Prevalence of equine onchocerciasis in southeastern and midwestern United States.

Of 664 horses reported to have originated in southeastern and midwestern United States, 341 (51.4%) were positive for Onchocerca cervicalis cutaneous microfilariasis. The highest prevalence of 68.8% was recorded for horses from Virginia. Horses with the highest concentrations of microfilariae (as high as 19,770 microfilariae/mg in 1 horse) were from the Southeast (Georgia, Florida, Tennessee, and Alabama). For horses from South Carolina, the prevalence of infection was 57.1%; from the Midwest (Kentucky, Illinois, Ohio, and Indiana), 54.4%; from North Carolina, 45.5%; from Pennsylvania, 7.7%; and from Minnesota, 5%. Histories and ages of the horses were not known, but most horses were approximately 15 years old. Compared with horses from other states, horses from Pennsylvania were younger, which may have accounted for their lower prevalence of infection.