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

The study discusses an investigation into a suspected case of Rocky Mountain Spotted Fever (RMSF), a tick-borne disease, in a 20-year-old horse, based on its clinical symptoms. This is an unusual occurrence, as such previous cases are more commonly reported in dogs and humans than in horses.

Horse’s Condition and Treatment

• The horse, a Paint gelding, was evaluated for a fever of unknown origin.
• His medical history and clinical signs suggested a potential tick-borne disease.
• The gelding was subsequently treated with oxytetracycline for five days, followed by doxycycline for an additional five days, as a preventative measure for suspected tick-borne disease.

Diagnostic Procedures

• Samples from the horse were collected for analysis. This included, a complete blood count, a tick-borne disease panel, tests for herpes virus, and serum biochemistry.
• Serologic tests were primarily focused on identifying titers to different bacteria commonly found in tick-borne diseases such as Borrelia burgdorferi, Anaplasma phagocytophilum, and Neorickettsia risticii. All of which were negative.
• However, the blood titer of Rickettsia rickettsii, the bacteria that causes RMSF, showed an initially high level that quickly reduced over the course of the disease, reaching seronegative status by the 38th day.

Findings

• Despite the negative results from a polymerase chain reaction test for the Rickettsia rickettsii bacteria, the gelding’s clinical observation and serology were highly indicative of RMSF.
• This led the researchers to believe that, despite the rarity of such occurrences, the horse had a clinical infection of RMSF, making it one of the few documented cases in horses within the southeastern United States.
• The study also discusses other possible differential diagnoses, including infections like Rickettsia parkeri, Theileria equi, and Babesia caballi.