Veterinary Epidemiology and Zoonosis

SCRUB TYPHUS

Synonym

• Chigger typhus

Type of zoonosis

• Anthropo-metazoonosis

Definition

• Scrub typhus is caused by Orientia tsutsugamushi, which is transmitted by the bite of mite, Leptotrombidium deliense (chigger mite) and develops a characteristic black eschar lesion at the site of mite bites.

Etiology

• It is caused by Orientia tsutsugamushi.
• Antigenic variation in Orientia tsutsugamushi strains is very common.

Reservoir and incidence

• The endemic area of scrub typhus in the world is described as the "tsutsugamushi triangle", which extends from northern Australia in the south to northern Japan and far-eastern Russia in the north, to Pakistan and Afghanistan in the west. Trombiculid mites, Leptotrombidium deliense (chigger mite) found abundant in these areas of heavy scrub vegetation.
• Epidemics of scrub typhus occurred among US troops stationed in Japan after World War II, and was described as "Shichito fever" or "Hatsuka fever"
• Scrub typhus is often fatal and case fatality may be 4% to 40% if untreated.

Transmission

• Trombiculid mites, Leptotrombidium deliense (chigger mite) transmit scrub typhus.

Disease in man

• Clinical signs include fever, muscle pain, headache, cough and gastrointestinal problems.
• The typical signs and problems of maculopapular rash, eschar lesions at the site of mite bites, splenomegaly and lymphadenopathy are developed during clinical infection.
• Haemorrhages and intravascular coagulations may be caused by a virulent strain of O.tsutsugamushi.
• Pneumonia, encephalitis and myocarditis may be developed following clinical phase of infection

Diagnosis

• Based on history and clinical signs
• Leukopenia and abnormal liver function tests during first week of infection
• Serological diagnosis with paired sera: Weil-Felix test and indirect immunofluorescent assay (gold standard test)
• PCR

Treatment

• Doxycycline is most commonly used, but chloramphenicol can be used as an alternative drug.
• Rifampicin and azithromycin can be used as alternative drugs for doxycycline and chloramphenicol.
• In children and pregnant women with scrub typhus, azithromycin can be used.

Prevention and control

• Control of mites
• Over-crowding should be avoided in certain circumstances like army camps and refugee camps.

Vaccination

• Vaccine was developed in 1937 in the United Kingdom by Wellcome Foundation and used in cotton rats (the project was called as “Operation Tyburn”). But this vaccine was not commercially used.
• The vaccine trial was planned in India for use by Allied Land Forces, South-East Asia Command (A.L.F.S.E.A.) in June, 1945. But, it was objected by the military commanders.
• The antigenic variation in Orientia tsutsugamushi strains was then identified, which hindered to produce a viable vaccine.