Etiology and Epidemiology

Tick-borne relapsing fever is normally contained within the relationship between tick and rodent host; human beings become affected only when they accidentally become involved in that relationship. For example, if human shelters such as log cabins attract rodents, they may in turn become tick habitats. Transmission of relapsing fever is through the infected saliva or coxal fluid of the tick, making it essentially a disease of locality. In the case of louse-borne relapsing fever, the only reservoir of B. recur-rentis is human beings, despite the fact that the disease is spread by lice, either in the bite, or by contact with the body fluids of the louse through scratching. The louse is infected by ingesting infected human blood; once infected, it remains so for the rest of its life which is generally about 3 weeks. The infection is not congenital in the offspring. As in typhus fever, the febrile condition of the patient encourages the departure of lice because they are sensitive to temperature and, consequently, prefer the temperature in the clothing of healthy persons.

Tick-borne relapsing fever tends to be more severe than the louse-borne variety, but both types vary greatly in severity and fatality. In 1912, for example, louse-borne relapsing fever was very severe in Indochina and India but very mild in Turkey and Egypt. There are also indications that levels of individual and residual immunity are important. Illustrative are Borrelia infections, which are severe in European populations in North and East Africa, but mild in the local populations. On the other hand, in West Africa the disease is equally severe among Europeans and the local inhabitants. Case fatality depends not only on the type of infection and the availability of treatment, but also on the individual's nutritional status and resilience. Thus after World War II, adult fatalities from the disease averaged 8.5 percent among the poorer classes but only 3.6 percent among the well-to-do. Children suffered the most, with death the outcome for 65 percent of cases.

Because mortality varies inversely with living conditions, louse-borne relapsing fever is a true famine fever, generally manifesting itself in times of distress, when overcrowding, diminished personal hygiene, and undernutrition encourage its spread and increase its deadliness. It is said to be "the most epidemic of the epidemic diseases" (Topley and Wilson 1984); and Alexander Collie (in 1887) noted that it rarely occurs except as an epidemic. The factors involved in the survival of the disease between epidemics are still not fully understood.

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