Etiology and Epidemiology

Occurring as a natural infection only in humans, epidemic typhus is caused by Rickettsia prowazekii. It is spread from host to host by the human body louse, P. humanus corporis, and less often by the human head louse, Pediculus humanus capitis. The body louse spends its entire existence in the clothes of humans. Eggs laid in the seams of the undergarments hatch after about 8 days, and the nymphs become adults in about 2 weeks, going through three molts. Each louse takes four to six blood meals a day from its host under natural conditions. Human blood constitutes its only food.

Once typhus organisms in infected human blood are ingested by a louse, they multiply rapidly in the cells lining the louse's intestines and are secreted in the feces of infected lice. Since rickettsiae are not found in other tissues (such as the salivary glands) of the louse, they are transmitted to new human hosts mechanically. This is usually accomplished by contact of infected louse feces with a small abrasion of the skin incurred when the human scratches the unpleasant itch caused by feeding lice. The disease spreads when lice leave feverish or dead victims for new hosts with normal temperatures. Unlike other rickettsial organisms, R. prowazekii is not passed from generation to generation in the eggs of its host arthropod. In fact, as Hans Zinsser of Harvard Medical School pointed out in his 1935 history of typhus fever, humans constitute a great threat to the health and happiness of these small creatures, for humans usually recover from typhus fever, whereas the disease is inevitably deadly for infected lice.

Typhus is widely known as a disease of cold climates, appearing in epidemics that usually reach their peaks in late winter and taper off in the spring. This pattern is clearly related to the ideal conditions for multiplication of lice and their rapid transmission to new hosts. Typhus flourishes when people are crowded together in unsanitary surroundings and lack fuel, circumstances that predispose them to wear the same garments day and night for months at a time.

Persons of all ages are susceptible to typhus. Mortality rates in untreated typhus fever vary between 5 and 25 percent, occasionally reaching 40 percent. In children under 15 years of age, however, the disease is generally mild. As age increases, so does mortality.

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