Abortion in a horse following Neorickettsia risticii infection
Elizabeth Coffman2008
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Abstract
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.
Key takeaways
- In an aborted equine fetus, especially one from a mare previously diagnosed with PHF, the combination of lymphohistiocytic enterocolitis, hepatitis, myocarditis, and mesenteric lymphadenitis may suggest fetal infection with Neorickettsia (formerly Ehrlichia) risticii, the etiologic agent of Potomac horse fever.
- There are several commercially available whole-cell killed vaccines for PHF; all are derived from the originally isolated strain of N. risticii.
- Whether subclinical infection can result in abortion has not been determined; however, characteristic pathological findings (lymphadenitis, hepatitis, myocarditis and, especially, lymphohistiocytic colitis) in the aborted fetus of a mare without a history of illness should prompt consideration of PHF, a diagnosis that can be confirmed by PCR analyses of fetal tissues.
- Fetal diarrhea is a poorly understood condition that is associated with abortion 8, ; given the marked inflammation in the colon in the case reported here and elsewhere, 16, perhaps some of those cases are related to PHF.
- In summary, PHF is a relatively common infection that can cause abortion months after infection and clinical recovery of the mare.
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