A Suspected Case of Rocky Mountain Spotted Fever in an Adult Horse in the Southeastern United States.
Authors: Freese Stephanie, Sheats M Katie
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Rocky Mountain Spotted Fever in an Adult Horse A 20-year-old Paint gelding presenting with fever of unknown origin and clinical signs compatible with tick-borne disease was investigated using a comprehensive serological panel alongside standard haematological and biochemical assessment. Empirical treatment with intravenous oxytetracycline (8 mg/kg twice daily for 5 days) followed by oral doxycycline (10 mg/kg twice daily for a further 5 days) was initiated based on physical examination findings and the horse's vaccination history. Serological titres to *Rickettsia rickettsii* demonstrated a characteristic declining pattern—decreasing from 1:1,600 on day 4 to 1:800 on day 8, 1:100 by day 21, and seronegative by day 38—whilst panels for *Borrelia burgdorferi*, *Anaplasma phagocytophilum*, and *Neorickettsia risticii* remained negative throughout. Although polymerase chain reaction testing for *Rickettsia rickettsii* was negative, the clinical presentation and serological profile align closely with Rocky Mountain Spotted Fever cases documented in dogs and humans, representing what appears to be the first suspected case in an adult horse in the southeastern United States. For practitioners in endemic regions, this case highlights the importance of including rickettsial diseases in the differential diagnosis for febrile horses with ambiguous presentations and reinforces the clinical utility of empirical tetracycline therapy pending serological confirmation.
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Practical Takeaways
- •Consider Rickettsia rickettsii in the differential diagnosis for fever of unknown origin in horses in the southeastern United States, particularly when tick exposure is likely
- •Serologic titers declining over time can support diagnosis of RMSF even when PCR testing is negative; paired serum samples on Days 4 and 21+ are valuable
- •Tetracycline antibiotics (oxytetracycline IV followed by doxycycline PO) are appropriate empiric therapy for suspected tick-borne rickettsial disease in horses pending confirmatory test results
Key Findings
- •A 20-year-old Paint gelding presented with fever consistent with tick-borne disease and serologic evidence of Rickettsia rickettsii infection (titer declining from 1:1,600 to seronegative over 38 days)
- •Clinical and serologic features were consistent with RMSF despite negative PCR for Rickettsia rickettsii
- •Tetracycline therapy (oxytetracycline followed by doxycycline) was administered based on clinical suspicion of tick-borne disease
- •This case documents the first suspected adult equine RMSF infection in the southeastern United States