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MI3.5-8 | Enteric Fever, PUO & Sepsis — SDL Guide

Learning Objectives

Define pyrexia of unknown origin (PUO) and classify its infective causes with appropriate diagnostic modalities
Classify enteric fever pathogens and describe the clinical course, pathogenesis, complications, and laboratory diagnosis of enteric fever
Select the most appropriate laboratory test for enteric fever based on the duration of illness and carrier status
Read and interpret serological test results (Widal test) in suspected enteric fever, noting their limitations

INSTRUCTIONS

Typhoid fever remains one of the top five causes of fever in Indian hospitals. PUO is a clinical puzzle where microbiology holds the key. This module follows the patient's journey — from Peyer's patches to bacteraemia to the Widal tube — equipping you with the systematic approach needed to investigate fever without an obvious source.

References

Ananthanarayan & Paniker's Textbook of Microbiology, Ch 28 (Salmonella) (textbook)
Harrison's Principles of Internal Medicine — Enteric (Typhoid) Fever, Fever of Unknown Origin (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 22-year-old college student from Puducherry presents with 10 days of continuous fever (38.8°C), headache, constipation, and a mild cough. On Day 8, his doctor noticed the fever was 'step-ladder' in pattern and ordered a Widal test which returned: O antigen 1:160, H antigen 1:80. He was labelled as 'enteric fever' and started on ciprofloxacin.

On Day 14, he returns — worse. Blood cultures (taken on Day 12) now flag positive after 48 hours: Salmonella Typhi, MDR, resistant to fluoroquinolones.

Two questions:
1. Was the Widal result truly diagnostic?
2. Why was the blood culture more useful than the Widal, and what was the ideal timing for each test?

By the end of this session you will know precisely which test to order on which day of enteric fever — and why.

WHY THIS MATTERS

India has an estimated 4–6 million enteric fever cases annually. Typhoid kills approximately 40,000 Indians per year — almost all preventable deaths. The emergence of Multi-Drug Resistant (MDR) and extensively drug resistant (XDR) Salmonella Typhi has made correct laboratory diagnosis critical — you cannot guess the antibiotic. PUO is the presenting diagnosis in up to 20% of inpatient fevers in Indian hospitals; a systematic microbiological approach catches the treatable causes before organ failure.

RECALL

Before proceeding, recall:
- Salmonella is a Gram-negative, facultative intracellular organism belonging to the family Enterobacteriaceae
- O antigens = somatic (lipopolysaccharide, LPS) cell wall antigens; H antigens = flagellar; Vi antigen = capsular (virulence)
- Peyer's patches — lymphoid aggregates in the terminal ileum
- The Widal test is an agglutination test (tube or slide) — if you are unclear on how agglutination reactions work, quickly revise before continuing

Pyrexia of Unknown Origin (PUO): Definition & Causes

Three-panel educational diagram defining PUO, classifying infective causes, and showing a microbiology-led diagnostic workflow including enteric fever testing by illness duration. — **Panel A:** Patient with fever, thermometer >= 38.3°C, duration >= 3 weeks, no diagnosis after evaluation, blood culture, chest X-ray, abdominal ultrasound. **Panel B:** Bacterial infections, tuberculosis, enteric fever, brucellosis, infective endocarditis, malaria, viral infections, fungal infections, deep abscess. **Panel C:** History and examination, CBC and inflammatory markers, blood cultures, urine culture, malaria smear or rapid test, serology, imaging-guided aspirate or biopsy, targeted diagnosis, week 1 blood culture for enteric fever, week 2 onwards Widal or serology, stool culture for carrier state.

PUO was defined by Petersdorf & Beeson (1961) as:
- Fever >38.3°C (101°F) on several occasions
- Duration >3 weeks
- No diagnosis after 1 week of thorough inpatient investigation

Modified Durack-Street classification (1991) adds sub-types: classic PUO (the original), nosocomial PUO, neutropenic PUO, and HIV-associated PUO.

Infective causes of PUO (the largest category — 30–40% in India):

Category	Examples
Bacterial	Enteric fever, tuberculosis (commonest in India), brucellosis, infective endocarditis, liver abscess, osteomyelitis
Viral	EBV/CMV (infectious mononucleosis), HIV seroconversion
Parasitic	Malaria (most common infective PUO in endemic areas), kala-azar (visceral leishmaniasis)
Fungal	Histoplasmosis, disseminated candidiasis (immunocompromised)

Diagnostic approach to infective PUO:
1. Detailed history — travel, animal exposure, sexual history, immunisation status, drug history
2. Complete examination — lymph nodes, spleen, fundus, murmurs
3. Repeated blood cultures (3–6 sets) — gold standard for bacterial PUO
4. Serology panel — based on epidemiology: Widal, Weil-Felix, Brucella agglutination, Paul-Bunnell, HIV ELISA
5. Bone marrow biopsy and culture — highest yield in enteric fever (90%), TB, kala-azar — valuable when blood cultures negative
6. Imaging — ultrasound (liver abscess, lymphadenopathy), CT, PET-CT for occult neoplasm vs infection
7. Molecular diagnostics — 16S rRNA PCR, Xpert MTB/RIF

Enteric Fever: Pathogens & Classification

Four-panel diagram classifying enteric fever pathogens and showing Salmonella morphology, intestinal invasion through Peyer's patches, bacteraemia, and clinical disease categories. — **Panel A:** Enteric fever pathogens, Typhoidal Salmonella, Salmonella enterica serovar Typhi, Paratyphoidal Salmonella, S. Paratyphi A, S. Paratyphi B, S. Paratyphi C, human reservoir, faeco-oral transmission. **Panel B:** Gram-negative bacillus, peritrichous flagella, O antigen: somatic LPS, H antigen: flagellar, Vi antigen: capsule-like virulence antigen in S. Typhi. **Panel C:** Contaminated food or water, terminal ileum lumen, M cell uptake, Peyer's patch, macrophage survival, lymphatic spread, primary bacteraemia, reticuloendothelial spread. **Panel D:** Typhoid fever caused by S. Typhi, paratyphoid fever caused by S. Paratyphi A/B/C, carrier state, gallbladder colonization, faecal shedding, systemic febrile illness.

Enteric fever = typhoid + paratyphoid fever.

Causative organisms:

Species	Biovar	Serovar	Disease
Salmonella enterica	Typhi	Typhi	Typhoid fever (most severe)
Salmonella enterica	Paratyphi A	Paratyphi A	Paratyphoid A (milder; common in India)
Salmonella enterica	Paratyphi B	Schottmuelleri	Paratyphoid B
Salmonella enterica	Paratyphi C	Hirschfeldii	Paratyphoid C (rare)

Salmonella Typhi characteristics:
- Gram-negative, motile (peritrichous flagella), non-sporing, non-capsulate (except Vi antigen — a polysaccharide virulence capsule that helps evade phagocytosis)
- Kauffmann-White scheme — serotyping based on O, H, Vi antigens:
- O antigen: group D (1, 9, 12)
- H antigen: phase 1 'd'
- Vi antigen: present (marker of virulence; lost in Vi-negative mutants)
- Reservoir: Exclusively humans (carrier state — chronic biliary carrier)
- Transmission: Faeco-oral route; contaminated water/food — the 5 Fs (Food, Fingers, Flies, Faeces, Fomites)

Drug resistance pattern (important for India):
- MDR S. Typhi: resistant to chloramphenicol, ampicillin, co-trimoxazole (all first-line agents)
- Fluoroquinolone resistance: nalidixic acid-resistant S. Typhi (NARST) — reduced susceptibility to ciprofloxacin
- XDR S. Typhi (Pakistan outbreak 2016–2019): also resistant to 3rd-gen cephalosporins + fluoroquinolones; only azithromycin and carbapenems remain
- Antibiotic of choice for susceptible strains: Fluoroquinolones; for MDR: azithromycin or 3rd-gen cephalosporins (ceftriaxone)

Pathogenesis & Clinical Course of Enteric Fever

A four-panel medical diagram showing how enteric fever spreads from intestinal Peyer's patches to bacteraemia, systemic organs, clinical stages, and duration-based laboratory diagnosis. — **Panel A:** Contaminated food or water, Salmonella Typhi/Paratyphi, stomach, terminal ileum, Peyer's patches, mesenteric lymph nodes, primary bacteraemia, liver, spleen, bone marrow, gallbladder, secondary bacteraemia. **Panel B:** Ileal lumen, M cell, Peyer's patch, macrophage, intracellular Salmonella survival, lymphatic vessel, cytokine-mediated inflammation. **Panel C:** Incubation period, week 1 step-ladder fever, week 2 sustained fever, relative bradycardia, abdominal pain, rose spots, hepatosplenomegaly, week 3 intestinal hemorrhage, ileal perforation, recovery or complications. **Panel D:** Blood culture week 1, bone marrow culture highest yield, Widal test after day 7, stool culture from week 2 onward, urine culture later illness, stool culture for chronic carrier state, gallbladder carrier reservoir.

Pathogenesis — step by step:

Ingestion — infective dose 10⁵–10⁷ organisms (lower for S. Typhi in contaminated water)
Ileal invasion — Salmonella crosses the intestinal epithelium via M-cells overlying Peyer's patches, enters the lamina propria
Intracellular survival — engulfed by macrophages in Peyer's patches; survives via Salmonella Pathogenicity Islands (SPI-1, SPI-2) — suppresses phagosome-lysosome fusion
Primary bacteraemia — organisms reach mesenteric lymph nodes → thoracic duct → bloodstream (end of incubation period, 1–2 weeks)
Systemic seeding — liver, spleen, bone marrow, gallbladder (gallbladder = reservoir for chronic carrier state)
Secondary bacteraemia — organisms re-enter bloodstream from liver/spleen (Week 2–3 of illness); correlates with peak clinical fever
Re-invasion of Peyer's patches — hypersensitivity reaction in sensitised lymphoid tissue → necrosis, ulceration → intestinal perforation (Week 3–4)

Clinical stages:

Week	Stage	Features
1	Incubation/early	Step-ladder fever, headache, constipation, dry cough, relative bradycardia
2	Bacteraemic	High fever (39–40°C), rose spots (faint pink macules on trunk — pathognomonic), splenomegaly, hepatomegaly
3	Complication	Intestinal ulcers at risk of perforation and haemorrhage; fever starts to decline
4	Resolution	Defervescence; risk of relapse if undertreated

Complications:
- Intestinal: Perforation (most feared, 1–3%), haemorrhage
- Systemic: Typhoid hepatitis, myocarditis, encephalopathy, DIC
- Carrier state: Chronic biliary carrier (usually female; chronic cholecystitis) — excretes organisms in bile/faeces for >1 year

A multi-panel diagram shows Salmonella Typhi entering Peyer's patches through M cells, surviving inside macrophages, spreading through primary and secondary bacteraemia, and seeding the gallbladder for faecal shedding. — **Panel A:** Ileal lumen, mucus layer, Salmonella Typhi, intestinal epithelium, M cell, Peyer's patch, follicle-associated epithelium.. **Panel B:** M-cell transcytosis, lamina propria, dendritic cell, macrophage uptake, lymphoid follicle, bacterial invasion arrows.. **Panel C:** Macrophage, Salmonella-containing vacuole, intracellular multiplication, inhibited phagolysosome killing, viable intracellular Salmonella Typhi.. **Panel D:** Mesenteric lymph node, lymphatics, primary bacteraemia, bloodstream, liver, spleen, bone marrow, reticuloendothelial multiplication, secondary bacteraemia, fever callout.. **Panel E:** Liver, bile ducts, gallbladder, bile, gallstones, chronic carrier state, intestine, faecal shedding, reinfection cycle arrow..

CLINICAL PEARL

Relative bradycardia (Faget's sign): In most fevers, heart rate rises ~10 bpm per degree Celsius. In typhoid fever, the heart rate paradoxically does NOT rise proportionately with the temperature — this relative bradycardia is caused by S. Typhi endotoxin acting on the cardiac conduction system. It is a classic bedside clue, though not pathognomonic (also seen in brucellosis, Legionella, dengue).