A Suspected Case of Rocky Mountain Spotted Fever in an Adult Horse in the Southeastern United States

Abstract

A 20-year-old Paint gelding was evaluated for fever of unknown origin. History and clinical signs were consistent with potential tick-borne disease. Samples were collected and submitted for tick-borne disease panel, herpes virus, complete blood count, and serum biochemistry. Based on physical examination findings and vaccination history, the gelding was treated for suspected tick-borne disease with oxytetracycline (8 mg/kg intravenously BID) for 5 days, followed by doxycycline (10 mg/kg PO BID) for an additional 5 days. Although titers to Borrelia burgdorferi, Anaplasma phagocytophilum, and Neorickettsia risticii on Days 4 and 8 were negative, the Rickettsia rickettsii titer went from 1:1,600 on Day 4 to 1:800 on Day 8, 1:100 on Day 21, and was seronegative by Day 38. Although complete blood polymerase chain reaction for Rickettsia rickettsii was negative, the clinical and serologic features of this case are extremely consistent with clinical cases of Rocky Mountain Spotted Fever (RMSF) described in both dogs and humans. Therefore, we submit this case report to document suspected clinical infection of an adult horse in the southeastern United States with Rickettsia rickettsii, the causative agent of RMSF. Other relevant differentials (i.e., Rickettsia parkeri, Theileria equi, and Babesia caballi) are also discussed.

Keywords: Equine; Fever; Lethargy; Rickettsia rickettsii; Rocky Mountain Spotted Fever; Tick-borne disease.