Larval chigger mites of the genus Leptotrombidium (relatives to Neotrombicula; family Trombiculidae) are also ectoparasitic on various small mammals including rodents and birds. In southern and eastern Asia a serious human disease called scrub typhus or tsutsugamushi fever is transmitted by Leptotrombidium larvae. The larval mites acquire the infection (Orientia tsutsugamushi = Rickettsia orientalis) by feeding on infected rodents, particularly Rattus spp. The infection can presumably be spread to new areas by infected mites being transported on birds. Since the infection in the mite population is transmitted transovarially, via the eggs, to the new generation the infection can be transmitted further to susceptible veretebrates. Man is an unnatural host for the mite larvae (and for O. tsutsugamushi). Human
infections can, however, occur when people visit the mite-infested 'typhus islands' and are attacked by hordes of infected chigger larvae. The infection is usually present in small and scattered foci in many different biotopes from flooded alluvial plains in Japan, scrub and disturbed forests in southeastern Asia, to semi-deserts in Pakistan and alpine areas in the Himalayas (Varma in WHO 1989b). Leptotrombidium deliense is the principal vector over most of the distributional area of tsutsugamushi fever. In Japan, L. akamushi is the main vector. In the Far East of Russia L. pavlovskyi is an important vector (Varma 1993).
A few days after the infection, in about 40% of cases, a small nodule can be seen at the site of the mite bite. The nodule will become necrotic and suppurate. About a week after the appearance of disease symptoms (general malaise, headache, fever, regional lymphadenitis, etc.) a spotted fever exanthema usually appears on the trunk, on the extremities and in the face. There is often bronchitis and encephalitis. Pneumonia, encephalitis, and heart failure are serious complications and may indicate fatal cases. During serious epidemics the mortality rate of untreated cases can exceed 60%. Scrub typhus has been, and may continue to be, of great importance in military medicine.
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