Tsutsugamushi Disease

parts of Japan, for example, cases known as Japanese river fever tend to occur during the warm, summer months along river terraces where larvae of L. akamtishi are present. Cases in Japan during the autumn and winter months, which may extend into late spring or early summer, are usually associated with two other chigger species, L. pallidum and L. scutellare. These seasonal differences are reflected in local Japanese names such as Umayado disease for the summer form of tsutsugamushi disease in Kagawa Prefecture and Shichito fever for the autumn/winter form in the Hachijo Islands of Izu Shichito. The nonsummer types of tsutsugamushi disease are usually characterized by relatively mild symptoms and low fatality rates. In other subtropical and tropical regions of Southeast Asia and the southwestern Pacific, cases occur independent of seasons, correlated with the presence of chigger vectors that are present year-round (e.g., L. arenieola, L. deliense, L. fletcberi).

More than 40 species of trombiculid mites (13 genera) are known or suspected to be vectors of O. tsutsugamushi. The major chigger vectors of this pathogen and their geographic occurrence are shown in Table III. The most important genus is Leptotrombidium, represented by approximately 25 vector species, most of which belong to the subgenus Leptotrombidium. Two or more vector species are known in each of the following 4 genera: Neotrombicula (6 spp.), Ascoschoengastia (2 spp.), Euschoengastia (2 spp), and Walehia (2 spp.). Other trombiculid genera that play a role in transmission of O. tsutsugamushi to humans are Acomatacarus, Eutrom-bieula, Gahrliepia, Leeuwenhoekia, Maekiena, Neoschoen-gastia, Odontacarus, and Shunsennia.

Chiggers that serve as vectors of tsutsugamushi disease are primarily parasites of wild rodents such as field mice (Apodemus spp.), voles (Microtus spp.), and rats (Leopoldamys, Maxomys, Rattus, and other genera). They also occur on a wide range of birds (including pheasants, pigeons, and chickens) which are susceptible to infection and can develop at least transient rickettsemia. Although it is conceivable that, there is no conclusive evidence to show that animals other than humans are involved in the transmission cycle rodents serve as a source of infection of O. tsutsugamushi. Instead, the mites themselves serve as the principal natural reservoirs of the disease agent. Certain strains of trombiculid mites of a given species effectively transmit the rickettsia transovarially and/or transstadially; thus, the pathogen is transmitted from the adult female to her larval offspring and to each developmental stage that follows. Consequently, O. tsutsugamushi is passed from generation to generation of mite and is maintained within local mite populations in endemic areas. Were this not the case, this mite-borne rickettsia could not persist in nature. Because trombiculid mites feed on only a single host, they do not have the opportunity to transmit an acquired pathogen at a subsequent feeding. For further information on the ecology and medical aspects of tsutsugamushi disease, see Kawamura et al. (1995).

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