Write a short note on scrub typhus and other rickettsial infections.

Aetiology and Classification

Rickettsial infections are caused by obligate intracellular Gram-negative bacteria transmitted by arthropod vectors, which invade and multiply in vascular endothelial cells, producing a widespread vasculitis — the pathological basis of every clinical feature.

Scrub typhus is endemic across India, particularly in the sub-Himalayan belt, the north-east, the Western Ghats and eastern states, with a marked post-monsoon peak. It is now recognised as one of the commonest causes of acute undifferentiated febrile illness in India — and one of the most frequently missed.

Clinical Features

• Incubation 6–21 days; abrupt onset of high fever with rigors, severe headache (often retro-orbital), myalgia and profound malaise.
• ESCHAR — the pathognomonic lesion of scrub typhus: a painless, punched-out ulcer with a black necrotic centre and an erythematous margin, resembling a cigarette burn, at the site of the mite bite. It is present in a variable proportion of Indian patients (often under half) and is found in hidden, moist areas — axilla, groin, inframammary fold, genitalia, neck, waistline — which is why a full undressed examination is essential. It is painless, so the patient will not point to it.
• Generalised lymphadenopathy — regional first, then generalised; a useful supporting sign.
• Maculopapular rash — appearing around day 5, beginning on the trunk and spreading centrifugally; often absent or difficult to see in dark skin.
• Hepatosplenomegaly; conjunctival suffusion; relative bradycardia.
• Complications (typically in the second week, when untreated): ARDS and pneumonitis (the commonest severe manifestation), acute kidney injury, meningoencephalitis, myocarditis, hepatitis and jaundice, septic shock, DIC, and haemophagocytic lymphohistiocytosis.

Investigations

• Weil–Felix test (OX-K agglutination) — cheap and widely available in India, but insensitive and non-specific; a negative result does not exclude the diagnosis and a positive result must be interpreted with caution. Its continued use as the sole test is a source of both missed and over-diagnosis.
• IgM ELISA — the most practical confirmatory test; becomes positive from the end of the first week.
• Immunofluorescence assay (IFA) — the reference standard, with paired sera.
• PCR on blood or eschar tissue — most useful in the first week, before antibodies develop; eschar samples have a high yield.
• Supporting findings: thrombocytopenia, leucocytosis or leucopenia, raised transaminases (very common), hypoalbuminaemia, raised CRP, hyponatraemia, and renal impairment. Chest X-ray for pneumonitis and ARDS.
• Exclude the differentials in parallel — malaria smear and rapid test, dengue NS1 and serology, leptospirosis IgM, blood cultures for enteric fever, and where relevant tuberculosis and HIV.

Management

• Doxycycline is the drug of choice — the clinical response is dramatic, with defervescence typically within 48 hours, and this rapid response is itself supportive of the diagnosis.
• Azithromycin is the preferred alternative in pregnancy and in children, and where doxycycline resistance is suspected.
• Chloramphenicol and rifampicin are further alternatives.
• START TREATMENT ON CLINICAL SUSPICION — do not wait for serology, which is often negative in the first week when treatment matters most. Empirical doxycycline is appropriate for any acute undifferentiated febrile illness in an endemic area once malaria has been excluded.
• Supportive care for complications: oxygen and ventilatory support for ARDS, renal replacement therapy, and management of shock and myocarditis.

Prevention

Avoidance of mite-infested scrub vegetation, insect repellents (DEET) on skin and permethrin on clothing, wearing protective clothing with tucked trousers, prompt bathing and inspection of skin after field exposure, clearing vegetation around dwellings, and rodent control. Doxycycline chemoprophylaxis may be considered for short-term high-risk exposure such as military deployment. There is no vaccine.