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Enteric Fever (Typhoid Fever)

Definition

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Etiology & Microbiology

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Epidemiology

• 9.2–21 million cases of typhoid fever and 5 million cases of paratyphoid fever annually; 110,000–280,000 deaths/year
• Highest burden: Indian subcontinent (India, Pakistan, Bangladesh, Nepal), Eastern Mediterranean, Africa — >1000 cases/100,000 children in some urban areas
• Risk factors: contaminated drinking water, flooding, street food, raw vegetables grown in sewage-fertilized fields, lack of hand washing, H. pylori infection (reduced gastric acidity)
• In the US: ~5700 cases/year; 78% travel-associated, predominantly from South Asia

Drug Resistance — Critical Update

• Multidrug-resistant (MDR) strains emerged in the 1980s: resistance to chloramphenicol, ampicillin, and trimethoprim (plasmid-mediated)
• Decreased susceptibility to ciprofloxacin (DSC) and full fluoroquinolone-resistant strains (clone H58) spread from the Indian subcontinent to Africa
• Extensively drug-resistant (XDR) S. Typhi emerged in Sindh, Pakistan in 2016 — resistant to MDR antibiotics plus fluoroquinolones and third-generation cephalosporins; susceptible only to azithromycin and carbapenems

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Pathogenesis & Morphology

1. Ingestion → gastric acid resistance → invasion of small intestinal M cells
2. Uptake by macrophages → spread to mesenteric lymph nodes → bacteremia
3. Systemic dissemination: liver, spleen, bone marrow, gallbladder

• Peyer's patches in terminal ileum enlarge into sharply delineated, plateau-like elevations up to 8 cm
• Mesenteric lymph nodes enlarged
• Oval ulcers oriented longitudinally along ileal axis → risk of perforation
• Spleen: enlarged, soft, pale red pulp; prominent phagocyte hyperplasia
• Liver: small foci of parenchymal necrosis — "typhoid nodules" (macrophage aggregates replacing hepatocytes); also found in bone marrow and lymph nodes

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Clinical Course

Week-by-week progression (untreated):

Key clinical signs:

• Relative bradycardia (Faget's sign): pulse inappropriately slow for the degree of fever — seen in <50%
• Rose spots: faint, salmon-colored, blanching maculopapular rash on trunk and chest; present in ~30% of patients; may show 2–3 crops; bacteria can be cultured from punch biopsies; difficult to detect in dark-skinned patients

• Hepatosplenomegaly: ~50% of patients
• Coated tongue: 51–56%

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Complications (~27% of hospitalized patients)

• Intestinal hemorrhage (~6%) — from necrosis of ileocecal Peyer's patches
• Intestinal perforation (~1%) → polymicrobial peritonitis, requires emergency surgery

• Meningitis, Guillain-Barré syndrome, neuritis, neuropsychiatric symptoms ("muttering delirium," "coma vigil")

• Hemophagocytic syndrome, DIC, pancreatitis, hepatitis, endocarditis, myocarditis, orchitis, glomerulonephritis, pneumonia, osteomyelitis, parotitis

• 2–5% of untreated patients develop chronic carriage (>1 year excretion in urine or stool)
• More common in women, infants, persons with biliary abnormalities or S. haematobium co-infection
• S. Typhi survives in gallbladder by forming biofilms on gallstones
• Associated with increased risk of gallbladder carcinoma
• Relapse: occurs in up to 10%, usually 2–3 weeks after fever resolution

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Diagnosis

• Widal test (agglutination of S. Typhi H and O antigens): Poor sensitivity and specificity; false positives from prior vaccination, other infections; not reliable in endemic areas — should not be used as the sole basis for diagnosis
• Modern rapid tests (Typhidot, TPTest) have variable performance
• PCR assays: increasingly used in research/reference settings

• Leukopenia and neutropenia (15–25% of cases)
• Leukocytosis more common in children, first 10 days, or with perforation/secondary infection
• Moderately elevated liver enzymes

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Treatment

Antibiotic Selection (guided by resistance pattern):

⚠️ Recent evidence (2024): A systematic review (PMID 39623850) confirms oral azithromycin is clinically effective vs. other antimicrobials for typhoid across all age groups — relevant given the XDR strain emergence in South Asia.

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Prevention

Vaccines:

General measures:

• Safe water and sanitation
• Hand hygiene
• Food safety (avoid raw fruits/vegetables, street food, ice in endemic areas)
• Identification and treatment of chronic carriers

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Summary

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Enteric Fever — Comprehensive Review

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1. Definition & Nomenclature

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2. Microbiology

• Full name: S. enterica subsp. enterica serovar Typhi = Salmonella Typhi
• Gram-negative, non-spore-forming, facultative anaerobic bacillus
• Humans are the ONLY natural reservoir — no animal reservoir
• Vi (virulence) capsular polysaccharide antigen is S. Typhi's most important virulence factor — unique to this serovar; protects against complement-mediated killing and phagocytosis; basis of Vi polysaccharide vaccines
• Possesses H (flagellar) and O (somatic) antigens — basis of Widal test

• S. Paratyphi A → most common, milder disease, no animal reservoir
• S. Schotmuelleri (formerly S. Paratyphi B)
• S. Hirschfeldii (formerly S. Paratyphi C)

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3. Epidemiology

• Fecally contaminated drinking water or ice
• Street food/beverages; raw fruits and vegetables grown in sewage-fertilized fields
• Flooding; lack of hand washing and toilet access
• Ill household contacts
• H. pylori infection (reduced gastric acid → lower infectious dose needed)
• Visiting friends and family in endemic regions

• Fecal-oral (food/water contamination by carriers or active cases)
• Sexual transmission (male-to-male — described)
• Healthcare worker occupational exposure (laboratory specimens)

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4. Pathogenesis

Step-by-step mechanism:

Ingestion of contaminated food/water
         ↓
Gastric acid barrier overcome (Vi antigen helps resistance)
         ↓
Attachment to M cells overlying Peyer's patches (terminal ileum)
         ↓
Transcytosis across epithelium → taken up by subepithelial macrophages
         ↓
Intracellular survival in macrophages (resist phagolysosomal killing)
         ↓
Transport via lymphatics to mesenteric lymph nodes
         ↓
PRIMARY BACTEREMIA (clinically silent, ~Week 1)
         ↓
Dissemination to liver, spleen, bone marrow, gallbladder
         ↓
Replication in reticuloendothelial system (RES)
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SECONDARY BACTEREMIA (symptomatic — fever, clinical illness)
         ↓
Gallbladder colonization → reinfection of intestinal Peyer's patches
         ↓
Weeks 3–4: Hyperplasia → ulceration → NECROSIS of Peyer's patches
         ↓
Intestinal hemorrhage or PERFORATION

• Vi antigen (polysaccharide capsule): Inhibits complement activation and phagocytosis; unique to S. Typhi; confers resistance to serum killing
• Type III secretion systems (T3SS): Two distinct systems (SPI-1 and SPI-2) mediate invasion of epithelial cells and intracellular survival in macrophages respectively
• Intracellular survival: S. Typhi replicates within a Salmonella-containing vacuole (SCV), evading lysosomal fusion

• Peyer's patches: Enlarge to plateau-like elevations up to 8 cm → neutrophilic infiltrate + macrophages containing bacteria, red cells, nuclear debris → oval ulcers oriented longitudinally along the ileal axis → perforation risk
• Mesenteric lymph nodes: Enlarged
• Spleen: Enlarged, soft, pale red pulp, obliterated follicular markings, prominent phagocyte hyperplasia
• Liver: Small, randomly scattered foci of parenchymal necrosis — "typhoid nodules" (macrophage aggregates replacing hepatocytes); also in bone marrow and lymph nodes

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5. Clinical Course

Week-by-Week Progression (Untreated)

Symptoms by Frequency (Prospective study, Kathmandu)

Hallmark Clinical Signs

• Sustained, 38.8–40.5°C (101.8–104.9°F)
• Documented at presentation in >75% of cases
• Can last 4 weeks untreated

• Pulse inappropriately slow relative to fever height
• Present in <50% of patients
• Mechanism: direct effect of bacteremia/endotoxemia on cardiac conduction

• Faint, salmon-colored, blanching maculopapular lesions, 1–4 mm
• Located primarily on trunk and chest
• Present in ~30% of patients (end of Week 1 – Week 2)
• May show 2–3 crops; resolve without scarring in 2–5 days
• Salmonella can be cultured from punch biopsy
• Difficult to detect in dark-skinned patients

• ~50% of patients
• Splenomegaly develops by Week 2

• Apathy, confusion, delirium ("muttering delirium"), "coma vigil" (patient appears unconscious but eyes remain open with picking at bedclothes/imaginary objects)
• Neuropsychiatric manifestations in 2–40% of patients

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6. Complications

Major Life-Threatening Complications

Other Complications

⚠️ HLH as a complication: A 2024 systematic review (PMID 38579699) characterizes enteric fever-associated HLH — a rare but life-threatening complication with fever, cytopenia, splenomegaly, and hyperferritinemia. Early recognition is critical.

Relapse

• Up to 10% of patients; typically 2–3 weeks after fever resolution
• Same organism and susceptibility profile
• Usually milder than primary episode
• Relapse rates lower with azithromycin than with fluoroquinolones or ceftriaxone

Chronic Carrier State

• 2–5% of untreated patients → chronic carriage (>1 year excretion in stool or urine)
• Gallbladder is the reservoir (biofilm formation on gallstones; invasion of gallbladder epithelium)
• Up to 10% excrete S. Typhi in feces for up to 3 months after acute illness
• Risk factors for chronic carriage: Women, infants, biliary abnormalities, Schistosoma haematobium co-infection
• Associated with gallbladder carcinoma (much higher incidence where S. Typhi is endemic)

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7. Diagnosis

Gold Standard: Culture

Non-Specific Laboratory Findings

• Leukopenia and neutropenia (15–25%) — contrasts with expected leukocytosis in sepsis
• Leukocytosis — more common in children, first 10 days, or with perforation/secondary infection
• Moderately elevated liver enzymes (ALT, AST)
• Mild thrombocytopenia
• Elevated CRP/ESR (nonspecific)
• Anemia (from GI losses or hemolysis)

Serological Tests

• Detects antibodies to S. Typhi H (flagellar) and O (somatic) antigens
• Diagnostic titer: O agglutinin ≥1:160 or H agglutinin ≥1:160 (varies by region and baseline titers)
• Problems:
  • Cross-reaction with other Salmonella strains → false positives
  • False negatives if collected too early (first week)
  • High background titers in endemic populations reduce specificity
  • Prior vaccination raises H titers → false positive
  • No standardization between laboratories
• Conclusion: Widal test alone is unreliable and should NOT be the sole basis for diagnosis. Culture remains essential.

• Typhidot (IgM/IgG against 50-kDa outer membrane protein): Better sensitivity in early disease
• TPTest, Tubex: Detect anti-O9 IgM
• Variable and suboptimal performance in endemic areas
• PCR assays: Increasing use in reference laboratories; not widely available

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8. Differential Diagnosis

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9. Treatment

Antibiotic Therapy (Harrison's 22E, Table 171-1)

⚠️ Fluoroquinolone caution: Due to high prevalence of decreased susceptibility to ciprofloxacin (DSC; MIC ≥0.125 µg/mL) on the Indian subcontinent and parts of Africa, fluoroquinolones should no longer be used as empirical first-line therapy for travel-acquired enteric fever from these regions. Use ceftriaxone or azithromycin empirically.

⚠️ XDR typhoid (Pakistan, 2016–present): Resistant to chloramphenicol, ampicillin, TMP-SMX, fluoroquinolones, AND 3rd-generation cephalosporins. Only susceptible to azithromycin (uncomplicated) and carbapenems (severe/complicated). >5000 cases documented; exported to UK, US, and multiple countries.

📌 2024 Evidence (PMID 39623850): Systematic review of RCTs confirms oral azithromycin is clinically equivalent to other antimicrobials for typhoid fever across all age groups, validating its central role especially for XDR typhoid.

Adjunctive Therapy

• For severe disease with delirium, obtundation, stupor, coma, or shock
• Regimen: IV dexamethasone 3 mg/kg loading dose, then 1 mg/kg q6h × 48 hours
• Shown to reduce mortality in severe typhoid (landmark RCT, Hoffman et al., NEJM)
• Should be reserved for critically ill patients only

• IV fluid resuscitation and electrolyte correction
• Blood transfusion if significant GI hemorrhage
• NG tube decompression for ileus

• Emergency laparotomy: bowel resection/repair + peritoneal lavage
• Broadened antibiotics to cover polymicrobial peritonitis

Chronic Carrier Treatment

• Oral fluoroquinolone (ciprofloxacin or norfloxacin) × 4–6 weeks — effective because fluoroquinolones are highly concentrated in bile
• Alternative: High-dose parenteral ampicillin (if susceptible and fluoroquinolone not tolerated)
• Cholecystectomy if antibiotic therapy fails (especially with gallstones serving as biofilm substrate), followed by another antibiotic course

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10. Prevention

Vaccines

• Unconjugated vaccines (Ty21a, Vi CPS) are poorly immunogenic in children <2 years — cannot elicit T cell-dependent memory
• TCV (conjugate vaccines) are effective from 6 months, elicit T-cell dependent immunity, and are WHO-recommended for routine childhood immunization in high-incidence countries
• Pakistan introduced TCV nationally in November 2019 — first country to do so
• No licensed vaccine exists for paratyphoid fever
• Vaccine protective efficacy can be overcome by high inocula (e.g., large food-borne exposures) — vaccines are an adjunct, not a substitute for food/water precautions

Travel Recommendations

• Vaccination recommended for travel to Southern Asia, Africa, the Caribbean, Central and South America
• Even short travel (<2 weeks) to high-risk areas warrants vaccination
• Laboratory workers exposed to S. Typhi should be vaccinated
• Household contacts of known carriers should be vaccinated

Public Health Measures

• Safe drinking water and sanitation (WASH — Water, Sanitation, Hygiene)
• Food handler screening and exclusion during illness
• Notifiable disease in the US — reporting enables identification of source patients and chronic carriers
• Contact precautions during hospitalization for diapered/incontinent patients (stool precautions until 3 consecutive negative stool cultures post-antibiotics)
• XDR typhoid → full contact precautions as for MDR organisms

WASH Evidence

📌 2023 Meta-analysis (PMID 37644449): Water, sanitation, and hygiene interventions significantly reduce typhoid fever risk, confirming WASH as a cornerstone of prevention alongside vaccination.

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11. Drug Resistance Timeline

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12. Summary: High-Yield Points

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