Scrub typhus, also known as Tsutsugamushi disease or Chigger borne typhus or Bush typhus, is an acute febrile illness caused by Orientia tsutsugamushi (earlier known as Rickettsia tsutsugamushi), an obligate intracellular Gram-negative bacterium.
Scrub typhus is endemic to a part of the world called the “tsutsugamushi triangle”, which extends from northeast Asia to Papua New Guinea and northern Australia in the southeast, Pakistan and Afghanistan in the northwest, and the Maldives and Réunion Islands in the southwest. However, evidence has shown that scrub typhus may not be restricted to the tsutsugamushi triangle and may be present outside this endemic zone also.
Scrub typhus is a re-emerging zoonotic disease in India. It is prevalent in many parts of India particularly in the sub-Himalayan belt, from Jammu to Nagaland. Outbreaks are frequent during the rainy season; but, in south India outbreaks occur during the cooler months of the year.
Here are some salient features about scrub typhus.
- The trombiculid mite is the reservoir of infection. The larva (chigger) feeds on the vertebrate hosts and acquires the infection. The larval stage of the mite acts as both the reservoir and the vector for infecting the humans and rodents due to transovarial transmission of the bacteria.
- Humans are accidental hosts in this zoonotic disease and acquire the infection through the bite of the infected larva of the trombiculid mite while walking, sitting, or lying on the infested ground. Human to human transmission does not occur.
- The incubation period is 10-12 days.
- The infection can range from a mild, self-limiting disease to a fatal infection, leading to multiorgan failure, if not diagnosed and treated in time.
- Symptoms are acute in onset and non-specific in nature with high fever and chills, headache, malaise, myalgia, cough and breathlessness, diarrhea, vomiting and a nonpruritic maculopapular rash. The rash typically begins on the abdomen and spreads to the extremities.
- An eschar at the site of chigger bite is pathognomonic of scrub typhus. It is a punched out ulcer with a black necrotic center and an erythematous border on the exposed body parts like legs, neck, axilla, chest, abdomen and groin along with regional lymphadenopathy. It appears few days after the chigger bite, but before the disease presents clinically making it an important early sign associated with scrub typhus. The eschar may be difficult to see in dark skinned people.
- Differential diagnosis: Other febrile illnesses like dengue fever, malaria, chikungunya, typhoid
- Lab diagnosis is by detection of IgM antibody on ELISA (positive within 3-4 days after the onset of illness), Weil Felix reaction (IgM titer ≥ 1:320 or a 4-fold rise in titer starting from 1:50), PCR from blood and eschar. Weil Felix test may be negative in the early stage of the infection as IgM antibodies appear only during the second week.
- The gold standard test for serologic diagnosis of scrub typhus is immunofluorescence assay (IFA), but the cost, need for specialized lab and training limit its use.
- Treatment: Doxycycline 200 mg / day in two divided doses for individuals above 45 kg for 7 days (orally or IV) is the drug of choice. Alternatively, azithromycin 500 mg single dose for 5 days (orally or IV), or tetracycline 500 mg in 4 divided doses for 7 days (orally or IV). In pregnant women, doxycycline is contraindicated. Azithromycin 500 mg in a single dose for 5 days is preferred.
- Patients treated with appropriate antibiotics typically become afebrile within 48 hours of starting treatment. Failure of defervescence within 48 hours rules out scrub typhus.
- Prevention: Chemoprophylaxis in endemic areas (doxycycline 200 mg single dose weekly, started before exposure to 6 weeks after exposure) and mite control (clearing the vegetation, application of insecticides to the ground and vegetation, application of insect repellents and miticide to both the exposed skin and clothing)
- There is current no vaccine for scrub typhus.