Acute Undifferentiated Febrile Illness

Separating the Five Common Causes

In South and South-East Asia dengue is the leading cause of acute undifferentiated fever, followed by leptospirosis, enteric fever, scrub typhus and other rickettsioses. No single feature is diagnostic, and co-infection is well described, particularly dengue with scrub typhus.

1. Where to Look for an Eschar

Reported frequency varies enormously, from 7 to 97 per cent across series, largely because it is missed rather than absent. It is usually single and sits on the neck, axilla, chest, abdomen or groin. On moist intertriginous surfaces such as the axilla, scrotum and perianal region it may lack the black scab entirely and appear only as a shallow yellow-based ulcer without surrounding erythema. A full undressed examination, including the groin and perineum, is the single highest-yield manoeuvre in this presentation.

2. Timing of Each Test

Evidence and Indian Context

1. The Staged Investigation Principle

ICMR acute fever guidance sequences investigation rather than ordering everything at first contact. On days 1 and 2 of an undifferentiated fever without danger signs, investigation and antimicrobials may reasonably be deferred. On days 3 and 4 a total leucocyte count with differential, a malaria smear with rapid test, and dengue testing where suspicion is high are appropriate. From day 5, paired blood cultures are added, together with testing for dengue, chikungunya, scrub typhus and leptospirosis as the clinical picture directs. The purpose is not to withhold care but to avoid a battery of tests taken at a point in the illness where they cannot yet be positive.

2. The Case Definition That Licenses Empirical Doxycycline

DHR-ICMR defines a suspected rickettsial case as an acute undifferentiated febrile illness of 5 days or more, with or without an eschar. If an eschar is present, a fever of less than 5 days should already be considered scrub typhus. Where fever has lasted 5 days or more and malaria, dengue and typhoid have been excluded, doxycycline should be started when scrub typhus is considered likely, without waiting for serological confirmation. Untreated rickettsial illness carries a case fatality of 30 to 45 per cent with multi-organ dysfunction, and antibiotic therapy is most effective early, precisely when diagnosis is least certain.

3. Why Rash Is a Poor Guide in Scrub Typhus

Rash is widely taught as the hallmark of rickettsial disease, but it is present neither at presentation nor in all patients. It is common in spotted fever and extremely rare in scrub typhus, and where it appears it does so only after 3 to 5 days. In Indian practice, where scrub typhus is by far the commonest rickettsiosis, waiting for a rash before considering the diagnosis will delay treatment in most patients who have it.

4. Complications Declare Themselves in the Second Week

The complications of scrub typhus characteristically develop after the first week: jaundice, acute kidney injury, pneumonitis progressing to acute respiratory distress syndrome, septic shock, myocarditis and meningoencephalitis. Pneumonitis is among the most frequent, presenting as non-productive cough and breathlessness with bilateral interstitial infiltrates that can progress to consolidation within 48 hours. A patient improving on day 6 is not yet safe.

5. Doxycycline in Children and the Dental Staining Question

Concern about enamel damage has caused reluctance to use doxycycline in children. Indian Academy of Paediatrics guidance is that doxycycline at the dose and duration used for rickettsial infection does not cause tooth staining or enamel damage. Withholding it on this basis exposes the child to a treatable illness with a substantial case fatality. Doxycycline remains contraindicated in pregnancy, where azithromycin is the drug of choice.

6. Supportive Laboratory Patterns

In scrub typhus the leucocyte count may be normal early and then rise above 11,000 per cubic millimetre, thrombocytopenia below 100,000 is seen in the majority, and transaminases are commonly raised. These are not diagnostic. Their value is in raising suspicion when the picture is otherwise undifferentiated, and in signalling severity: a falling platelet count with rising transaminases and a new oxygen requirement in the second week is the pattern that precedes deterioration.

7. Co-infection Is Not Rare

Dengue with scrub typhus co-infection is documented from Indian tertiary centres. A positive dengue test does not exclude a treatable rickettsial illness, and a patient with confirmed dengue who fails to improve beyond the expected defervescence, or who develops transaminitis and an oxygen requirement in the second week, warrants reconsideration and empirical doxycycline.

8. Regional Note for Coastal Karnataka and the Western Ghats

Scrub typhus has been reported across Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Bihar, West Bengal, Meghalaya, Rajasthan, Maharashtra, Karnataka, Tamil Nadu and Kerala, and in some regions accounts for up to half of all undifferentiated fever presenting to hospital. Endemic foci sit in specific habitats: abandoned plantations, overgrown forest clearings, shrubby field margins, river banks and poorly maintained kitchen gardens. During and after the monsoon in coastal Karnataka, leptospirosis and scrub typhus co-circulate with dengue, and exposure history rather than season alone should guide the differential.