Onchocerca cervicalis

The nematode Onchocerca cervicalis occurs in horses in many regions of the world where suitable midge intermediate hosts (Culicoides species) are also present.  The parasite has been found in horses in western Canada and occurs commonly in the United States.  Adult O. cervicalis live in the nuchal ligament.  Microfilariae (first-stage larvae) produced by the females make their way to the dermis and tend to migrate to areas of the body favoured by the local midge species - in British Columbia this seems to be the face but in many areas of the world it is the ventral midline.  Infection of the intermediate host occurs during blood-feeding.  In the midge the microfilariae develop into infective third-stage larvae which gain entry to the horse during blood feeding.

Many horses infected with O. cervicalis never show any clinical signs, but in some animals skin lesions develop.  Typically these include alopecia, crusting and bleeding and pruritus is a common clinical sign.  The problem is often more obvious in the summer.  Even though the presence of O. cervicalis is confirmed by teasing microfilariae from a skin biopsy, the significance of the parasite can be difficult to assess.  The biting activity of the midges, and of other blood-feeding arthropods, can result in clinical signs similar to those of  cutaneous onchocerciasis.  For example, Culicoides is associated with a specific disease affecting the skin (Queensland itch or sweet itch) that is caused by a hypersensitivity to antigens in midge saliva and which occurs in Canada.

Onchocerca cervicalis is not known to be zoonotic.  A related species, Onchocerca volvulus, is a major pathogen of people in equatorial west and central Africa and also occurs in scattered foci in South and Central America.  The major problem with O. volvulus is invasion of the cornea by the microfilariae, which can result in a loss of vision - river blindness.  The vectors for O. volvulus are black flies (Simulium species) that lay their eggs in white water.

Phylum: Nematoda
Class: Rhabditea
Order: Spirurida
Suborder: Spirurina
Superfamily: Filaroidea
Family: Onchocercida

The Family Onchocercidae contains many species of Onchocerca, some of which parasitize ruminants, primarily cattle, and horses in many parts of the world. Another species within the genus is Onchocerca volvulus, which causes onchocerciasis and "river blindness" in people in Africa and Central and South America.

Adult O. cervicalis are up to approximately 30 cm (females) and 7 cm (males) in length, and are very narrow and hair-like. It is difficult to see any details of the structures without very careful microscopic examination.

The microfilariae produced by the adult female O. cervicalis measure up to approximately 250 µm in length, are unsheathed and have a short tail.

Onchocerca cervicalis occurs in horses and mules throughout the world, including areas of Canada, where environments support parasite development in the midge intermediate host.

Life Cycle – Indirect

Adult O. cervicalis live in the nuchal ligament. The female parasites produce microfilariae (first-stage larvae) that migrate to the dermis. The areas of concentration of these microfilariae in the dermis is related to the biting habits of the local midge (Culicoides species) intermediate hosts. For example, in some regions they are most common around the face, in others on the ventral body surface. The microfilariae are ingested by the midge during feeding, and develop to the infective third-stage. Under ideal conditions this development takes up to three to four weeks. Infection of the horse occurs when the larvae invade the horse during a subsequent feeding by the midge. The larvae then migrate to the nuchal ligament and mature.

Life Cycle: Onchocerca cervicalis

Transmission of O. cervicalis occurs during periods of the year when the midges are active; in western Canada this is the summer. Little is known of the details of any seasonal transmission patterns.

Many horses infected with O. cervicalis show no clinical signs. In some horses, however, microfilariae may be associated with pruritic skin lesions characterised by inflammation, scaling and depigmentation. These lesions, known as "cutaneous onchocerciasis", are usually worse during the summer. In western Canada, the condition has been seen on the face and neck and in the ventral mid-line. The exact association between the presence of O. cervicalis microfilariae and the development of skin lesions is not clear, in part because many infected horses never show any clinical signs. Further, the pathogenic mechanisms underlying the skin lesions are not fully understood.

In some horses, hypersensitivity to antigens in the saliva of midges can also produce pruritic skin lesions, usually along the caudal back and tail head. This condition, which has been reported in western Canada, is known as "Sweet Itch" or "Queensland Itch" and is distinct from cutaneous onchocerciasis. Additional information about Sweet Itch is available under Culicoides.

In symptomatic horses, history and clinical signs can be helpful. Microfilariae of O. cervicalis can be recovered from small, macerated skin biopsy incubated for at least 12 hours in normal saline at 37C. Histological examination of skin biopsies can also be useful.

Ivermectin (VARIOUS) and moxidectin (QUEST, QUEST PLUS) are approved in Canada for their efficacy against microfilariae of Onchocerca cervicalis in the skin and for the associated prutitus, although these products may not be fully successful in all affected horses.

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

Other than treatment of horses and prevention of contacts between midges and horses, there are no specific control measures for O. cervicalis. One of the problems is that many horses infected with the parasite show no clinical signs. Although the infection in these "carrier" horses can be detected by examining skin biopsies, this procedure is difficult to justify in the context of control.

Onchocerca cervicalis is not known to be zoonotic.

Lees MJ et al. (1983) Cutaneous onchocerciasis in the horse: five cases in southwestern British Columbia. Canadian Veterinary Journal 24: 3-5.