Mite parasites of small mammals from scrub typhus foci in Australia

Abstract

Over 45,000 parasitic mites collected from 732 small mammals live-trapped between September 1961 and March 1965 at scrub typhus foci in north Queensland (Atherton Tableland and the coastal strip from Sarina to Cape Tribulation) are systematically reviewed. The laelapids comprised 38% of the total, the trombiculids 35%, and five other families (mostly Listrophoridae) 27%. A total of 21 genera and 53 species were represented: Laelapidae 7 genera 18 spp. Myobiidae 1 genus 4 spp. Spinturnicidae 1 genus 1 sp. Trombiculidae 8 genera 24 spp. Ereynetidae 1 genus 2 spp. Psoroptidae 1 genus 1 sp. Listrophoridae 2 genera 3 spp. A total of 17 host species were examined, comprising one monotreme (Tachyglossus), one marsupial mouse (Antechinus), two bandicoots (Perameles and Isoodon), nine rats and mice (Hydromys, Uromys, two Melomys spp., four Rattus spp., and Mus), and four bats (Nyctimene, Syconyctevis, and two Pteropus spp.). In all, 13 of the 18 laelapid species are host-specific, two have a wide host range (Mesolaelaps australiensis and Laelaps nuttalli), two are possibly nidophiles (Gymnolaelaps and M. bandicoota), and one is a rare intranasal parasite (Pneumonyssus). A similar host dependence is true of the members of the five minor families. Chiggers (larval Trombiculidae) are, on the contrary, habitat-limited, being restricted to the moist rain-forest floor by their postlarval stages, which are delicate, free-living predators. Two exceptions are EutrombicuIa hirsti and Guntherana perameles, which prefer to attach to bandicoots. The preference of chiggers for the ear as an attachment site in rodents, but the perineum in bandicoots, is confirmed (a few species attach internally to mucous membranes). No marked seasonal variation in numbers occurs in the laelapid and miscellaneous species, presumably due to their intimate association with their hosts. Certain chiggers, however, presumably with one generation a year, show marked seasonal variation, e.g. Odontacarus and the subgenus Guntherana in spring and early summer. The mite vector of scrub typhus, Leptotrombidium deliense, was found in good numbers, and throughout the year, at all six of the central coastal areas examined (Cape Tribulation to Ingham). It was not found at the southerly limit of the disease (Mackay- Sarina area), and it did not prove practicable to study the northerly Cape York area. On the Atherton Tableland, it was found only in small numbers at one, and that atypical, of the several foci studied. The causative agent, Rickettsia tsutsugamushi, was isolated from small ground mammals at the sites where the vector was found. Both the disease and its vector are concentrated on the wetter side of the 60-in. isohyet, and in areas of rain-forest or humid marginal vegetation. This is true even of districts where the sandy soil will not support rain-forest. Cases of scrub typhus have not been linked directly to established cultivation (sugar-cane is the principal crop), nor does L. deliense occur in this habitat.