (e.g. to ensure efficacy against roundworms and tapeworms) or to increase the chance that at least one active ingredient is effective against gastrointestinal worms that have become resistant, or to delay resistance development by those worms that are still susceptible.

Depending on the country most of these anthelmintics are available for oral administration as pastes, gels or other formulations for oral administration (e.g. tablets, drenches, feed additives). Very few are available as injectables. Many horse owners complain about the high prices of ivermectin formulations  (mostly pastes or gels) for horses when compared with injectables for cattle and other livestock. The reason why ivermectin injectables are usually not used on horses is apparently that shortly after introduction, it was noticed that horses were more prone to develop severe clostridial infections at the injection site (due to contamination of the needles) and other undesired side effects than cattle or sheep. In addition, the pharmacokinetic behavior of ivermectin on horses is different than in ruminants. For these reasons oral pastes were developed for horses that do not show such side effects.

Most wormers containing benzimidazoles (e.g. febantel, fenbendazole, oxibendazole, etc), tetrahydropyrimidines (e.g. morantel, pyrantel) and other classic anthelmintics kill the worms shortly after treatment and are quickly metabolized and/or excreted within a few hours or days. This means that they have a short residual effect, or no residual effect at all. As a consequence treated animals are cured from worms but do not remain protected against new infections. To ensure that they remain worm-free the animals have to be dewormed periodically, depending on the local epidemiological, ecological and climatic conditions. Exceptions to this are macrocyclic lactones (e.g. ivermectin, moxidectin) that are stored in the body fat and progressively released. They may offer several weeks protection against re-infestation, depending on the dosage, the delivery form and the specific parasite.

Resistance of Oxyuris equi to anthelmintics

There are a few reports (e.g. in the US, Germany, Czech Reupblic, New Zealand) on surviving adult Oxyuris adult worms after treatment of horses with macrocyclic lactones (mainly ivermectin and moxidectin) or pyrantel. In some cases ivermectin to ivermectin (an possibly other macrocyclic lactones) seems to be confirmed, but in other cases it is unclear whether failure was due to resistance development by the worms or to inherent incomplete efficacy of these compounds at the recommended dose.

This means that if an anthelmintic fails to achieve the expected efficacy against Oxyuris worms, chance is rather high that failure was not due to resistance but to incorrect use, which is the most frequent cause of product failure, but resistance cannot be excluded. Or the product used was unsuited for the control of Oxyuris worms.

Learn more about parasite resistance and how it develops.

Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Oxyuris equi control.


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