Rocky Mountain Spotted Fever

Rocky Mountain spotted fever is caused by Rickettsia rickettsii, a gram-negative, coccobacillary, obligate intracellular bacterium. The disease is transmitted through bites of infected ticks, primarily Dermacentor variabilis and Dermacentor andersoni. These ticks attach to exposed skin areas and transmit the pathogen into the bloodstream during feeding.

After entering the bloodstream, R. rickettsii uses host cell surface proteins to bind to vascular endothelial cells and invades them through receptor-mediated endocytosis. Once inside, the bacterium escapes the phagosome into the cytoplasm, where it replicates. It hijacks host actin filaments to move within and between cells, evading extracellular immune defenses.

Infection compromises endothelial cell membrane integrity, increasing vascular permeability and causing capillary leak with extravasation of leukocytes and plasma. This allows fluid and immune cells to infiltrate surrounding tissues, resulting in localized edema, reduced tissue perfusion, and systemic hypotension from widespread vascular leakage.

Early symptoms appear within 2 to 14 days of infection and include high fever, severe headache, muscle aches, and nausea. These symptoms are followed by a characteristic maculopapular rash that typically begins at the wrists and ankles before spreading centripetally to the trunk and other body areas.

The initial maculopapular rash may progress to petechiae—small red or purple spots caused by capillary hemorrhages. In severe cases, this petechial rash can spread widely across the body. Without prompt treatment, the disease can advance to life-threatening complications including multiorgan failure and encephalitis.

Doxycycline is the antibiotic of choice and should be administered immediately upon suspicion of RMSF, even before laboratory confirmation. Delayed treatment significantly increases the risk of severe outcomes. If left untreated, the case fatality rate can reach 20 to 30%, particularly in young children and individuals with G6PD deficiency.

Unlike bacterial gastroenteritis that affects the gastrointestinal tract, RMSF is a systemic vascular disease targeting endothelial cells. Its hallmark petechial rash and rapid progression to multiorgan failure distinguish it from localized bacterial infections, requiring immediate recognition and treatment to prevent fatal complications.