Undifferentiated Fever: The Evidence Behind the Pathway

It is September in Dakshina Kannada, and the rains have only just eased. A 34-year-old man who works in an areca plantation near Sullia comes to the casualty on the sixth day of fever. He has a headache and severe body ache. On day 3 a clinic did a malaria rapid test and a dengue NS1, both negative, and gave him paracetamol. Today his platelet count is 90,000 and his transaminases are raised. This is an illustrative case, not a real patient.

“Dengue, typhoid, leptospirosis, scrub typhus or malaria. Do we send everything?” By the end of this article you will know which test is worth sending on which day, when to treat without waiting for a result, and why the day the fever breaks is not the day to relax.

1. The day of fever decides the test

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. So the ICMR acute fever guidance does not send every test at first contact. It sequences them, so that no test is sent on a day it cannot yet be positive.

“Why not send them all on day 1, to be safe?” Because a test sent too early is not safe; it is falsely reassuring. On days 1 and 2, in a 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 a rapid test, and dengue testing where suspicion is high. From day 5, add paired blood cultures and testing for dengue, chikungunya, scrub typhus and leptospirosis as the picture directs.

Each test also has its own window. Dengue NS1 is informative on days 1 to 5, and its sensitivity falls steeply after day 5. Dengue IgM is informative from day 5. Scrub typhus IgM ELISA turns useful from the end of the first week. Weil-Felix is worth sending only after 5 to 7 days of fever. So our patient’s negative NS1 on day 3 was a fair test for dengue then, but it does not tell us what the fever is now, on day 6.

2. The case definition that lets you treat

This is the most useful rule in the pathway. DHR-ICMR defines a suspected rickettsial case as an acute undifferentiated febrile illness of 5 days or more, with or without an eschar. At 5 days or more, with malaria, dengue and typhoid excluded, doxycycline is started on suspicion, without waiting for serological confirmation. With an eschar, a fever of less than 5 days is already scrub typhus.

“Isn’t it better to confirm before treating?” Usually, yes. Here, no. Untreated rickettsial illness carries a case fatality of 30 to 45 per cent with multi-organ dysfunction, and treatment works best early, which is exactly when the diagnosis is least certain.

So we examine our patient again, fully undressed, including the groin and perineum. The eschar is in his right groin: a small black scab. The reported frequency of an eschar ranges from 7 to 97 per cent across series, largely because it is missed rather than absent. On moist skin it may have no black scab at all, only a shallow yellow-based ulcer.

3. The dengue grades you were taught

Many of us learnt dengue fever, dengue haemorrhagic fever and dengue shock syndrome. The National Guidelines for Clinical Management of Dengue Fever 2023 replaced them with three groups: Group A, mild dengue with no warning sign and no risk factor; Group B, dengue with warning signs and, or, risk factors; Group C, severe dengue.

“Why change something everyone knew?” Because the old grading looked backwards. The old grades called a patient severe only after a haemorrhagic manifestation and a documented plasma leak, which is a diagnosis made in retrospect. Warning signs are what is visible in the hours before the leak, when admission can still change the outcome. The assessment tab collects all eight.

One point to note. The tool also prints influenza categories A, B(i), B(ii) and C, from the MoHFW scheme. They share the letters with the dengue groups and nothing else, so the tool names the scheme every time it prints a letter.

4. The critical phase begins as the fever settles

NCVBDC places the critical phase of dengue after the third or fourth day of fever, lasting about 24 to 48 hours. It is the period of plasma leakage, and it opens as the temperature falls. A common mistake, and an easy one to make: the patient whose fever broke on day 5 looks better, and the family asks to go home. He is entering the window in which shock happens.

Watch the haematocrit. A rise of 10 per cent above the patient’s own baseline is an early sign of plasma leakage; a rise of 20 per cent or more is marked haemoconcentration. The reference values are population figures, and where anaemia is common, a haematocrit that reads as normal may already be a large rise for that patient. Record a baseline on admission.

5. Why waiting for a rash delays treatment

We are taught that rickettsial disease has a rash. Rash is common in spotted fever, but in scrub typhus IAP guidance puts it at only 30 to 43 per cent of cases. In India, where scrub typhus is by far the commonest rickettsiosis, waiting for a rash will delay treatment in most patients who have it.

The complications of scrub typhus come after the first week: jaundice, acute kidney injury, pneumonitis progressing to acute respiratory distress syndrome, septic shock, myocarditis and meningoencephalitis. A patient improving on day 6 is not yet safe. A falling platelet count with rising transaminases and a new oxygen requirement in the second week is the pattern that comes before deterioration.

6. Two questions residents ask

“Doxycycline in a child? What about the teeth?” IAP guidance is that doxycycline, at the dose and duration used for rickettsial infection, does not cause tooth staining or enamel damage. Withholding it on that ground exposes the child to a treatable illness with a substantial case fatality. It stays contraindicated in pregnancy, where azithromycin is the drug of choice.

“The dengue test is positive. Can I stop looking?” No. Dengue with scrub typhus co-infection is documented from Indian tertiary centres. If a patient with confirmed dengue fails to settle as expected, or develops transaminitis and a new oxygen requirement in the second week, reconsider and treat empirically.

7. Where scrub typhus lives

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.

Its foci are 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 circulate together with dengue. Ask where the patient works and walks, not only what month it is.

Back to our patient

He has had fever for 6 days. His malaria test was negative, and dengue and typhoid need excluding with tests that fit day 6, not day 3. He works in a plantation, and he has an eschar. He meets the DHR-ICMR definition, and with an eschar he would meet it even before day 5. He starts doxycycline today, without waiting for the IgM ELISA, and we watch him into the second week for the complications that come then.

Try this

A 22-year-old woman, a student in Mangaluru, has had fever for 3 days. There is no danger sign, no eschar on a full examination, and no unusual exposure. The family wants “all the fever tests” today. What do you send? Illustrative case.

Answer. On days 3 and 4: a total leucocyte count with differential, a malaria smear with a rapid test, and dengue testing if suspicion is high (NS1 is informative now). Scrub typhus IgM and Weil-Felix are not yet worth sending; they belong to the end of the first week. Explain the danger signs, and review on day 5, sooner if the fever settles and she feels worse.

Take-home points

• Match the test to the day: NS1 in the first 5 days, dengue IgM from day 5, scrub typhus serology from the end of the first week.
• Fever of 5 days or more, with malaria, dengue and typhoid excluded, licenses doxycycline without waiting for serology; an eschar licenses it sooner.
• Undress the patient and look in the groin, axilla and perineum for the eschar.
• The fever settling in dengue opens the critical phase; watch the haematocrit against the patient’s own baseline.
• A positive dengue test does not exclude scrub typhus.

For the fever that is still unexplained after three weeks, see the queue’s article Pyrexia of Unknown Origin: The Evidence Behind the Pathway. For the empirical antibiotic choices by syndrome, see Empirical Antibiotics in India: The Evidence Behind the Pathway. For scrub typhus that reaches the brain, see Neuro-Infections: The Evidence Behind the Pathway.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried in its Features and Evidence tabs on 5 November 2026, each from the source the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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