Scrub typhus

An infection caused by Orientia tsutsugamushi - a small intracellular bacterium related to the family of Rickettsiaceae - the organism is classified on its own and not with other rickettsiaceae as it has differences in genes and cell wall structure.¹ The name scrub typhus was applied after discovery of increased frequency of the disease in scrub or wasteland areas.²

Transmission of scrub typhus

Orientia tsutsugamushi is transmitted by the ovaries of trombiculid mites. The offspring mites or larvae are then infected and these are known as "chiggers". These mites then pass the infection to humans by feeding on the fluid in skin cells.

Any surrounding which supports the chiggers will be rife in infection eg near riverbanks (especially if forest cleared close by). It follows that infections are greatest in the rainy season.

Epidemiology

• Scrub typhus is endemic in Eastern and Southern Asia, Northern Australia but is also found in other regions eg India, Thailand, Tibet, Japan, Russia and mountainous regions of Nepal.

• Scrub typhus can cause outbreaks of pyrexia of unknown origin eg in a school in India.³

• Not surprisingly, infection is commonest in the indigenous population and immunity after infection varies and may last 1 - 3 years. However, people visiting endemic areas may also be infected.

• In endemic areas up to 5% of the population are infected.

Presentation

Infection with scrub typhus is most often self-limiting but can occasionally be severe and even fatal.

The incubation period is up to ten days and the commonest features are:

• Papule followed by an eschar at the site of chigger feeding (only in 50%)

• Fever

• Headache

• Myalgia

• Cough

• Abdominal pain, nausea and vomiting

• Regional lymphadenopathy

• Maculopapular rash

Indigenous patients do not commonly develop rash or lymphadenopathy which is thought to be related to previous exposure.³

Severe cases can develop encephalitis and interstitial pneumonia and this may be fatal. If concomitant G6PD deficiency is present then the severity is increased.

Investigations

• Indirect immunofluorescence

• PCR for Orientia tsutsugamushi from blood of feverish patients

• Some studies have used PCR (polymerase chain reaction) on specimens obtained from eschars.⁴⁵ This involves obtaining a small piece of the eschar and then amplifying the DNA to look for the genetic sequence of scrub typhus.

• CXR may show lower zone lung infiltrates in interstitial pneumonia

Complications

• Pneumonitis

• Hepatitis

• Myocarditis

• Meningoencephalitis

• Disseminated intravascular coagulation⁶

• Multi-organ failure

Management

• Antibiotics: doxycycline orally or chloramphenicol in more severe cases.

• There may be antibiotic resistance in some areas eg Thailand.

• Azithromycin has been used in resistant cases and may be better than doxycycline - especially in children and pregnant women.⁵

Prognosis

Fatality of untreated cases is between 5-10 % and is even higher in adults.⁶

Prevention

Vector control and systemic acaracides have been used. No vaccine or chemoprophylaxis is available.