Abortion in a horse following Neorickettsia risticii infection.

A pregnant 18-year-old Quarterhorse mare presented with fever, anorexia, tachycardia, tachypnea, and gastrointestinal hypermotility at day 68 of gestation. Potomac horse fever was diagnosed based on polymerase chain reaction (PCR) analysis of whole blood and a high antibody titer to Neorickettsia risticii. The mare made a rapid clinical recovery following antibiotic therapy, but aborted 98 days later. Necropsy on the aborted fetus revealed lymphohistiocytic colitis, lymphadenitis, myocarditis, and hepatitis. The placenta was grossly and histologically normal. Formalin-fixed lymph node, thymus, liver, and colon taken from the aborted fetus were positive by PCR for N. risticii DNA. Potomac horse fever is a common disease in horses that may result in delayed abortion. The microscopic lesions in the fetus are characteristic, and the diagnosis can be confirmed by PCR on formalin-fixed tissues.