Rocky Mountain Spotted Fever

What Is It?

Rocky Mountain spotted fever is a severe illness caused by tiny bacteria called Rickettsia rickettsii, which are transmitted through the bite of an infected tick. In the eastern United States and in California, the infected tick is usually Dermacentor variabilis, the American dog tick. In most of the western United States, the tick is more likely to be Dermacentor andersoni, the Rocky Mountain wood tick. Humans typically become infected in the spring and early summer.

Once someone is bitten by an infected tick, Rocky Mountain spotted fever bacteria can spread throughout the bloodstream and lymphatic system. The tick must remain attached and be actively feeding to transmit the bacteria. Not everybody who is bitten by an infected tick develops Rocky Mountain spotted fever. It is not clear why some people get the disease and others do not.

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