General Description: Onchocerca species are long and coiled, the male being 6 to 7 cm. and the female up to 30 cm. in length. Microfilariae are 0.25 mm. long.

Life Cycle: The indirect life cycle of Onchocerca involves a Culicoides biting midge as intermediate host. Microfilariae are found in subcutaneous tissue spaces, where they are picked up by feeding Culicoides. Infective larvae are present in the mouth parts of the insect 24 to 25 days later.

Location: Onchocerca cervicalis occurs in the ligamentum nuchae of the neck. O. reticulata is found in the flexor tendons and suspensory ligaments, particularly of the forelegs.

Geographical Distribution: Worldwide.

Significance: Heavy infections can be debilitating to horses and can restrict freedom of movement.

Effect on Host: Adult worms in ligaments and tendons cause swelling and pain. Nodules resulting from calcification of dead worms can double the thickness of ligaments. Microfilariae may infiltrate and irritate the cornea and iris of the eye, sometimes causing blindness. Lameness, swellings in ligaments, and eye irritation are among the clinical signs of onchocerciasis. Local dermatitis can occur as a reaction to the antigens of the microfilariae.

Diagnostic Information: A small skin sample is soaked in saline for several hours, and the solution is examined for microfilariae.

Control: Surgical removal of debilitating nodules may be considered. Treatment can reduce the incidence of microfilariae in the eyes. The most effective preventative measure is to use insecticides and screening to control Culicoides midge.

Microfilariae of Onchocerca in tissues		Culicoides fly		Horse with ventral abdominal oedema