Review Article: The Ecology of Chigger-Borne Rickettsiosis (Scrub Typhus),

Abstract

A critical review of the ecology of chigger-borne rickettsiosis (scrub typhus) is presented, based upon a synthesis of both published and unpublished information. All the epidemiological characteristics of chigger-borne typhus are attributable to the habits and other features of the vector chiggers. This rickettsiosis is found in a remarkably broad range of habitats, presumably in all areas where members of the Leptotrombidium deliense-group of chiggers and wild rats of the genus Rattus coexist, ranging from semideserts to alpine reaches in the mountains, and from sandy beaches to dense but disturbed forest. The habitats are characterized by the presence of changing ecological conditions, wrought by man or nature, and expressed by transitional types of vegetation, even if these are in circumscribed foci as along jungle streams. The intimate association between the rats, chiggers and secondary vegetation (which together with Rickettsia tsutsugamushi, the etiological agent, constitute the “zoonotic tetrad”) is an ancient one. We postulate that as Rattus and associated chiggers of the subgenus Leptotrombidium penetrated new areas and habitats, they diversified, and as the chiggers adapted to other hosts in the new regions, such mammals became secondarily involved in the ecology of the rickettsiosis. Chiggers in the L. deliense-group constitute the major vectors of this disease, and we consider that they also serve as the primary reservoir of the infection in nature, perpetuating the cycle by transovarian transmission of the rickettsiae from the naturally infected female mite to the next generation. While chiggers can acquire rickettsiae from a rodent host by feeding, laboratory data suggest that only rarely can this result in infection in another mammal, whether by a chigger that has become infected while feeding and then reattaching to a second host (which is unusual in itself) or by transovarially infected progeny. However, while such an event may be difficult to demonstrate in the laboratory, the same low percentage of success in the field would be significant in the natural cycle because of the vast numbers of chiggers involved, and because of the amplification that would ensue by transovarian transmissionof acquired infection over a period of generations. Small mammals (“theraphions”) like rats, voles, and tree-shrews which are primarily ground-dwelling are important in the ecologyof the rickettsiosis by serving as major hosts of the vector chiggers. Arboreal rodents and commensal rats play little role in the infection, apparently because they seldom come in contact with ground infested with the L. deliense-group. Rickettsia tsutsugamushi differs significantly from other members of the genus, and the similarities between them apparently represent convergence towards existence as intracellular parasites with cycles in both mammals and arthropods. The so-called serotypes of the agent, in contrast, are quite divergent. The widespread range of certain serotypes, and the fact that they may be transmitted by several species of chiggers suggest that infection acquired from theraphions may be significant in the ecology of the rickettsiosis. By using chiggers of the L. deliense-complex and wild rats as indicators of the presence of this infection, “oasis chigger-borne rickettsiosis” has been demonstrated among relict faunae in “ecological islands.” Such faunal data indicate that the infection may be present in areas where it has not yet been demonstrated, as in northern Iran and some southern republics of the U.S.S.R., parts of Tibet, Sinkiang, etc.