Scrub Typhus in India — The Re-Emerging Threat of a Forgotten Zoonosis

Scrub typhus, an acute febrile illness caused by Orientia tsutsugamushi, is an underdiagnosed but re-emerging vector-borne zoonosis in India. The disease is endemic in several Indian states and is characterised by non-specific symptoms, often leading to diagnostic delays and high mortality in untreated cases. This comprehensive article delves into its pathophysiology, clinical manifestations, epidemiological trends, diagnostic challenges, and current treatment strategies.

Introduction

Scrub typhus, a type of rickettsial disease, has experienced a resurgence in India over the past two decades. The disease derives its name from the vegetation ('scrub') where its vectors, the larvae of trombiculid mites (chiggers), thrive. Historically reported during World War II, its incidence declined but has now re-emerged, particularly in hilly and agricultural regions. Despite its high morbidity and potential mortality, awareness remains low among clinicians, especially in rural healthcare settings.

Epidemiology

Scrub typhus is endemic in the "tsutsugamushi triangle," which extends from northern Japan and eastern Russia to the north of Australia and Pakistan, encompassing the Indian subcontinent. In India, outbreaks are most commonly reported from:

● Himachal Pradesh: High-altitude villages, apple orchards

● Tamil Nadu and Kerala: Post-monsoon, particularly among agricultural workers

● Northeast India: Assam and Arunachal Pradesh

● Maharashtra and Gujarat: Sporadic rural outbreaks

Recent data from the Indian Council of Medical Research (ICMR) estimates nearly 1 million cases annually, though underreporting is a significant issue. Children and elderly patients, as well as immunocompromised individuals, are particularly vulnerable.

Pathophysiology

Orientia tsutsugamushi is an obligate intracellular, Gram-negative bacterium. Upon chigger bite, the pathogen enters the bloodstream and disseminates, with a predilection for endothelial cells.

● Primary site of infection: The skin, where an eschar may develop.

● Immune evasion: Suppression of host cell apoptosis and manipulation of immune signalling pathways.

● Endothelial infection: Leads to vasculitis, increased vascular permeability, and multi-organ dysfunction.

● Cytokine storm: In severe cases, overproduction of TNF-alpha, IL-6, and other cytokines exacerbates inflammation and tissue damage.

Clinical Manifestations

Scrub typhus presents with non-specific symptoms that mimic other tropical infections:

● Fever (94–100%)

● Headache (60–80%)

● Myalgia

● Gastrointestinal symptoms (nausea, vomiting, diarrhoea)

● Eschar (pathognomonic, but only in 10–50% of Indian cases)

● Lymphadenopathy and splenomegaly

Severe Complications

● Acute Respiratory Distress Syndrome (ARDS)

● Meningoencephalitis

● Myocarditis

● Acute Kidney Injury (AKI)

● Disseminated Intravascular Coagulation (DIC)

Diagnostic Challenges

Common Diagnostic Tools

● Weil-Felix Test: Inexpensive but low specificity

● IgM ELISA: Most widely used with good sensitivity

● PCR: Detects bacterial DNA; highly specific but limited to research and tertiary care

Challenges

●       Lack of awareness among clinicians

●       Limited availability of advanced diagnostics in rural India

●       Misdiagnosis as dengue, typhoid, or leptospirosis

Treatment

First-line Therapy

● Doxycycline: 100 mg BID for 7–10 days

● Azithromycin: 500 mg daily for 3–5 days (used in pregnancy and pediatric cases)

Alternative

● Chloramphenicol: Effective but with potential for bone marrow suppression

Response to therapy is usually rapid (defervescence within 48–72 hours).

Prevention and Control

● Personal protective measures: Long-sleeved clothing, repellents

● Vegetation control around human settlements

● Community education in endemic areas

● No vaccine is available; vaccine development is ongoing but challenging due to antigenic variation.

Conclusion

Scrub typhus is an underrecognized but significant contributor to acute febrile illness in India. Early recognition and prompt antibiotic therapy are essential to prevent life-threatening complications. Increased surveillance, improved diagnostics, and physician training are imperative to combat this neglected disease.

References:

1. Rathi N, Rathi A. Rickettsial infections: Indian perspective. Indian Pediatr. 2010;47(2):157-64.

2. ICMR Bulletin on Rickettsial Diseases, 2023.

3. Silpapojakul K. Scrub typhus in the Asia-Pacific region. Clin Infect Dis. 2016;63(7):e121-6.