Undifferentiated Febrile Illness
What to send on which day of illness, and when to start doxycycline · v1.2- Enter the day of fever, the danger signs, and whatever findings and results you already have.
- You get the investigations for that day of illness, following ICMR acute fever guidance, with a level of care and a weighted differential.
- It says whether the DHR-ICMR case definition for empirical rickettsial therapy is met, and prints the regimen.
- Where the picture fits dengue, it grades Group A, B or C on the 2023 national guideline.
- Any danger sign overrides the day-of-illness sequence and opens the admission pathway, whatever the day of fever.
- Fever past the first two weeks. Beyond three weeks without a diagnosis, use the Pyrexia of Unknown Origin Pathway.
- Infants below 1 year, where neonatal sepsis, urinary tract infection, meningitis and pneumonia dominate the differential.
- Fever with a localising source, such as pneumonia, cellulitis or a urinary tract infection.
- Antimalarial treatment. Malaria has to be excluded for the rest of the pathway to hold, and the tool names it rather than dosing it.
- Cover for bacterial sepsis, severe leptospirosis and enteric fever. The regimen printed here treats rickettsial illness only, and in organ dysfunction the broad-spectrum decision is a separate one.
1. Patient Characteristics
2. Day of Illness
Enteric fever serology is the same problem. Widal needs an agglutinin response that is rarely present before the end of the first week, and a single titre where baseline antibodies are common cannot establish a diagnosis. Typhidot and similar rapid IgM tests carry high false positivity in the same setting, particularly after recent typhoid vaccination or a previous infection. Neither replaces blood culture, the only test that confirms enteric fever and the one with the highest yield in the first week.
3. Danger Signs
Any single item here overrides the day-of-illness sequence and mandates admission with empirical therapy.
4. Influenza-Like Illness Screen
An ILI case is an acute respiratory infection with measured fever of 38°C (100.4°F) or more, plus a respiratory symptom, with onset in the last 10 days. Categorisation follows the MoHFW Category A, B and C scheme.
5. Discriminating Clinical Features
6. Results Already Available
7. Dengue Severity, if This Is Dengue
The National Guidelines for Clinical Management of Dengue Fever 2023 (NCVBDC) retired the dengue fever, dengue haemorrhagic fever and dengue shock syndrome grades. Group A, Group B and Group C replace them, decided on warning signs, risk factors and severe features.
These items are graded only where the rest of the form points to dengue, and the grade is conditional until dengue is confirmed. Persistent vomiting comes from section 3, the platelet count and haematocrit from section 6.
Age below 10 or above 65, pregnancy and the chronic disease question in section 4 are read from what you have already entered.
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.
| Cause | Features that raise suspicion | Features that argue against |
|---|---|---|
| Dengue | Retro-orbital pain, severe myalgia, marked thrombocytopenia, rising haematocrit, leucopenia, saddleback fever | Neutrophilic leucocytosis, eschar, prolonged fever beyond 10 days |
| Scrub typhus | Eschar, fever 5 days or more, rural or scrub exposure, dry cough with bilateral infiltrates, transaminitis, thrombocytopenia | Rash is uncommon in scrub typhus despite being taught as a hallmark |
| Leptospirosis | Conjunctival suffusion, calf tenderness, flood or sewage exposure, jaundice with renal failure, raised creatine kinase | Absence of any water contact makes it substantially less likely |
| Enteric fever | Stepwise fever, relative bradycardia, coated tongue, abdominal tenderness, normal or low leucocyte count | Eschar, conjunctival suffusion, very short illness |
| Malaria | Paroxysmal fever with chills and rigors, splenomegaly, travel to or residence in an endemic district | A negative smear and rapid test on two occasions during fever |
1. Where to Look for an Eschar
Reported frequency ranges from 7 to 97 per cent across series, largely because it is missed rather than absent.
- Usually single, on the neck, axilla, chest, abdomen or groin.
- On moist intertriginous surfaces, 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
| Test | Informative window | Note |
|---|---|---|
| Dengue NS1 antigen | Days 1 to 5 | Sensitivity falls steeply after day 5 |
| Dengue IgM | Day 5 onward | Combine with NS1 in the overlap period |
| Malaria smear and rapid test | Any day, ideally during fever | Repeat if the first is negative and suspicion persists |
| Scrub typhus IgM ELISA | End of first week onward | Optical density cut-off 0.5; regional baselines vary |
| Weil-Felix | After 5 to 7 days of fever only | Titre 1:80 suggests possible infection; low sensitivity and specificity |
| Rickettsial PCR | First week (blood), any time (eschar) | Rickettsaemia lasts 7 to 10 days |
| Blood culture | Day 5 onward, paired sets | Yield highest in the first week of enteric fever |
1. The Staged Investigation Principle
ICMR acute fever guidance sequences investigation rather than ordering everything at first contact, so that no test is sent on a day it cannot yet be positive.
| Day of fever | Investigation, in an undifferentiated fever without danger signs |
|---|---|
| Days 1 and 2 | Investigation and antimicrobials may reasonably be deferred |
| Days 3 and 4 | Total leucocyte count with differential, malaria smear with rapid test, and dengue testing where suspicion is high |
| Day 5 onwards | Add paired blood cultures, with testing for dengue, chikungunya, scrub typhus and leptospirosis as the clinical picture directs |
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.
- With an eschar, a fever of less than 5 days is already scrub typhus.
- At 5 days or more, with malaria, dengue and typhoid excluded, doxycycline is started on suspicion and without serological confirmation.
- Untreated rickettsial illness carries a case fatality of 30 to 45 per cent with multi-organ dysfunction, and treatment works best early, which is when the diagnosis is least certain.
3. The Dengue Grades You Were Taught Have Been Retired
The National Guidelines for Clinical Management of Dengue Fever 2023 replaced dengue fever, dengue haemorrhagic fever and dengue shock syndrome 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, meaning shock, or fluid accumulation with respiratory distress, or severe bleeding, or severe organ dysfunction, or an AST or ALT of 1000 units per litre or more, or impaired consciousness at a Glasgow Coma Scale below 9.
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. The assessment tab collects all eight.
4. Two Different A, B and C Schemes Sit in This Tool
The influenza categories are the MoHFW scheme: A, B(i), B(ii) and C. The dengue groups are the NCVBDC scheme: A, B and C. They share the letters and nothing else, so the tool names the scheme every time it prints a letter.
5. The Critical Phase Begins As the Fever Settles
NCVBDC places the critical phase after the third or fourth day of fever, lasting about 24 to 48 hours. It is the period of vasculopathy and plasma leakage, and it opens as the temperature falls. The patient who looks better because the fever broke on day 5 is entering the window in which shock happens.
- A haematocrit 10 per cent above the patient's own baseline is an early objective indicator of plasma leakage. A rise of 20 per cent or more is marked haemoconcentration.
- The reference values, below 40 per cent in children and adult females and below 45 per cent in adult males, are population figures.
- Where anaemia is common, a haematocrit that reads as normal may already be a substantial rise in that patient. Record a baseline on admission rather than reasoning from one value.
6. Why Rash Is a Poor Guide in Scrub Typhus
Rash is common in spotted fever and substantially less common in scrub typhus, where IAP guidance puts it at 30 to 43 per cent of cases. In Indian practice, where scrub typhus is by far the commonest rickettsiosis, waiting for a rash will delay treatment in most patients who have it.
7. Complications Declare Themselves in the Second Week
The complications of scrub typhus develop after the first week, so a patient improving on day 6 is not yet safe.
- Jaundice, acute kidney injury, pneumonitis progressing to acute respiratory distress syndrome, septic shock, myocarditis and meningoencephalitis.
- Pneumonitis is among the most frequent: non-productive cough and breathlessness, with bilateral interstitial infiltrates that can consolidate within 48 hours.
- 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.
- None of these is diagnostic. A falling platelet count with rising transaminases and a new oxygen requirement in the second week is the pattern that precedes deterioration.
8. Doxycycline in Children and the Dental Staining Question
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.
9. 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.
- Reconsider, and treat empirically, where a patient with confirmed dengue fails to defervesce as expected, or develops transaminitis and a new oxygen requirement in the second week.
10. 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. Exposure history rather than season alone should guide the differential.
References
- Department of Health Research and Indian Council of Medical Research. Guidelines for Diagnosis and Management of Rickettsial Diseases in India. DHR-ICMR; February 2015.
- Indian Council of Medical Research. Treatment Guidelines for Antimicrobial Use in Common Syndromes. 2nd ed. ICMR; 2019.
- Chrispal A, Boorugu H, Gopinath KG, et al. Acute undifferentiated febrile illness in adult hospitalised patients: the disease spectrum and diagnostic predictors. Trop Doct. 2010;40(4):230-234.
- Abhilash KPP, Jeevan JA, Mitra S, et al. Acute undifferentiated febrile illness in patients presenting to a tertiary care hospital in South India: clinical spectrum and outcome. J Glob Infect Dis. 2016;8(4):147-154.
- Rathi N, Rathi A. Rickettsial infections: Indian perspective. Indian Pediatr. 2010;47(2):157-164.
- Indian Academy of Pediatrics. IAP Guidelines on Rickettsial Diseases in Children. Indian Pediatr. 2017;54:223-229.
- Varghese GM, Trowbridge P, Janardhanan J, et al. Clinical profile and improving mortality trend of scrub typhus in South India. Int J Infect Dis. 2014;23:39-43.
- Blacksell SD, Bryant NJ, Paris DH, et al. Scrub typhus serologic testing with the indirect immunofluorescence method as a diagnostic gold standard: a lack of consensus leads to a lot of confusion. Clin Infect Dis. 2007;44(3):391-401.
- Ministry of Health and Family Welfare, Government of India. Guidelines on Categorisation of Seasonal Influenza A H1N1 Cases During Screening for Home Isolation, Testing, Treatment and Hospitalisation. National Centre for Disease Control; category A, B(i), B(ii) and C definitions.
- Centers for Disease Control and Prevention. Influenza Antiviral Medications: Summary for Clinicians. CDC; 2024.
- Centers for Disease Control and Prevention. Dengue Virus Antigen Detection. CDC; 2024.
- National Center for Vector Borne Diseases Control, Ministry of Health and Family Welfare, Government of India. National Guidelines for Clinical Management of Dengue Fever. NCVBDC; 2023.
- World Health Organization. Dengue: Guidelines for Diagnosis, Treatment, Prevention and Control. New edition. WHO; 2009.
How to Cite This Tool
DOIhttps://doi.org/10.5281/zenodo.22401576
AMA Style:Umakanth S. Undifferentiated Febrile Illness. Version 1.2. MEDiscuss Clinical Decision Support System. Published 2026. Accessed . https://mediscuss.org/cdss/febrile-illness-pathway. doi:10.5281/zenodo.22401576
Vancouver Style:Umakanth S. Undifferentiated Febrile Illness [Internet]. Version 1.2. MEDiscuss.org; 2026 [cited ]. Available from: https://mediscuss.org/cdss/febrile-illness-pathway. doi:10.5281/zenodo.22401576
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