Etiology and Epidemiology

The severity with which Rocky Mountain spotted fever treats its victims underscores its natural existence as an infection of ticks and their mammalian hosts. The microbial cause of the disease, Rickettsia rickettsii, normally inhabits ixodid, or hard shell, ticks, apparently causing little harm to the host. Although small mammals are susceptible to a mild infection with R. rickettsii and may transmit it to uninfected ticks, the principal means by which the organism is maintained in nature is from one generation to the next in the eggs of the female tick.

The epidemiology of Rocky Mountain spotted fever is linked to areas favorable for the habitation of the vector ticks. The Rocky Mountain wood tick, Derma-centor andersoni, and the American dog tick, Derma-centor variabilis, are the most common vectors in the United States, although the Lone Star tick, Ambly-omma americanum, also transmits the disease in the south central and southeastern parts of the United States. Two other ticks, Rhipicephalus sanguineus and Amblyomma cajennense, also carry the disease in

Mexico, Central America, and South America. Only a small percentage of ticks - generally less than 5 percent - are usually infective.

Humans typically contract Rocky Mountain spotted fever when they accidentally become a part of the disease's biosystem. In the western United States, hikers, backpackers, and the like may become subjected to infection when traveling in areas where ticks are plentiful, especially during the spring months. In the eastern United States, where most cases now occur, changing land-use patterns have brought humans into the habitat of the tick. The development of suburban housing developments and the transformation of agricultural land into wooded recreation areas are two examples.

Rocky Mountain spotted fever characteristically appears in "islands" of infection. During early research on a particularly virulent form in Montana's Bitterroot Valley, for example, investigators were baffled by the fact that Rocky Mountain spotted fever appeared on the west side of the Bitterroot River but not on the east side (see Map VIII.120.1). Recently, Rocky Mountain Laboratory investigator Willy Burgdorfer has shown that this peculiar epidemiological occurrence is related to an antigenic "interference phenomenon." Nonpathogenic rickett-siae in the ovaries of ticks on the east side of the river "interfere" with the establishment of pathogenic rickettsiae in these tissues, thus preventing the pathogenic R. rickettsii from being passed on to the next generation of ticks.

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