Oxyuris equi

The pinworm nematode Oxyuris equi occurs in horses around the world, including Canada.


The pinworm nematodeOxyuris equioccurs in horses around the world, including Canada. The adults live in the large intestine, but after mating there the males die. The females then migrate to the terminal rectum from where they protrude their tails and lay eggs in an adhesive on the peri-anal skin. Over a few days an infective third-stage larva develops in each egg. Infection of the horse is by ingestion of the infective eggs, which can float around in the environment. The pre-patent period of O. equiis approximately five months.

Many horses infected withO. equinever show any clinical signs but in some animals the egg-laying activities of the female parasites causes sometimes intense pruritus ani, which leads to affected horses attempting to relieve the itching, usually by rubbing their tail areas against something, for example gate, fence, riding instructor or small child! Because of its life cycle and route of transmission pinworm can thrive in groups of horses kept in sub-optimal (dirty) conditions.

Oxyuris equiis not known to be zoonotic.


Phylum: Nematoda
Class: Rhabditea
Order: Oxyurida
Superfamily: Oxyurida
Family: Oxyuridae

Among nematodes of veterinary importance, the closest relative toOxyuris equiisSkrjabinema, the pinworm of ruminants (seen occasionally in Canada). It is also closely related toEnterobius vermicularis, the pinworm of people.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasitesis based on that in the seventh edition ofFoundations of Parasitologyby Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education,Boston, 2005.


Adult O. equimeasure up to approximately 150 mm (mature females) and 12 mm (males) in length. The males, which are rarely seen other than at post mortem, have a single spicule. The females, when mature, have a characteristic pointed tail which may occupy three quarters of the total length of the parasite.

Eggsof0O. equiare approximately oval, slightly flattened on one side, with a thick, smooth shell and a plug at one end. It is usually difficult to see the internal structure of the eggs. The eggs ofO. equiare easily distinguishable from those of other intestinal parasites of horses.

Host range and geographic distribution

The pinworm Oxyuris equi occurs in horses and other equids throughout the world, including Canada. The parasite is particularly common in groups of horses managed with sub-optimal cleanliness.

Life cycle - direct

Adult O. equi live in the large intestine. After mating, the males die and the females move to the rectum. With (or without) defecation, the females extend their long tails beyond the anus and lay eggs in clumps on the peri-anal skin. These clumps are held in place with an adhesive produced by the female nematode. An infective larva develops within each egg over a minimum of a few days, during which period the adhesive loses its stickiness, and the egg drops to the ground. Infection of the horse is by ingestion of infective, larvated eggs. The eggs hatch within the small intestine and the larvae released undergo a mucosal migration in the large intestine. The pre-patent period for O. equi is approximately five months.

Life Cycle: Oxyuris equi


A population of O. equi can produce large numbers of eggs which quickly become infective and widely distributed in the environment, for example on the horse and in bedding, food and water. Crowding together of inadequately treated horses facilitates transmission, which can easily occur in stables and yards, as well as at pasture. Poor grooming practices can also transmit O. equi by moving infective eggs from the peri-anal region to the mouth, for example with a damp cloth.

Epidemiological studies have recovered eggs of Enterobius (human pinworm) from amazing locations, including the spouts of hospital coffee machines and cinema light fixtures. It seems probable that the eggs of O. equi may be equally adventurous.

Pathology and clinical signs

Many horses infected with O. equi show no adverse effects. The most notable clinical signs are associated with egg-laying by females, which causes pruritus ani ("itchy bum"), which leads to attempts by affected horses to relieve the itching, usually by backing into something or someone.


Clinically, the attempts by an infected horse to relieve the itching may lead to unusual and sometimes alarming behaviour, particularly rubbing the tail head against a wall or gate or door. The large mature adult females of O. equi, and sometimes typical eggs, may be seen in the feces (the latter by fecal flotation). More usually, eggs are detected in swabs or scrapings from the anus or peri-anal skin.


Treatment and control

There are several products approved in Canada for treatment of Oxyuris equi.  There are a few isolated reports of O. equi that are resistant to the macrocyclic lactones, but the distribution and severity of this resistance have not been fully elucidated. There are no reports from Canada.







Ivermectin  with Praziquantel




Moxidectin  with Praziquantel






Pyrantel pamoate



Additional information is on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Effective control of O. equi depends on appropriate treatment protocols and on maintaining a high level of overall management, bearing in mind particularly the characteristics of the eggs – they rapidly become infective, they are light and float in the air to many locations, and they survive relatively well in adverse environmental conditions.

Public health significance

Oxyuris equi is not known to be zoonotic.


Reinemeyer CR (2012) Anthelmintic resistance in non-strongylid parasites of horses. Veterinary Parasitology 185: 9-15.