ROCKY MOUNTAIN SPOTTED FEVER
(Alternate Names = Mexican Spotted Fever & São Paulo Spotted Fever)
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Around 1890 certain regions in western North America experienced outbreaks of a frequently fatal human illness. The symptoms involved a high fever, severe arthritic and muscular pain and skin eruptions, which began on the ankles, wrists and forehead, but then would spread all over the body. Matheson (1950) reported that in fatal cases the disease worsens quickly with the patient dying from the 6th to the 12th day. If the fever reduces and the patient lives two weeks, recovery was usually quick. Later, the disease was found to exist in eastern North America and South America as well.
Two strains of the disease, a mild and a virulent type, exist through most regions where the disease is a problem. Mortality varies from around 80 percent for the virulent strain and about 4-6 percent for the mild strain. The disease was named, Rocky Mountain Spotted Fever" because of its apparent origin. The causative agent is Rickettsia rickettsii. It is not contagious, but highly infectious and transmitted to humans by ticks. Service (2008) noted that is both transovarial and transstadial. Early investigations revealed that the disease is mainly an infection of small mammals, and large mammals, excluding humans, are not susceptible. Principal tick species involved include Dermacentor andersoni, and to a restricted area Dermacentor sanguineus, which are the vectors for humans in western North and Central America. Dermacentor variabilis is the vector tick in eastern North America, while in South America Amblyomma cajennense is the principal vector. Other vector species undoubtedly exist, such as Dermacentor occidentalis and the rabbit tick, Haemaphysalis leporis-palustris, which does not attack humans but can maintain the Rickettsia in the rodent population. Laboratory transmission trials have succeeded with additional vector species as well.
Studies have shown that the incubation period in humans after the bite of infectious ticks varies from 2 to 12 days. Service (2008) reports that as an infective tick has to feed on a host for at least two hours before sufficient Rickettsiae are injected for the host to become infected, early tick removal may prevent transmission. Although Matheson (1950) reported on a vaccine that gave some protection for almost one year, there was no mention of it by Service (2008).
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