Scrub typhus

Aetiology and epidemiology. Orientia tsutsugamushi, an obligate intracellular Gram-negative bacterium, transmitted by the bite of the larval stage (chigger) of trombiculid mites (Leptotrombidium species) — which are both the vector and the reservoir. Endemic across the "tsutsugamushi triangle" of South and South-East Asia and the Pacific. India carries a very large burden, with post-monsoon (August–November) outbreaks in the sub-Himalayan belt, the north-east, Tamil Nadu, Kerala, Karnataka and Odisha, and it is one of the commonest identifiable causes of acute undifferentiated febrile illness in India.

Clinical features. Incubation 6–21 days; then abrupt fever with rigors, severe headache, myalgia and a maculopapular rash. The pathognomonic sign is the eschar — a painless, black, necrotic, cigarette-burn-like lesion with an erythematous halo — which is found in only about 20–50% of Indian patients and characteristically lies in hidden, moist, covered sites: axilla, groin, inframammary fold, perineum, waistline and behind the ears. It must be actively hunted for by fully undressing and examining the patient, which is the single highest-yield bedside act in this disease. Also generalised lymphadenopathy and hepatosplenomegaly.

Complications — why it kills older adults. ARDS is the commonest severe complication; also acute kidney injury, myocarditis, meningoencephalitis, hepatitis with jaundice, septic shock, multi-organ dysfunction, haemophagocytic lymphohistiocytosis and sensorineural hearing loss. Older age, delayed treatment, jaundice, hypoalbuminaemia and ARDS predict death. Untreated mortality reaches 30–70%, but falls to under 5% with prompt doxycycline — which is why empirical treatment matters so much.

Investigations. IgM ELISA is the most widely used and practical confirmatory test in India, becoming positive from about day 5–7. The Weil–Felix (OX-K agglutination) test is cheap and still used in peripheral settings but is insensitive and non-specific. The indirect immunofluorescence assay (IFA) is the reference standard. PCR from eschar tissue or whole blood is the most useful test in the first week, before antibodies appear. Supportive findings: thrombocytopenia, raised transaminases, hypoalbuminaemia, raised CRP, a normal or raised white cell count, and bilateral infiltrates on chest radiograph.

Treatment. Doxycycline 100 mg twice daily for 7 days is the drug of choice, and defervescence within 48 hours is characteristic and supports the diagnosis. Alternatives: azithromycin 500 mg daily for 3–5 days (preferred in pregnancy, in doxycycline intolerance and where resistance is suspected), with intravenous doxycycline or azithromycin in severe disease; chloramphenicol and rifampicin are further options. Supportive care for ARDS, acute kidney injury and shock determines outcome in severe cases. In older adults, doxycycline is safe and does not need renal adjustment, but must be taken upright with water to avoid oesophagitis, and it interacts with warfarin (raises INR) and is chelated by antacids, calcium and iron.

Prevention, and the geriatric presentation. There is no vaccine. Prevention is avoidance of chigger-infested scrub and long grass, protective and permethrin-treated clothing, DEET repellent, and clearing vegetation around dwellings. The geriatric point to make: in older adults the illness frequently presents non-specifically — delirium, functional decline, falls, or unexplained ARDS or acute kidney injury without a recognised fever. Therefore, in the post-monsoon season in India, scrub typhus should be actively considered in any older patient with acute undifferentiated febrile illness or unexplained deterioration, an eschar hunted for, and empirical doxycycline started while awaiting serology.

The landmark Indian randomised controlled trial by Varghese and colleagues (New England Journal of Medicine, 2023) compared intravenous doxycycline, azithromycin, and the combination in severe scrub typhus with organ dysfunction, and showed that combination intravenous doxycycline plus azithromycin was superior to either drug alone, reducing the composite endpoint of death at day 7, persistent organ complications and persistent fever. Combination therapy is now the preferred regimen for severe scrub typhus, while oral doxycycline monotherapy remains standard for uncomplicated disease. Empirical doxycycline for acute undifferentiated febrile illness during the post-monsoon season in endemic Indian districts continues to be recommended practice.