rickettsii

Rickettsia rickettsii is a gram-negative, obligate intracellular bacterium in the spotted fever group of the genus Rickettsia. It is the etiologic agent of Rocky Mountain spotted fever (RMSF), a tick-borne illness that affects humans and other animals, primarily in the Americas.

Transmission occurs through the bite of infected ticks, most commonly Dermacentor species such as Dermacentor variabilis and Dermacentor andersoni, depending on the geographic region. Ticks acquire the bacteria from an infected host and can transmit it during subsequent feeds. Human-to-human transmission is not typical.

Pathogenesis involves invasion of vascular endothelial cells, leading to vasculitis and multisystem involvement. The incubation period is usually 2 to 14 days after a bite. Clinical features often begin with high fever, severe headache, myalgia, and fatigue. A maculopapular or petechial rash typically appears days after fever, starting on the wrists and ankles and spreading to the trunk; some cases may have little or no rash.

Diagnosis relies on clinical suspicion in areas where the disease is common, supported by laboratory tests such as serology (immunofluorescent antibody, IFA), polymerase chain reaction (PCR), or immunohistochemistry. Early in illness, tests may be negative, and diagnosis is often confirmed by paired sera.

Treatment is most effective when started promptly. Doxycycline is the first-line therapy for RMSF in patients of all ages; alternatives such as chloramphenicol are used only in specific circumstances. Prevention focuses on tick avoidance and removal, including protective clothing, repellents, prompt tick checks, and environmental measures to reduce tick habitats.

Epidemiology varies by region with RMSF reported across the Americas.