Rocky Mountain Fever: A Review

Rocky Mountain spotted fever (RMSF) is a disease caused by the bacterium Rickettsia rickettsii which is spread to humans by ticks. Symptoms include the sudden onset of fever, headache and muscle pain followed by the development of a rash. In the laboratory, rickettsiae cannot be cultivated on agar plates or in broth, but only in viable eukaryotic host cells. Rocky Mountain spotted fever and Mediterranean spotted fever are rickettsial infections primarily of endothelial cells that normally have a potent anticoagulant function. As a result of endothelial cell infection and injury, the hemostatic system is perturbed and shows changes that vary widely from a minor reduction in the platelet count to severe coagulopathies, such as deep venous thrombosis and disseminated intravascular coagulation. Animals probably become infected by aerosol and by the bite of any of the 40 species of ticks that carry the organisms. From the portal of entry in the skin, rickettsiae spread via the bloodstream to infect the endothelium and sometimes the vascular smooth muscle cells, brain, lungs, heart, kidneys, liver, gastrointestinal tract and other organs. Rickettsia species enter their target cells, multiply by binary fission in the cytosol and damage heavily parasitized cells directly. The target cells are macrophages in the lungs, liver, bone marrow, spleen, heart valves and other organs. Clinico-epidemiologic diagnosis is ultimately a matter of suspicion. Empirical treatment and later laboratory confirmation gives specific diagnosis. Some laboratories are able to identify rickettsiae by immunohistology in skin biopsies as a timely, acute diagnostic procedure, but to establish the diagnosis; physicians usually rely on serologic demonstration of the development of antibodies to rickettsial antigens in serum collected after the patient has recovered. Currently, assays that demonstrate antibodies to rickettsial antigens themselves are preferable to the nonspecific, insensitive Weil-Felix test that is based on the crossreactive antigens of OX-19 and OX-2 strains. Tetracycline, Doxycycline and chloramphenicol are the drugs of choice for treatment. Control of the tick population on the property, keeping pets tick-free are some measures to control the disease.