The treatment of choice for Onchocerca spp. microfilaria is an avermectin-family drug, with retreatment at four-month intervals. Treated horses show marked improvement. Although adult filaria are not killed by avermectins, in practice most horses are free of disease within 6-12 months.
Adult filaria live in calcified nodules in the ligamentum nuchae and produce microfilaria that migrate to the ventral midline, face, neck, and chest.
The resulting dermatitis is thought to be a hypersensitivity to microfilarial antigens. Accumulation of microfilaria in the eye may also lead to conjunctivitis and uveitis.
Note that Culicoides spp. (no-see-ums, biting midges) carry the onchocerca microfilariae, but do not cause the problems listed above themselves.
Instead, biting midges cause sweet itch, a very pruritic warm weather dermatitis that is also secondary to hypersensitivity.
Doing nothing is a poor choice in this symptomatic animal. But finding microfilaria in skin biopsies of asymptomatic horses is a common and incidental finding.
Because of this, some references debate the role of onchocera in the pathogenesis. It seems likely that variations in immune response play a role in severity of disease presentation.
In humans, onchocerciasis is a well-known cause of hypersensitivity-related superficial keratitis (river blindness) and dermatitis.
Image courtesy of Daniel J. Drew at the Yale Peabody Museum.
