Onchocerciasis (Ventral Midline dermatitis) is a skin disease caused by a hair-like worm called Onchocerca cervicalis. The adult worm lives in connective tissue of the horse's neck. The majority of horses in the United States are infected, but a few develop a dermatitis. It is believed that the skin response is due to an allergic reactionn to the dying microfilaria (prelarvae stage of worms released by female worm into the bloodstream).
The filaria migrate under the skin and settle primarily on the midline of the abdomen from the chest to the groin, especially around the umbilicus. Other sites are the withers, face, eyelids, and legs. At these sites the parasites produce an itchy skin disorder with redness, moist shallow sores, crusting and scaling, and patchy hair loss. Spots up to 10 inches in diameter can develop. Scarring and loss of skin pigmentation may also develop.
These open sores attract Culicoides gnats and other flying insects. Gnats feeding on the open sores pick up filaria and introduce them to a new host. Biting flies and other insects aggravate the skin disorder and create pyoderma.
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Ivermectin paste is completely effective in ridding the horse of filaria within 2 to 3 weeks. Minor reactions can occur with the use. Veterinary supervision is advised.
Adult worms are not affected by deworming agents and therefore serve as a reservoir for recurring infection. To keep the skin free of disease, ivermectin must be repeated at 4-month intervals. A deworming program incorporating ivermectin will effectively control the onchocerciasis. The filaria are capable of penetrating the eye and producing uveitis, a leading cause of blindness in horses.