TYPHOID FEVER - Agent, Incubation Period, Mode of Transmission & Control Measures

I. INTRODUCTION

II. AGENT FACTORS (Causative Organism)

• A Gram-negative, non-spore-forming, facultatively anaerobic bacillus belonging to the family Enterobacteriaceae.
• S. typhi is the major causative agent; S. paratyphi A, B, and C cause paratyphoid, which is milder.
• The organism possesses three main antigenic components:
  • O antigen (somatic, cell wall lipopolysaccharide)
  • H antigen (flagellar)
  • Vi antigen (virulence antigen - a polysaccharide capsule that inhibits phagocytosis and complement activation)
• At least 80 phage types have been identified; phage typing is a useful epidemiological tool for tracing the source of outbreaks.
• S. typhi can survive intracellularly in tissues of the liver, spleen, bone marrow, and lymph nodes - this is key to its pathogenesis and persistence.
• Resistance: It is readily killed by drying, pasteurization (60°C for 30 minutes), and common disinfectants (phenol, chlorine). However, it can survive for days to weeks in contaminated water, ice, and food, especially in cold conditions.
• The infecting dose and virulence of the organism are the key factors influencing onset of disease; as few as 10^3 - 10^7 organisms can cause infection depending on the vehicle.

• Cases (active cases): May be mild, subclinical, or severe. A case is infectious as long as bacilli appear in stools or urine.
• Carriers: These are critically important in perpetuating the disease:
  • Temporary (incubatory) carriers: Excrete organisms before onset of illness.
  • Convalescent carriers: Excrete bacilli for 6-8 weeks after illness; numbers diminish rapidly. By 3 months, only ~4% still excrete; by 1 year, ~3%.
  • Chronic carriers: Those who excrete bacilli for more than 1 year after a clinical attack. Organisms persist in the gallbladder and biliary tract (often in association with gallstones). Chronic carriage develops in 2-5% of cases and may last for several decades (50 years in extreme cases). The notorious "Typhoid Mary" (Mary Mallon) was a chronic carrier responsible for >1300 cases in her lifetime.
  • Fecal carriers are more common than urinary carriers; chronic urinary carriers often have an underlying structural abnormality of the urinary tract or concurrent Schistosoma haematobium infection.
  • Chronic carriage is more common in women, the elderly, and those with biliary abnormalities.

• Primary sources: Faeces and urine of cases or carriers.
• Secondary sources: Contaminated water, food, fingers ("the 5 Fs"), and flies.
• There is no evidence that typhoid bacilli are excreted in sputum or milk.

III. INCUBATION PERIOD

• The incubation period of typhoid fever is typically 1 to 3 weeks (7-21 days), with an average of 10-14 days.
• It can range from 3 days to 60 days depending on:
  • The inoculum size (infecting dose) - smaller doses lead to longer incubation
  • The virulence of the strain
  • The host's immune status and gastric acid level (low gastric acidity = shorter incubation)
  • The vehicle of infection (waterborne outbreaks tend toward longer incubation)
• The disease is communicable as long as the bacilli appear in excreta - typically from the first week through the convalescent period, and indefinitely in carriers.

IV. MODE OF TRANSMISSION

A. Water-Borne Transmission (MOST IMPORTANT)

• The most common and important mode of transmission globally.
• Contamination of drinking water supplies with sewage is the classic mechanism.
• Responsible for most large-scale epidemics (e.g., "explosive epidemics" with a characteristic epidemic curve peaking rapidly).
• Ice made from contaminated water, and floods contaminating water supplies, are important vehicles.
• Risk correlates directly with mixing of drinking water with human sewage.

B. Food-Borne Transmission

• The most common mode in developed countries.
• Any food can be contaminated by:
  • Carriers handling food (cooks, food handlers - e.g., Typhoid Mary)
  • Raw fruits and vegetables grown in fields fertilized with sewage ("night soil")
  • Shellfish and seafood from contaminated water
  • Milk and milk products contaminated by carriers
  • Street vendor foods
• Food-borne outbreaks tend to be smaller and more prolonged compared to waterborne outbreaks.

C. Fly-Borne Transmission

• Flies can mechanically transfer the organism from infected faeces to food.
• Important in areas with open defecation and poor sanitation.
• Flies carry organisms on their legs, body, and in their vomit.

D. Contact Transmission (Person-to-Person)

• Direct feco-oral contact - unhygienic handling
• Fomites - contaminated utensils, latrines, etc.
• Sexual transmission between male partners has been described.
• Healthcare workers can acquire infection from infected patients or clinical specimens.
• Relatively less common than water/food-borne routes.

E. Fomites

• Contaminated clothing, bedding, utensils, and other objects can serve as vehicles, especially in institutional settings.

Conditions Favoring Transmission:

• Absence of safe drinking water
• Inadequate sanitation and sewage disposal
• Open defecation practices
• Lack of handwashing facilities
• Overcrowding
• Poor food hygiene practices
• Previous H. pylori infection (associated with reduced gastric acidity, lowering the infective dose needed)

V. CONTROL MEASURES

A. MEASURES DIRECTED AT THE SOURCE (Reservoir / Cases / Carriers)

1. Case Management

• Early diagnosis and prompt treatment with appropriate antibiotics (see below) reduces the duration of infectivity and prevents spread.
• Isolation: Cases should be subject to enteric precautions - isolation of excreta (faeces and urine), disinfection of contaminated articles, and strict handwashing. Full ward isolation is not required, but barrier nursing is essential.
• Treatment options:
  • Fluoroquinolones (ciprofloxacin 500 mg BD x 10-14 days) - first line for sensitive strains
  • Third-generation cephalosporins (ceftriaxone IV, cefixime oral) for MDR and fluoroquinolone-resistant cases
  • Azithromycin (1g on day 1, then 500 mg OD x 6 days) - effective for uncomplicated cases and XDR strains
  • Chloramphenicol, ampicillin, and cotrimoxazole are unreliable due to widespread resistance (first-line in the past but no longer recommended as empiric therapy)
  • Dexamethasone is added for severe cases with meningitis, encephalitis, or shock (reduces mortality)
• Notification: Typhoid is a notifiable disease in India and most countries. Cases must be reported to public health authorities for epidemiological investigation.

2. Carrier Detection and Management

• Food handlers, water workers, milk handlers, and healthcare workers should be screened for carrier status.
• Stool and urine cultures (minimum 3 negative cultures) are required to certify freedom from carriage.
• Carriers must be excluded from handling food, water supplies, and patient care until declared free.
• Treatment of carriers:
  • Ciprofloxacin 750 mg twice daily for 4 weeks can eliminate carriage in ~80% of cases.
  • Amoxicillin + probenecid for 3 months was previously used.
  • In cases where carriage is due to gallbladder disease (cholelithiasis), cholecystectomy may be needed for eradication.
• Health education on handwashing and personal hygiene for carriers who cannot be treated.

3. Disinfection of Excreta

• Faeces, urine, and vomit from cases must be disinfected with bleaching powder, cresol, or carbolic acid before disposal.
• Soiled linen and articles should be disinfected by boiling or autoclaving.
• Terminal disinfection of the patient's room after recovery or death.

B. MEASURES DIRECTED AT THE ROUTES OF TRANSMISSION (Environmental)

1. Safe Water Supply

• Provision of safe, potable water is the single most important measure in controlling typhoid.
• Chlorination of public water supplies (residual chlorine 0.2-0.5 mg/L at the consumer's end).
• Boiling of water at household level in endemic areas.
• Regular testing and bacteriological surveillance of water sources.
• Prevention of cross-connections between water mains and sewage pipes.
• Protection of wells, tanks, and reservoirs from contamination.
• During floods and natural disasters, emergency chlorination and distribution of safe water.

2. Sanitary Disposal of Sewage (Excreta Disposal)

• Construction and use of sanitary latrines (water seal latrines, pour flush latrines).
• Sewage treatment plants to prevent fecal contamination of water bodies and agricultural fields.
• Prevention of open defecation, especially near water sources.
• Promotion of Total Sanitation Campaigns (Swachh Bharat Mission in India).
• Preventing use of untreated human excreta as agricultural fertilizer.

3. Food Hygiene and Safety

• Supervision of food handlers - regular health examinations, periodic stool cultures.
• Thorough cooking of food; avoiding raw vegetables fertilized with sewage.
• Proper pasteurization of milk and dairy products.
• Control of street food vendors and public eating establishments.
• Proper storage and refrigeration of food to prevent multiplication of organisms.
• Handwashing with soap before handling food and after using the toilet.

4. Control of Flies

• Fly control measures: fly-proof latrines, screening of windows and food, use of insecticides.
• Elimination of fly breeding sites (proper garbage disposal, composting pits with covers).
• Keeping food covered.
• Use of fly traps and sticky strips.

5. Personal Hygiene

• Thorough handwashing with soap after defecation and before eating - the single most effective personal hygiene measure.
• Safe disposal of infant stools (often overlooked).
• Avoiding ice, raw foods, and unpeeled fruits in endemic areas.

C. MEASURES DIRECTED AT THE HOST (Immunization)

1. Typhoid Vaccines Currently Available:

• TCV (Typhoid Conjugate Vaccine) is now the WHO-preferred vaccine and is recommended for routine immunization of children ≥6 months in endemic countries. It is the only typhoid vaccine approved for children under 2 years. India introduced TCV in the national immunization program.
• ViPS vaccine provides no protection in children under 2 years (T-cell independent response).
• Ty21a is contraindicated in immunocompromised individuals and children under 5.

2. Target Groups for Vaccination:

• Children 6 months to 15 years in endemic areas (highest risk group)
• Travelers to endemic areas
• Laboratory workers handling S. typhi cultures
• Household contacts of known carriers
• Military personnel
• Food handlers in high-risk settings

3. Mass Immunization Campaigns:

• Targeted vaccination campaigns in epidemic settings and endemic urban areas have shown significant impact.
• School-based vaccination programs are highly cost-effective.

D. OTHER GENERAL PUBLIC HEALTH MEASURES

1. Health Education: Community education on safe water use, food hygiene, handwashing, recognition of symptoms, and importance of seeking early treatment.
2. Surveillance and Epidemiological Investigation:
   • Mandatory notification and reporting of all cases.
   • Investigation of every case/cluster to identify the source and route of transmission.
   • Contact tracing and surveillance of household contacts for 21 days (the maximum incubation period).
   • Stool cultures from contacts who are food handlers.
3. International Measures:
   • Under International Health Regulations (IHR), typhoid is a reportable condition.
   • Surveillance of travelers from endemic to non-endemic areas.
   • Vaccination of travelers to high-risk areas.
   • Inspection of ships and aircraft originating from endemic areas.
4. Multidrug Resistance (MDR) and Extensively Drug-Resistant (XDR) S. typhi:
   • MDR strains (resistant to chloramphenicol, ampicillin, cotrimoxazole) emerged in the 1980s and are widespread in Asia.
   • XDR S. typhi (additionally resistant to fluoroquinolones and third-generation cephalosporins) first emerged in Hyderabad, Pakistan (2016) and is spreading.
   • XDR typhoid is treated with azithromycin (oral) or carbapenems (parenteral).
   • Combating MDR/XDR requires antibiotic stewardship, rational prescribing, and accelerated TCV rollout.

VI. SUMMARY TABLE