Scrub typhus

Scrub typhus or bush typhus is a form of typhus caused by the intracellular parasite Orientia tsutsugamushi, a Gram-negative α-proteobacterium of family Rickettsiaceae first isolated and identified in 1930 in Japan. Scrub typhus or bush typhus is a form of typhus caused by the intracellular parasite Orientia tsutsugamushi, a Gram-negative α-proteobacterium of family Rickettsiaceae first isolated and identified in 1930 in Japan. Although the disease is similar in presentation to other forms of typhus, its pathogen is no longer included in genus Rickettsia with the typhus bacteria proper, but in Orientia. The disease is thus frequently classified separately from the other typhi. Signs and symptoms include fever, headache, muscle pain, cough, and gastrointestinal symptoms. More virulent strains of O. tsutsugamushi can cause hemorrhaging and intravascular coagulation. Morbilliform rash, eschar, splenomegaly, and lymphadenopathies are typical signs. Leukopenia and abnormal liver function tests are commonly seen in the early phase of the illness. Pneumonitis, encephalitis, and myocarditis occur in the late phase of illness. It has particularly been shown to be the most common cause of acute encephalitis syndrome in Bihar, India. Scrub typhus is transmitted by some species of trombiculid mites ('chiggers', particularly Leptotrombidium deliense), which are found in areas of heavy scrub vegetation. The bite of this mite leaves a characteristic black eschar that is useful to the doctor for making the diagnosis. Scrub typhus is endemic to a part of the world known as the tsutsugamushi triangle (after O. tsutsugamushi). This extends from northern Japan and far-eastern Russia in the north, to the territories around the Solomon Sea into northern Australia in the south, and to Pakistan and Afghanistan in the west. It may also be endemic in parts of South America. The precise incidence of the disease is unknown, as diagnostic facilities are not available in much of its large native range, which spans vast regions of equatorial jungle to the subtropics. In rural Thailand and Laos, murine and scrub typhus account for around a quarter of all adults presenting to hospital with fever and negative blood cultures. The incidence in Japan has fallen over the past few decades, probably due to land development driving decreasing exposure, and many prefectures report fewer than 50 cases per year. It affects females more than males in Korea, but not in Japan, which may be because sex-differentiated cultural roles have women tending garden plots more often, thus being exposed to vegetation inhabited by chiggers.The incidence is increasing in the southern part of the Indian subcontinent and in northern areas around Darjeeling. In endemic areas, diagnosis is generally made on clinical grounds alone. However, overshadowing of the diagnosis is quite often as the clinical symptoms overlap with other infectious diseases such as dengue fever, paratyphoid, and pyrexia of unknown origin (PUO). If the eschar can be identified, it is quite diagnostic of scrub typhus, but this is very unreliable in the native population who have dark skin, and moreover, the site of eschar which is usually where the mite bites is often located in covered areas. Unless it is actively searched for, the eschar most likely would be missed. History of mite bite is often absent since the bite does not inflict pain and the mites are almost too small to be seen by the naked eye. Usually, scrub typhus is often labelled as PUO in remote endemic areas, since blood culture is often negative, yet it can be treated effectively with chloramphenicol. Where doubt exists, the diagnosis may be confirmed by a laboratory test such as serology. Again, this is often unavailable in most endemic areas, since the serological test involved is not included in the routine screening tests for PUO, especially in Burma (Myanmar). The choice of laboratory test is not straightforward, and all currently available tests have their limitations. The cheapest and most easily available serological test is the Weil-Felix test, but this is notoriously unreliable. The gold standard is indirect immunofluorescence, but the main limitation of this method is the availability of fluorescent microscopes, which are not often available in resource-poor settings where scrub typhus is endemic. Indirect immunoperoxidase, a modification of the standard IFA method, can be used with a light microscope, and the results of these tests are comparable to those from IFA. Rapid bedside kits have been described that produce a result within one hour, but the availability of these tests is severely limited by their cost. Serological methods are most reliable when a four-fold rise in antibody titre is found. If the patient is from a nonendemic area, then diagnosis can be made from a single acute serum sample. In patients from endemic areas, this is not possible because antibodies may be found in up to 18% of healthy individuals.