Etiology and Epidemiology

The etiologic agent of scrub typhus, Rickettsia tsutsugamushi, is a natural infection of several trombiculid mites, most commonly L. deliensis. Maintained in

Map VIII.153.1. Known geographic distribution of mite (scrub) typhus in Asia, July 1948.

Map VIII.153.1. Known geographic distribution of mite (scrub) typhus in Asia, July 1948.

nature by generational transmission through the eggs of the female, the disease is communicated to humans only during the larval stage of the mite's life cycle. At this time, the six-legged larval mite, often called a "chigger," seeks an animal on which to find a meal of tissue juices, or lymph. Usually they feed on field mice, rats, tree-shrews, and other small mammals, but humans are satisfactory if they happen into the mite's environment. Ground-frequenting birds may also become infected and transport infected mites to a new location. In later stages of their lives, the mites live in the soil and not in animals.

Because of the wide geographic area across which tsutsugamushi is spread, its epidemiological pattern varies. In the Niigata region of Japan, for example, the disease is most common during the summer months, associated with increased incidence of the mite L. akamushi, whereas in the Chiba and the Kanagawa prefectures and the Izu Schichito Islands, it occurs in mild form throughout autumn and winter. In Malaya, a hot, tropical climate supports the disease throughout the year, and near the India-Burma border it is usually associated with the monsoon season.

Similarly, a variety of terrains may support the development of the vector mites, including grassy fields, river banks, neglected or abandoned rice fields, overgrown clearings, forests, jungles, and border areas between field and forest. The key requirements in any area include a suitable rodent population and sufficient ground moisture to support the mites. The "islands of infection" in which scrub typhus typically occurs apparently represent favorable ecological conditions for the mites.

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