Equine ocular pathology ascribed to Onchocerca cervicalis infection: a re-examination

Eyes from 292 old (15-20 years) horses originating in the eastern, southeastern and midwestern United States were examined for the presence of Onchocerca cervicalis microfilariae (mf) and concurrently for anterior and posterior segment ocular pathological changes. One-hundred-fifty-three animals (52.4%) were positive for dermal mf (range 0.03-5,364/mg). Of these, 60 animals had anterior segment changes. An additional 58 animals with pathological changes had no dermal mf. Mf were recovered from the ocular tissues of 18% of animals (range 0.07-29/mg). All animals with ocular mf were positive for dermal mf. Lesions most frequently encountered were punctate scleral opacities (in 14.0% of horses--7.2% with mf), temporal scleral viteligo (in 11.6%--6.8% with mf) and uveitis (11.0% of horses--6.2% with mf). Few significant posterior segment changes were seen. These data suggest that no ocular pathological changes represent reliable indicators for the presence of O. cervicalis infection.