The most consistent laboratory finding with RMSF is thrombocytopenia, with platelet counts as low as 20,000 /µL. Resolution of thrombocytopenia can be used to gauge the animal's response to treatment. Other laboratory findings include: moderate leukocytosis; normocytic, normochromic anemia; elevated glucose and cholesterol concentrations, and increased alkaline phosphatase and alanine aminotransferase. Hypoalbuminemia from vasculitis is often present. If the kidneys are involved, the BUN may be elevated a along with proteinuria.

Serologic testing is the most useful means to detect infection with R. rickettsii. The presence of a four-fold increase in IgG antibody titers to R. rickettsii along with appropriate clinical signs of RMSF is considered diagnostic for the disease. A single high IgG titer (>1024) is suggestive of exposure within the last tick season. Positive IgG titers may persist for 3–10 months post-infection. Cross reactivity to other spotted fever rickettsia exists and may complicate interpretation of test results.

Other less practical tests used to detect RMSF include PCR of a biopsy or skin lesion sample. Direct fluorescent antibody staining can be used to detect organisms in frozen sections of tissues or ticks, but the test has a high false negative rate especially if the animal is receiving antibiotics.

The treatment of choice for RMSF is tetracycline (22-30 mg/kg body weight given TID) or doxycycline (10-20 mg /kg body weight given BID). Chloramphenicol (15-30 mg /kg body weight given TID) should be used in pregnant bitches and may be preferred in young puppies (<6 mos old) to avoid the dental staining from tetracycline. Fluoroquinolones (enrofloxacin at 3 mg /kg body weight given BID) can also be used.

Response to antibiotics usually is seen within 24-48 hrs, although advanced cases with signs of necrosis or thrombosis may fail to respond. Supportive care should be instituted concomitantly with antibiotic administration. Conservative use of fluids and systemic or topical corticosteroids may be needed. Dogs recovered from RMSF have effective protective immunity to further reinfection.

References: Clifton DA et al, PNAS USA 95: 4646, 1998; Stiles J, Vet Clin N Am Sm An Pract 30: 1144-1148, 2000; Hinrichsen VL et al, JVIM 7:1092-1096, 2001;Warner RD, Marsh WW, JVIM, 10:1413-1417, 2002; Demma LJ, N Eng J Med 353:587-594, 2005; Otis WL et al, Univ Georgia Coll Vet Med, 2006.