Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Onchocerca Skin &/or Eye Inflammation

Synonyms: Onchocerciasis, Ventral Midline Dermatitis, Neck Threadworm Infestation, Onchocerca cervicalis Onchocerca Conjunctivitis


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.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis for skin lesions caused by Onchocerca is good if the causative agent is recognized. The prognosis for eye inflammation and recurrent uveitis is always guarded.

Horses that have a good worming history with ivermectin or moxidectin almost never have a problem with this condition.

These worms and larvae can often be found in asymptomatic skin and eye tissues. This makes it difficult to know when these parasites actually cause disease or are just present.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • Are the eye signs my horse is experiencing caused by this parasite?
  • Would treatment with ivermectin or moxidectin help?

Good facilities management to decrease the population of flies is recommended, including the use of fans, screens, fly repellant and fly masks and sheets to protect horses from biting. Keep horse in protected stalls during times of high fly activity.

further reading & resources

Related References:

Pascoe RRR, Knottenbelt DC Manual of Equine Dermatology WB Saunders 1999

Higgins AJ, Snyder JR eds. The Equine Manual. 2nd Ed. Edinburgh: Elsevier Saunders 2006.

Author: Doug Thal DVM Dipl. ABVP