Scrub typhus

Scrub typhus, also known as bush typhus, is an acute febrile infectious disease caused by the bacterium Orientia tsutsugamushi, transmitted to humans through the bite of infected larval mites (chiggers) belonging to the family Trombiculidae. It is a zoonotic disease widely prevalent in the so-called “tsutsugamushi triangle”, which extends from Southeast Asia and the Pacific Islands to parts of northern Australia and East Asia, including India. The disease is characterised by high fever, rash, eschar formation, lymphadenopathy, and multi-organ involvement in severe cases.

Causative Agent and Vector

Scrub typhus is caused by Orientia tsutsugamushi, an obligate intracellular Gram-negative bacterium that primarily infects the endothelial cells lining blood vessels. The bacterium exhibits antigenic diversity, with several strains such as Karp, Gilliam, and Kato, complicating vaccine development and diagnosis.
The infection is transmitted by the bite of infected chigger mites, which are the larval stage of Leptotrombidium species. These mites inhabit scrub vegetation, grassy areas, and forest floors. The infection cycle is maintained in nature between mites and small rodents, particularly field mice and rats, with humans acting as accidental hosts.

Geographical Distribution

Scrub typhus is endemic to a large area known as the “tsutsugamushi triangle”, encompassing:

• South and Southeast Asia (India, Nepal, Myanmar, Thailand, Indonesia, and Vietnam).
• East Asia (China, Japan, Korea).
• Pacific Islands and parts of northern Australia.

In India, it is particularly prevalent in Himalayan foothills, North Eastern states, Himachal Pradesh, Uttarakhand, Tamil Nadu, Kerala, and parts of Maharashtra and Rajasthan. Seasonal outbreaks often coincide with the monsoon and post-monsoon periods due to increased vegetation and mite activity.

Mode of Transmission

The disease spreads through the bite of infected chiggers, which inject the bacteria into the skin while feeding. The mites typically attach to areas of the body where the skin is thin and moist, such as the axilla, groin, waist, and genital region.
There is no direct person-to-person transmission. Transmission occurs only through exposure to infected chiggers in endemic areas.

Pathogenesis

Once inside the human body, O. tsutsugamushi multiplies in the endothelial cells of blood vessels, leading to:

• Vasculitis (inflammation of blood vessels).
• Increased vascular permeability, causing tissue damage and organ dysfunction.
• Systemic inflammatory response, which contributes to fever and other symptoms.

Severe cases may involve multi-organ failure due to widespread endothelial injury.

Clinical Features

The incubation period ranges from 6 to 21 days (typically 10–12 days). The disease presents with non-specific symptoms initially, which can resemble other tropical fevers.
Common symptoms include:

• Sudden onset of high fever and chills.
• Severe headache, myalgia, and fatigue.
• Cough and mild respiratory distress in some cases.
• Lymphadenopathy (swollen lymph nodes).
• Maculopapular rash, appearing after a few days of fever.
• Gastrointestinal symptoms such as nausea, vomiting, and abdominal pain.

A pathognomonic feature of scrub typhus is the eschar — a dark, scab-like lesion at the site of the chigger bite, resembling a cigarette burn. The eschar is often surrounded by erythema and accompanied by regional lymph node enlargement. However, eschars may not always be visible, especially in dark-skinned individuals or in less accessible body regions.

Complications

If left untreated, scrub typhus can progress to severe disease involving multiple organ systems. Common complications include:

• Pneumonitis and Acute Respiratory Distress Syndrome (ARDS).
• Meningoencephalitis (inflammation of the brain and meninges).
• Hepatitis and jaundice.
• Renal failure due to vasculitic damage.
• Myocarditis (inflammation of the heart muscle).
• Septic shock and multi-organ failure in advanced cases.

Mortality rates can range from 1% to 30%, depending on early diagnosis and treatment.

Diagnosis

Accurate and early diagnosis is essential, as clinical symptoms can mimic other febrile illnesses such as dengue, malaria, typhoid, or leptospirosis.
Diagnostic methods include:

• Clinical examination for eschar and rash in endemic areas.
• Weil–Felix test: A traditional agglutination test using Proteus OXK antigen; inexpensive but nonspecific.
• Indirect Immunofluorescence Assay (IFA): Gold standard for serological confirmation.
• Enzyme-Linked Immunosorbent Assay (ELISA): Detects IgM antibodies; widely used for rapid diagnosis.
• Polymerase Chain Reaction (PCR): Detects bacterial DNA and confirms infection early.

Treatment

Scrub typhus responds well to specific antibiotic therapy if started early.
First-line drugs:

• Doxycycline – 100 mg twice daily for 7–14 days (drug of choice for adults).
• Azithromycin – preferred for children and pregnant women due to better safety profile.
• Chloramphenicol – an alternative where doxycycline resistance or contraindication exists.

Early initiation of antibiotics typically leads to rapid defervescence (fall in fever) within 48 hours, confirming clinical diagnosis. Supportive therapy, including fluid management and treatment of complications, is critical in severe cases.

Prevention and Control

Since no widely available vaccine exists, preventive strategies focus on reducing exposure to chiggers and promoting early diagnosis.
Preventive measures include:

• Avoiding mite-infested areas, especially scrublands and grassy fields.
• Wearing protective clothing, such as long sleeves, full trousers, and boots.
• Using insect repellents containing DEET or permethrin on skin and clothing.
• Maintaining vegetation clearance around human dwellings in endemic regions.
• Health education and community awareness about identifying eschar and early reporting of fever.

Chemoprophylaxis: Doxycycline (200 mg weekly) has been used in high-risk groups, such as military personnel or agricultural workers in endemic zones, though this is not recommended for routine use.

Epidemiology in India

Scrub typhus is an important cause of acute undifferentiated febrile illness in India. Outbreaks have been reported in states such as Himachal Pradesh, Uttarakhand, Jammu and Kashmir, Assam, Tamil Nadu, and Kerala. Increased awareness, improved diagnostic capacity, and vector surveillance have led to more frequent detection in recent years.

Public Health Importance

Scrub typhus represents a major public health challenge due to:

• Widespread distribution in rural and forested areas.
• Non-specific clinical presentation, often leading to misdiagnosis.
• Lack of vaccine and poor awareness among communities.
• Potential for severe complications and high mortality if untreated.