Adults: Doxycycline $300 \text{ mg}$ single oral dose. Alternative: Ciprofloxacin $1 \text{ g}$ single oral dose.

Children: Azithromycin $20 \text{ mg/kg}$ single oral dose. Alternative: Erythromycin.

Resistance patterns vary geographically.

Zinc supplementation: For children.

Prevention:

Source: Safe water supply, proper sanitation (safe disposal of human feces).
Mode of Transmission: Food hygiene (cooking food thoroughly, avoiding raw foods in endemic areas), handwashing.
Host: Oral Cholera Vaccines (OCV) for targeted populations or travelers, health education.

3. Typhoid Fever (Enteric Fever)

Agent: Salmonella Typhi (most common) and Salmonella Paratyphi A, B, C. Gram-negative bacillus.
Pathogenesis: Ingestion $\rightarrow$ invade Peyer's patches $\rightarrow$ regional lymph nodes $\rightarrow$ bloodstream (primary bacteremia) $\rightarrow$ reticuloendothelial system (liver, spleen, bone marrow) $\rightarrow$ secondary bacteremia $\rightarrow$ recolonization of Peyer's patches and gallbladder (carrier state).
Transmission: Feco-oral route, contaminated food/water, chronic carriers (e.g., "Typhoid Mary"). Incubation period: 7-14 days.
Clinical Features (Classic presentation, may be atypical):
- Week 1: Gradual onset of "step-ladder" fever (rises daily), headache, malaise, anorexia, relative bradycardia, constipation (more common than diarrhea initially).
- Week 2: Sustained high fever, apathy, "rose spots" (faint, salmon-colored maculopapular rash on trunk/abdomen, blanch with pressure), hepatosplenomegaly, "pea-soup" diarrhea, abdominal tenderness.
- Week 3: If untreated, complications arise. Worsening toxemia, delirium.
- Complications: Intestinal hemorrhage (most common, due to necrosis of Peyer's patches), intestinal perforation (most serious, leads to peritonitis), encephalopathy, myocarditis, cholecystitis, osteomyelitis, meningitis.
- Chronic carrier state: Excretion of bacteria for >1 year, usually from gallbladder.
Diagnosis:
- Blood culture: Gold standard. Positive in 70-80% in first week.
- Bone marrow culture: Highest sensitivity (90%), useful in partially treated cases.
- Stool culture: Positive later, useful for identifying carriers.
- Urine culture: Less sensitive.
- Widal test: Detects agglutinating antibodies (O and H antigens). Retrospective value, poor sensitivity/specificity due to prior infection, vaccination, or cross-reactivity. A four-fold rise in titer in paired sera is more significant.
- Typhidot/Typhidot M: Detects IgM/IgG antibodies to S. Typhi (IgM suggests acute infection). Rapid test.
Treatment:
- Antibiotics (key to treatment): Duration 7-14 days, depending on severity and agent.
  - Drug-Sensitive Strains: Ciprofloxacin $500 \text{ mg}$ BID OR Azithromycin $500 \text{ mg}$ once daily.
  - Multidrug-Resistant (MDR) Strains (resistant to chloramphenicol, ampicillin, cotrimoxazole):
    - Fluoroquinolones (Ciprofloxacin, Ofloxacin) if susceptible.
    - Third-generation Cephalosporins: Ceftriaxone $50-75 \text{ mg/kg/day}$ IV/IM for 10-14 days.
    - Azithromycin $1 \text{ g}$ on day 1 then $500 \text{ mg/day}$ for 6 da