Rickettsia rickettsii is a species of obligate intracellular Gram‑negative bacterium that infects endothelial cells and causes Rocky Mountain spotted fever, a severe tick‑borne disease.
Biology and Pathogenesis
Rickettsia rickettsii is a member of the spotted fever group of rickettsiae. It is a small coccobacillus that lacks many metabolic pathways and therefore depends on the host cell for nutrients and ATP. After transmission by a tick bite, the organism enters endothelial cells via induced phagocytosis, escapes from the phagosome and replicates in the host cytosol. It spreads cell to cell via actin polymerization. Infection of the vascular endothelium leads to widespread vasculitis, increased vascular permeability and edema. This vascular damage manifests clinically as fever, headache and a petechial or maculopapular rash. Because the bacteria replicate inside host cells and produce few extracellular antigens, early serologic diagnosis can be challenging.
Disease and Epidemiology
Rocky Mountain spotted fever was first described in the western United States but is now more common in the southeastern and south central states and parts of South America. It is transmitted primarily by ticks such as Dermacentor variabilis, Dermacentor andersoni and Rhipicephalus sanguineus. Symptoms appear after an incubation period of 2–14 days and include high fever, severe headache, myalgia, nausea and vomiting. A characteristic rash typically begins on the wrists and ankles and spreads to the trunk and extremities, including palms and soles. Without treatment, vascular injury can lead to hypotension, disseminated intravascular coagulation, acute renal failure or encephalitis. Early administration of doxycycline significantly reduces mortality and is recommended for all suspected cases, including in children. Preventive measures include avoiding tick habitats, using repellents and promptly removing attached ticks.
Rickettsia rickettsii exemplifies the pathogenicity of rickettsial bacteria and underscores the importance of early recognition and treatment of tick‑borne illnesses.
Related Terms: Rickettsia, Rocky Mountain spotted fever, Tick‑borne disease, Doxycycline, Rickettsial