Ecological considerations in scrub typhus. 1. Emerging concepts

Abstract

Scrub typhus infection is now known to occur in geographical and ecological areas where its presence was hitherto unsuspected-north and west of the Indus River and in south-eastern Siberia; in semi-desert, montane desert, and alpine terrain high in the Himalayas, as well as in primary jungle. Additional data are presented to support the hypothesis that "ecological islands", containing basically similar faunas of rodents and ectoparasites, exist on scattered mountains in the Pakistan Himalayas, despite the intervening "barriers" of desert, broad rivers and massive peaks. Since scrub typhus has been demonstrated in some of these isolated areas, it is felt that the infection may exist, unrecognized, in neighbouring countries as well.A number of larval trombiculid mites, largely species of the subgenus Leptotrombidium, are believed to be vectors, in addition to the well-known L. (L.) deliense and L. (L.) akamushi. The host-range of natural infection in ground-dwelling small mammals, especially rodents, is very broad in endemic areas. An important factor of time, and not only of space, may be involved in an endemic locus, the disease undergoing a sequential evolution involving different chiggers and rodents over a period of years. It is pointed out that new irrigation schemes, road construction, and agricultural projects may introduce scrub typhus into an area or greatly increase its endemicity if the infection is already present.