This is a parasitic infection in which worm larvae of the genus Onchocerca are introduced through the skin by tiny biting flies (midges, gnats). These larvae penetrate the skin, get into the bloodstream and migrate into the nuchal ligament (the large ligament along the crest of the neck) and other large ligaments. Here they mature into adults, which grow into long thin worms; hence the name Threadworm.

The adult worms can reside in these locations for years without causing signs and are immune to the effects of our worming agents. These adults produce larvae that go back into the bloodstream and burrow into the skin around the lower chest, abdomen, head, neck and withers, creating itchy bumps that may drain and result in skin irritation and itchiness.

The larvae may also invade the tissues of the eye resulting in inflammation, uveitis or conjunctivitis. Onchocerca infection cause a characteristic pale plaque at the outside corner (lateral limbus) of the eye that persists through the life of the horse whether or not there is active infection.

Firm "mineralized" skin lesions can also develop along the crest of the neck (or elsewhere) resulting from an allergic response to dead or dying adults.

Diagnosis requires biopsy and histopathology. However, in some cases diagnosis can be presumed by appearance of skin and eye lesions, and improvement after deworming.

Treatment includes ivermectin or moxidectin wormers for the skin lesions. Symptomatic treatment of eye signs also requires use of anti-inflammatory and antibacterial ophthalmic medications. These anthelmintic drugs only kill the larvae, not the adults. Treatment may temporarily worsen signs as the body reacts to dead or dying larvae, but these signs should improve over subsequent weeks.