What is the causative agent of Rocky Mountain spotted fever?

The causative agent of Rocky Mountain spotted fever is the bacterium Rickettsia rickettsii.

R. rickettsii is an obligate intracellular bacterium, meaning it reproduces inside host cells. In humans, it primarily infects the endothelial cells lining blood vessels. Damage to these vessels helps produce the disease’s characteristic fever, headache, and spotted rash, while severe infections can affect multiple organs.

The bacterium is transmitted by infected ticks, including the American dog tick, Rocky Mountain wood tick, and brown dog tick. The illness was recognized in the western United States during the late 19th century, but its infectious agent and tick-borne route were worked out through early 20th-century investigations.

A common mix-up is confusing R. rickettsii with Borrelia burgdorferi, the bacterium that causes Lyme disease. Ehrlichia chaffeensis causes ehrlichiosis, while Coxiella burnetii causes Q fever. These are all bacterial diseases associated with vectors or animals, but they are distinct infections.