ROCKY MOUNTAIN SPOTTED FEVER
Rocky Mountain spotted fever (RMSF) is the most severe and most commonly reported tick-borne rickettsial disease in the United States. RMSF has been reported in every state but occurs more frequently in the south-Atlantic states. There are numerous “spotted” diseases, caused by different species of Rickettsia, but only RMSF occurs in the United States.
ETIOLOGY: BACTERIAL
Rickettsia rickettsii is the bacterium responsible for causing RMSF in people. It is a gram-negative, obligate intracellular bacterium. In the case of RMSF, the cells invaded initially are the cells that line small- to medium-size blood vessels. When R. rickettsii multiplies, it kills the cells, causing blood to leak out of the vessels into the surrounding tissue. Since blood vessels are virtually everywhere in the body, R. rickettsii can cause blood seepage into just about any tissue. This makes RMSF a potential multiorgan disease. The leakage also causes a rash, or spotted fever, to develop in 85% to 90% of people who become ill.
HOSTS
Ticks are both reservoirs and vectors for RMSF. Hard, or ixodid, ticks spread the disease to humans. So far the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni) are the most common culprits in the United States. The brown dog tick (Rhipicephalus sanguineus), which transmits RMSF more commonly in Mexico, has been implicated in an RMSF outbreak in Arizona. These ticks do not feed exclusively on people, so R. rickettsii is found in many vertebrates.
TRANSMISSION
People are infected primarily by the bite of a tick. Rarely, people become infected by crushing an infected tick with their fingers, allowing tick fluids to enter through cuts or broken skin.
The tick life cycle has three active stages after the egg hatches: larva, nymph, and adult (Appendix 1). The larva must have a blood meal before it can molt to the nymph stage, and the nymph must have a blood meal before it can molt to the adult stage. The tick becomes infected when it takes blood from an infected animal or person. The bacteria stay with the tick when it molts to the next stage. As it feeds before molting, and at any stage, it can infect another animal or person by depositing R. rickettsii when it bites. Ticks in all three stages feed on people. A tick needs to be attached to a person for at least 6 hours before R. rickettsii is transmitted (Figure 30).
Once infected, a tick can carry R. rickettsii for its entire life. Female ticks can also become infected when male ticks transmit the organism through body fluids or spermatozoa. The female tick can pass R. rickettsii to her eggs through transovarian transmission.
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