Typhoid & Enteric Fever

Enteric fever is a systemic febrile illness caused by Salmonella serotypes Typhi and Paratyphi, transmitted by the faecal-oral route. For Community Medicine it is a classic "common-source water-borne" disease — high-yield for Widal interpretation, the chronic carrier concept, vaccine comparison, and outbreak investigation.

Definition & Classification

Enteric fever is the umbrella term covering:

• Typhoid fever — caused by Salmonella enterica serotype Typhi (S. Typhi).
• Paratyphoid fever — caused by S. Paratyphi A, B, C (milder, shorter course; A is commonest in India).

These are strictly human-restricted pathogens (no animal reservoir), which is epidemiologically critical: eradication is theoretically possible because man is the only reservoir (cases + carriers).

Feature	Typhoid (S. Typhi)	Paratyphoid (S. Paratyphi)
Reservoir	Man only	Man only
Severity	More severe, longer	Milder, shorter
Antigen for Vi vaccine	Vi capsular antigen present	Paratyphi A lacks Vi → not covered
Commonest in India	—	Paratyphi A

High-yield: Man is the only reservoir for S. Typhi — there is no animal or environmental reservoir. This makes the carrier the linchpin of community transmission.

Agent, Reservoir & Source

• Agent: S. Typhi — Gram-negative, motile, non-lactose-fermenting bacillus. Antigens: O (somatic, heat-stable LPS), H (flagellar, heat-labile), Vi (virulence/capsular — inhibits phagocytosis, basis of carrier detection and Vi vaccine).
• Reservoir of infection: cases and carriers.
• Source: faeces and urine of cases/carriers; secondary vehicles are contaminated water, food, milk, and flies.
• Infective dose: large (~10⁵–10⁹ organisms) — hence usually a water-borne (low dose, many exposed → propagated) or food/milk-borne (high dose → explosive) pattern.

The carrier state (very high-yield)

Carrier type	Definition	Notes
Incubatory carrier	Sheds organism during incubation period	Before symptoms
Convalescent carrier	Sheds during recovery	Up to 6–8 weeks
Temporary carrier	Excretes < 1 year	Most resolve
Chronic carrier	Excretes for > 1 year	Definition cut-off

• Chronic carrier = person excreting S. Typhi for more than 1 year after acute illness (or with no history of illness).
• Commonest site = gall bladder (urinary carriers in Schistosoma co-infection).
• More common in women and in those with gall stones / chronic cholecystitis.
• Mary Mallon ("Typhoid Mary") — classic eponymous chronic carrier (cook) who caused multiple outbreaks.

High-yield: Chronic carrier cut-off is >1 year of faecal/urinary excretion; gall bladder is the commonest reservoir; women > men.

Pathophysiology (brief, exam-relevant)

Ingestion → gastric acid barrier (large dose needed) → small-intestinal mucosa → invade Peyer's patches → mesenteric nodes → primary bacteraemia (asymptomatic, incubation) → multiply in reticuloendothelial system (liver, spleen, bone marrow) → secondary bacteraemia (symptomatic, end of incubation) → re-seeding of gall bladder & gut → faecal shedding.

• Incubation period: usually 10–14 days (range 3–21 days), dose-dependent.
• Peyer's patch hyperplasia → necrosis → ulceration (longitudinal ulcers) → risk of haemorrhage / perforation in the 3rd week.

Clinical Features

• Week 1: stepladder rise of fever, malaise, headache, relative bradycardia (Faget sign), constipation (adults) or diarrhoea (children).
• Week 2: sustained high fever, rose spots (blanching pink macules on trunk), hepatosplenomegaly, coated tongue, abdominal distension, toxaemia ("typhoid state").
• Week 3: complications — intestinal perforation, haemorrhage, deterioration.
• Week 4: convalescence (if untreated and survives).

High-yield: Relative bradycardia (Faget sign) and rose spots are the classic typhoid clinical pearls.

Diagnosis & Investigation of Choice

Diagnostic yield shifts with the week of illness:

Week of illness	Best sample / test	Sensitivity comment
Week 1	Blood culture	Investigation of choice; positive ~80% in week 1
Week 1–2	Bone marrow culture	Most sensitive (~90–95%), unaffected by prior antibiotics
Week 2–3	Widal test (antibody)	Rises 2nd week onward
Week 2–4	Stool & urine culture	Positive later; carrier detection

• Investigation of choice (acute): Blood culture — gold-standard confirmation, highest early yield.
• Most sensitive: Bone marrow culture (positive even after antibiotics).
• Newer: Typhidot (IgM/IgG), antigen detection — supplementary.

Widal test interpretation (exam favourite)

The Widal test measures agglutinating antibodies against O and H antigens of S. Typhi.

Significant titres (single sample, endemic area): O ≥ 1:160 and H ≥ 1:160 (cut-offs vary by region; a four-fold rise in paired sera 7–10 days apart is the most reliable evidence).

Interpreting the two antigens:

Antibody	Appears	Meaning	Persistence
O (anti-somatic)	Earlier (~end week 1)	Indicates recent/active infection	Falls in months
H (anti-flagellar)	Later	May reflect past infection, vaccination, or anamnestic recall	Persists long

Flow for a "true positive" vs "anamnestic" decision:

Rising O titre on paired sera → true active infection. Isolated high H (esp. with intercurrent unrelated fever, no O rise) → anamnestic reaction.

Anamnestic reaction (must-know)

• A transient, non-specific rise in pre-existing antibody titres (usually H more than O) triggered by an unrelated febrile illness (e.g., another infection) in a person previously exposed/vaccinated to S. Typhi.
• Distinguished from true infection because it is transient and not sustained — a second (paired) Widal shows no further rise / falling titre, whereas true infection shows a four-fold rising titre.

High-yield: A four-fold rise in titre on paired sera = true infection. An isolated raised H titre that does not rise on repeat = anamnestic reaction. The O antigen rise is the more reliable marker of active infection.

Causes of false Widal results:

• False positive: other Salmonella, malaria, other enterobacteriaceae, chronic liver disease, prior vaccination, endemic background titres.
• False negative: early disease (before antibody rise), prior antibiotics, carriers, immunocompromise, prozone phenomenon.

High-yield: Widal is not the investigation of choice — it has poor specificity. It is an antibody (indirect) test, reliable only with paired sera; blood culture remains confirmatory.

Management / Drug of Choice

• Empirical DOC (uncomplicated, in much of South Asia with quinolone resistance): Ceftriaxone (IV) or Cefixime (oral); Azithromycin for uncomplicated/MDR.
• Historically chloramphenicol was the classic DOC (now largely abandoned due to marrow toxicity and resistance).
• Fluoroquinolones (ciprofloxacin) — once first-line, now limited by widespread nalidixic-acid-resistant / DCS (decreased ciprofloxacin susceptibility) strains in India.
• XDR typhoid (Pakistan, emerging) — resistant to first-line + fluoroquinolones + third-gen cephalosporins → treat with azithromycin / carbapenems.
• Carrier treatment: prolonged ciprofloxacin (or amoxicillin); cholecystectomy if gallstones and persistent carriage.

High-yield: With rising cephalosporin resistance (XDR strains), azithromycin is increasingly important; chronic carriers with gallstones may need cholecystectomy.

Complications

• Intestinal: haemorrhage and perforation (3rd week, ileal Peyer's patches) — most feared.
• Typhoid state / encephalopathy, myocarditis.
• Relapse (~5–10%), chronic carrier state.
• Metastatic foci: osteomyelitis (notably in sickle cell disease — Salmonella osteomyelitis), abscesses, cholecystitis.

Prevention & Control (Community Medicine core)

Three pillars: (1) control of reservoir, (2) control of transmission (sanitation/water), (3) protection of susceptibles (vaccine).

• Sanitation & safe water = single most important long-term measure (the "F-diagram": Fluids, Fingers, Flies, Fomites, Food).
• Identify and treat carriers, especially food handlers.
• Health education, hand hygiene, safe food.
• Vaccination for high-risk groups, travellers, outbreak control.

Typhoid Vaccines (very high-yield comparison)

Vaccine	Type	Route	Age	Schedule / Booster	Duration	Key points
Vi polysaccharide (ViPS)	Subunit (Vi capsular antigen)	IM/SC	≥ 2 years	Single dose; booster every 3 years	~3 years	T-cell independent, no herd effect, poor < 2 yr
Ty21a	Live attenuated oral	Oral	≥ 5–6 years	3–4 capsules alternate days; booster ~3 yrs	~3–5 years	Avoid in immunocompromised, pregnancy; no antibiotics/antimalarials around dosing
Vi-conjugate (TCV)	Vi conjugated to carrier protein (e.g., TT/CRM197)	IM	≥ 6 months	Single dose	Long (≥ 5+ years)	T-cell dependent → memory, usable in infants, WHO-recommended for routine immunisation in endemic areas

High-yield: The Vi-conjugate vaccine (TCV) is the major advance — given as a single dose from 6 months of age, gives long-lasting protection and immunological memory (T-dependent), suitable for routine childhood immunisation. WHO prequalified (Typbar-TCV, an Indian product).

High-yield: Ty21a is the only live oral typhoid vaccine — contraindicated in immunocompromised and pregnancy; antibiotics and mefloquine/chloroquine interfere with the live organisms, so separate their timing.

Old vaccine (historical): killed whole-cell TAB (Typhoid-Paratyphoid A & B) — abandoned due to high reactogenicity.

Mnemonic — vaccine minimum ages: "Conjugate is for the Cradle (6 months), Polysaccharide for the Pre-schooler (2 yr), Oral for the Older child (≥5–6 yr)."

Outbreak Investigation — Common-Source Water Cluster

When a cluster of enteric fever appears, treat it as a probable common-source, water-borne outbreak. Stepwise approach:

1. Confirm the diagnosis (clinical + blood culture) and confirm existence of an outbreak (compare current vs expected/endemic frequency).
2. Define a case (clinical + lab criteria, time-place-person).
3. Search for cases — line listing of all cases.
4. Describe by Time, Place, Person:
   • Time → epidemic curve. A point (common) source gives a single sharp peak with cases clustered within one incubation period; a continuous common source (contaminated water supply) gives a plateau/sustained curve; propagated (person-to-person) gives successive peaks one incubation period apart.
   • Place → spot map (clustering around a particular water source/well/tank).
   • Person → attack rates by age/sex/exposure.
5. Formulate & test hypothesis — calculate attack rates in exposed vs unexposed (e.g., users vs non-users of a particular water source); a food-specific / water-specific attack rate table identifies the vehicle.
6. Sanitary survey & lab confirmation — inspect water source, test water (faecal coliforms, E. coli as indicator of faecal contamination), look for cross-connection/leak between sewage and supply.
7. Control measures — chlorinate/repair water supply, treat cases, find carriers, health education, vaccinate if needed.
8. Report & follow-up.

High-yield: The shape of the epidemic curve distinguishes a point-source (single sharp peak) from a propagated outbreak (multiple peaks ~1 incubation apart). Water-borne typhoid is typically a continuous common-source outbreak with a sustained/plateau curve and a low-grade insidious rise.

Indicator of faecal contamination of water: coliforms / E. coli (thermotolerant faecal coliforms) — water should have zero coliforms per 100 mL.

Key Differentials

Condition	Distinguishing features
Malaria	Periodic fever with chills/rigor, splenomegaly, peripheral smear/RDT positive, relative bradycardia absent
Brucellosis	Undulant fever, animal/occupational exposure, arthralgia
Dengue	Acute fever, retro-orbital pain, thrombocytopenia, leukopenia, NS1/serology
Leptospirosis	Conjunctival suffusion, myalgia (calf), jaundice, renal failure, water exposure
Tuberculosis / abdominal TB	Chronic fever, weight loss, evening rise
Amoebic liver abscess	Tender hepatomegaly, RUQ pain, ultrasound
Typhus / rickettsial fever	Eschar, rash, occupational/vector exposure

The combination of stepladder fever + relative bradycardia + rose spots + leukopenia with positive blood culture clinches typhoid over these.

Recently asked / exam angle

• Widal interpretation: Given paired sera titres, identify true infection (four-fold rise, esp. O) versus anamnestic reaction (isolated H, no rise on repeat) — perennial favourite.
• Chronic carrier definition: excretion > 1 year; commonest site gall bladder; commoner in women; "Typhoid Mary" eponym.
• Vaccine comparison: match vaccine to type, route, minimum age, T-dependence. The Vi-conjugate (TCV) from 6 months, single dose, long duration is the new high-yield point; Ty21a is the only live oral vaccine.
• Investigation of choice: blood culture (acute); bone marrow culture most sensitive even after antibiotics.
• Outbreak: identify pattern of epidemic curve for water-borne common source; coliform count as faecal-contamination indicator.
• Reservoir: man only — single best answer to "reservoir of S. Typhi."
• Resistance trend: emergence of XDR typhoid, role of azithromycin/ceftriaxone.

Rapid revision

1. S. Typhi — man is the only reservoir; faecal-oral, large infective dose → water-borne or food-borne.
2. Antigens: O (somatic), H (flagellar), Vi (capsular/virulence) — Vi is the basis of vaccine and carrier detection.
3. Incubation 10–14 days; stepladder fever, relative bradycardia (Faget sign), rose spots, splenomegaly.
4. Investigation of choice = blood culture; most sensitive = bone marrow culture (positive even post-antibiotics).
5. Widal: significant titre O & H ≥ 1:160; a four-fold rise in paired sera is most reliable; O rise = active infection.
6. Anamnestic reaction = transient rise (usually H) due to an unrelated fever in a previously exposed/vaccinated person — no further rise on repeat.
7. Chronic carrier = excretion > 1 year; commonest site gall bladder; women > men; Typhoid Mary.
8. DOC now: ceftriaxone / cefixime / azithromycin (fluoroquinolone resistance widespread; XDR strains emerging).
9. Most feared complication = intestinal perforation/haemorrhage in the 3rd week (Peyer's patch ulcers); Salmonella osteomyelitis in sickle cell disease.
10. Vi-conjugate (TCV): single dose, from 6 months, T-dependent, long-lasting — WHO-recommended for endemic-area routine immunisation.
11. Ty21a: only live oral vaccine, ≥ 5–6 yr, contraindicated in pregnancy/immunocompromise; keep antibiotics away from dosing.
12. Outbreak: epidemic-curve shape distinguishes point-source vs propagated; coliform/E. coli = indicator of faecal contamination of water; safe water + carrier control are the mainstays.