There are thousands of species of trombiculid mites in many genera but only about 20 species commonly attack people. The family Trombiculidae has a more or less worldwide distribution, but the medically most important species, such as Leptotrombidium deliense, L. akamushi and L. fletcheri, which are vectors of scrub typhus Orientia (= Ricksettsia) tsutsugamushi, are found in Asia, the Pacific regions and the north-east coast of Australia.

Other trombiculid mites in many parts of the world cause itching and a form of dermatitis known as scrub itch, autumnal itch or trombidiosis. In northern Europe larvae of Neotrombicula autumnalis (harvest mites) and in North America and parts of Central and South America larvae of Eutrombicula alfreddugesi (red bugs) commonly attack people and cause considerable discomfort.

External morphology

Adults and nymphs

Adults are small (1–2 mm), usually reddish and covered dorsally and ventrally with numerous feathered hairs giving them a velvety appearance. The four pairs of legs end in paired claws. The body is distinctly constricted between the third and fourth pairs of legs, giving it an outline resembling a figure of eight. Palps and mouthparts project in front of the body and are clearly visible (Fig. 19.1).

Nymphs resemble the adults but are smaller (0.5–1.0 mm) and the body is less densely covered with feathered hairs.

Neither adults nor nymphs are of direct medical importance; they do not bite humans or animals but feed on small arthropods and their eggs. Only the larvae are parasitic and hence disease vectors.