Fever, Zoonoses and the Undiagnosed Patient

Microbiology — Fever and Zoonoses, NMC MBBS licence examination syllabus (Nepal Medical Council).

Fever, zoonoses and the undiagnosed patient

"Fever" is not a diagnosis. It is an instruction to take a better history.

An undifferentiated fever is one of the commonest presentations in this region and one of the least satisfying, because the list of possible causes is enormous and most of the early tests come back unhelpful. What narrows it is not more investigation but better questioning — where the patient has been, what they have touched, and what the skin shows when you actually look at it.

Two things run underneath the whole chapter. First, before any of the reasoning, decide whether this patient is sick now: sepsis and severe malaria kill within hours and do not wait for a diagnosis. Second, several of the infections here are identified by a single physical finding that is only found if you go looking — an eschar in a skinfold, conjunctival suffusion, a bite mark the patient has forgotten.

🩺 Where this lives: Rabies is almost invariably fatal once symptoms begin, and almost entirely preventable before they do. Everything therefore depends on what happens in the hours after a bite — and the single most effective act is the least medical-sounding one: thorough washing of the wound with soap and running water for several minutes, which substantially reduces the risk on its own. In settings where dog bites are common and post-exposure prophylaxis may be delayed or unavailable, teaching that one measure to families and health workers saves lives that no later intervention can.

💡 A note on specifics. This chapter gives no doses, no named antibiotics, no rabies exposure categories, no vaccine schedules and no incubation periods. Rabies categories determine whether immunoglobulin is given — a life-or-death decision — and are set by the national rabies programme; incubation periods differ between published sources. Take all of them from current national guidance. Malaria and dengue are covered in their own chapter, enteric fever in the Enteric Infections chapter, tuberculosis and HIV in theirs, and sepsis recognition in the Medicine chapter.

The approach

The order matters. Physiology before diagnosis — a patient who is shocked, confused or poorly perfused needs resuscitation and empirical treatment before the differential is complete. Then geography, exposure and examination, in that sequence, because each narrows the list more sharply than any blood test taken blind. And in any febrile patient from a malaria-endemic area, exclude malaria first: it is common, rapidly fatal, and treatable.

Zoonoses

THE EXPOSURE HISTORY THAT MAKES THE DIAGNOSIS LEPTOSPIROSIS Exposure: fresh water contaminated with ANIMAL URINE — flooding, paddy fields, canals, sewers, and occupations involving them. Common after heavy rain and flooding. Picture: abrupt fever, severe MYALGIA (especially calves), headache, and CONJUNCTIVAL SUFFUSION — red eyes without discharge, which is easy to overlook. Severe disease involves jaundice, renal failure and bleeding. SCRUB TYPHUS and rickettsial infections Exposure: mite or tick bite in scrub, bush or grassland. THE FINDING TO HUNT FOR IS AN ESCHAR — a painless black scab with a red rim, often in the axilla, groin, under a breast or at the waistband, where neither the patient nor a hurried examiner will see it. Undress and look. BRUCELLOSIS Exposure: unpasteurised milk and dairy, or animal handling. Prolonged undulating fever, sweats, joint and back pain. JAPANESE ENCEPHALITIS Exposure: rural rice-growing areas with pigs and water birds. Encephalitis in an endemic area. Vaccine- preventable — see the national immunisation programme. ANTHRAX Exposure: animal hides, wool, carcasses. Cutaneous form gives a painless ulcer with a black eschar. THE PRACTICAL POINT: none of these is diagnosed by looking at the fever chart. They are diagnosed by asking what the patient does and where they have been, and then examining the skin properly.

Rabies

💡 Exam angle: the examinable content is the immediate management of the bite, not the treatment of established disease — because there effectively is none. Wash the wound thoroughly with soap and running water for several minutes, apply an antiseptic, do not suture immediately, assess for post-exposure prophylaxis per national category, check tetanus status, and consider antibiotics for the bite wound itself. Note two traps: a lick on broken skin counts as an exposure, and a bite from an apparently well animal does not exclude risk.

Fever plus one finding

Classical fever patterns — continuous, remittent, intermittent — are traditionally taught and are far less useful in practice than combining fever with a single other finding. Fever with a non-blanching rash means antibiotics now. Fever with jaundice reshapes the differential entirely. Fever with confusion demands consideration of meningitis, encephalitis and cerebral malaria together. Each pairing narrows the list faster than watching a temperature chart for three days.

Prolonged fever

WHEN FEVER PERSISTS WITHOUT A DIAGNOSIS THE THREE BROAD GROUPS INFECTION — still the commonest, particularly occult MALIGNANCY — lymphoma and others INFLAMMATORY / connective tissue disease IN THIS REGION, THINK TUBERCULOSIS FIRST AND REPEATEDLY. Extrapulmonary and disseminated TB can present as prolonged fever with weight loss and no respiratory symptoms whatever, and it is the single most commonly missed cause of prolonged fever here. THEN LOOK FOR A HIDDEN COLLECTION Intra-abdominal or pelvic abscess, dental infection, infective endocarditis, osteomyelitis, empyema. The source-control principle again: an undrained collection will not resolve with antibiotics. AND DO NOT FORGET DRUG FEVER — review every medication, including recently started ones; the fever settles when the drug stops. HIV — both as a cause and as the reason an unusual infection is present. Test. THE MOST PRODUCTIVE INVESTIGATION IS USUALLY REPEATING THE HISTORY AND EXAMINATION. New findings appear over time, patients remember exposures they did not mention, and a physical sign missed on day one is frequently obvious on day four. More diagnoses are made this way than by adding another blood test to the list.

Clinical reasoning: four presentations

🔍 Case 1 — fever after the floods

PresentationA farmer presents during the monsoon with abrupt fever, severe calf pain and headache. His eyes are red without discharge. He has been working in flooded fields. He is diagnosed with a viral illness.

Key cluesFlood water exposure, severe myalgia, conjunctival suffusion.

ReasoningThis combination suggests leptospirosis, acquired from water contaminated with animal urine. Severe disease progresses to jaundice, renal failure and bleeding, so early recognition matters.

AnswerTreat per national guidance while investigating, monitor renal and hepatic function, and exclude malaria and dengue, which share the setting and season.

🔍 Case 2 — the scab nobody found

PresentationA woman has a week of fever, headache and myalgia after working in scrubland. Examination is documented as unremarkable, and no source is found. She was examined with her clothes on.

Finding missedAn eschar.

ReasoningScrub typhus produces a painless black eschar that characteristically hides in the axilla, groin, under a breast or at the waistband — precisely the places a partial examination misses.

AnswerUndress and examine the whole skin, including skinfolds, in any unexplained fever. If an eschar is found, treat per national guidance.

🔍 Case 3 — the dog was fine

PresentationA child is bitten on the hand by a neighbour's dog. The wound is cleaned briefly with an antiseptic wipe and sutured. No prophylaxis is arranged because the dog appears healthy and is known to the family.

Three errorsInadequate washing, immediate suturing, and no risk assessment.

ReasoningThorough washing with soap and running water for several minutes substantially reduces risk and is the single most valuable immediate act. An apparently well animal does not exclude rabies, and immediate suturing is avoided.

AnswerWash thoroughly, apply antiseptic, avoid immediate closure, assess exposure category and arrange post-exposure prophylaxis per the national rabies programme, and check tetanus status.

🔍 Case 4 — six weeks of fever

PresentationA man has six weeks of fever, night sweats and weight loss. Chest X-ray is normal and he has no cough. Multiple cultures are negative. Broad-spectrum antibiotics have been given twice without benefit.

Leading considerationExtrapulmonary or disseminated tuberculosis.

ReasoningTB can present as prolonged fever with weight loss and no respiratory symptoms at all, and a normal chest film does not exclude extrapulmonary disease. It is the most commonly missed cause of prolonged fever in this region.

AnswerInvestigate for TB per national guidance including extrapulmonary sites, test for HIV, look for an occult collection, review every drug for drug fever, and re-take the history and examination rather than adding further blind antibiotics.

Commonly confused

Confusion	The distinction	Why it matters
Fever pattern vs fever plus a finding	Combinations narrow the list faster	Charts rarely make the diagnosis.
Examining vs undressing the patient	An eschar hides in skinfolds	The diagnosis is on the skin.
Red eyes vs conjunctivitis	Suffusion has no discharge	Points towards leptospirosis.
Healthy-looking dog vs no risk	Appearance does not exclude rabies	Prophylaxis is assessed regardless.
Antiseptic wipe vs thorough washing	Soap and running water for minutes	The single most effective immediate act.
Normal chest X-ray vs no TB	Extrapulmonary TB may spare the lungs	The commonest missed prolonged fever.
More tests vs re-examining	New findings appear over time	Repetition outperforms escalation.
Persistent fever vs drug fever	Review every medication	It settles when the drug stops.

Rapid revision

MUST-KNOW FACTS 1. Decide FIRST whether the patient is sick NOW — resuscitate before diagnosing. 2. In any fever from an endemic area, EXCLUDE MALARIA FIRST. 3. Ask WHERE they have been and HOW LONG AGO. 4. Ask WHAT THEY WERE EXPOSED TO — animals, water, food, bites, work. 5. EXAMINE THE WHOLE SKIN, undressed. 6. LEPTOSPIROSIS: water contaminated with ANIMAL URINE, often after flooding. 7. Fever, severe MYALGIA, CONJUNCTIVAL SUFFUSION (red eyes, no discharge). 8. Severe leptospirosis: jaundice, renal failure, bleeding. 9. SCRUB TYPHUS: mite bite in scrub — hunt for an ESCHAR. 10. The eschar is PAINLESS, black, and hides in skinfolds. 11. BRUCELLOSIS: unpasteurised milk or animal handling. 12. JAPANESE ENCEPHALITIS: rural rice areas, pigs — vaccine-preventable. 13. RABIES is essentially ALWAYS FATAL once symptoms begin. 14. It is almost entirely PREVENTABLE after exposure. 15. WASH THE WOUND WITH SOAP AND RUNNING WATER FOR SEVERAL MINUTES. 16. This alone substantially reduces risk. 17. Apply antiseptic; do NOT suture immediately. 18. A LICK ON BROKEN SKIN counts as an exposure. 19. An apparently WELL animal does not exclude risk. 20. Assess category and arrange prophylaxis per the NATIONAL programme. 21. Check TETANUS status after any animal bite. 22. Fever + NON-BLANCHING RASH → meningococcal — antibiotics immediately. 23. Fever + JAUNDICE → malaria, leptospirosis, hepatitis, cholangitis, sepsis. 24. Fever + BLEEDING or low platelets → dengue, leptospirosis, sepsis. 25. Fever + CONFUSION → meningitis, encephalitis, cerebral malaria. 26. Fever + ESCHAR → scrub typhus and related infections. 27. PROLONGED FEVER: infection, malignancy, inflammatory disease. 28. In this region TUBERCULOSIS leads the list. 29. Extrapulmonary TB may have NO respiratory symptoms and a normal X-ray. 30. Look for a HIDDEN COLLECTION — it will not resolve on antibiotics. 31. Consider DRUG FEVER — review every medication. 32. TEST FOR HIV in prolonged or unusual fever. 33. RE-TAKE THE HISTORY AND RE-EXAMINE — repeatedly.

💡 Exam angle: three habits carry this chapter. Ask about exposure before ordering tests. Undress the patient and look for an eschar. And in prolonged fever, think tuberculosis, think hidden collection, think drug fever — then go back and examine again.

Syllabus points

Deciding whether the patient is sick now
Geography, exposure and examination
Excluding malaria first
Leptospirosis and flood exposure
Conjunctival suffusion and myalgia
Scrub typhus and hunting for the eschar
Brucellosis, Japanese encephalitis and anthrax
Why rabies is always fatal once symptomatic
Wound washing as the critical first act
Assessing exposure and prophylaxis
Fever plus one finding
Prolonged fever and the three groups
Why tuberculosis leads the list here
Hidden collections and drug fever

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