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.
Once in the body, the bacteria attack and severely damage the linings of blood vessels. The injured vessels leak a watery fluid, which causes swelling. The blood vessels also can lose blood, which causes potentially life-threatening hemorrhages (serious bleeding). As damaged blood vessels continue to leak, blood pressure can decrease. If this happens, blood flow to the kidneys can decrease, causing kidney damage and, possibly, kidney failure. Rocky Mountain spotted fever bacteria also may attack the brain directly, causing symptoms of encephalitis (brain infection) or meningoencephalitis (infection of the brain and its surrounding membranes). Rocky Mountain spotted fever also can cause jaundice (yellowing of the skin and eyes) as a result of liver injury. In the lungs, Rocky Mountain spotted fever can lead to an accumulation of fluid in the lungs (pulmonary edema) and severe breathing difficulties.
The U.S. Centers for Disease Control and Prevention (CDC) reports about 800 cases of Rocky Mountain spotted fever strikes in the United States each year. Children aged 5 to 9 are more likely to be infected than any other age group.