Chain of Transmission in Infectious Disease

Link 1: Infectious Agent

• Pathogenicity - the ability to produce disease in a susceptible host
• Virulence - the severity of disease it can cause
• Infective dose - the minimum number of organisms needed to establish infection
• Ability to survive outside the host in the external environment

Link 2: Reservoir (Source of Infection)

Types of Reservoirs:

• Cases (clinical disease): Infectious during incubation (e.g., measles), during overt illness, or in convalescence
• Carriers: Persons who harbor the agent without apparent illness yet can transmit it. Carriers are classified by:
  • Type: Incubatory (shed before symptoms - measles, mumps, hepatitis B), Convalescent (shed during recovery - typhoid, cholera), Healthy/subclinical (never show symptoms - polio, meningococcal meningitis)
  • Duration: Temporary vs. Chronic (e.g., typhoid carriers for years via gall bladder colonization; hepatitis B carriers)
  • Portal of exit: Urinary, intestinal, respiratory, nasal carriers

Link 3: Portal of Exit

Link 4: Mode of Transmission

A. Direct Transmission

• Direct contact: skin-to-skin (STIs, leprosy), kissing, sexual intercourse
• Droplet spread: large respiratory particles (>5 µm) - project up to 30-60 cm during coughing/sneezing - diphtheria, pertussis, COVID-19, meningococcal meningitis
• Contact with soil: hookworm, tetanus, mycoses (direct exposure of skin/mucosa to soil)
• Inoculation: rabies via dog bite, hepatitis B via contaminated needles
• Transplacental (vertical): TORCH agents (Toxoplasma, Rubella, CMV, Herpes), varicella, HIV, syphilis

B. Indirect Transmission

Link 5: Portal of Entry

• Respiratory tract - most respiratory and many viral infections
• Gastrointestinal tract - feco-oral pathogens (typhoid, cholera, polio)
• Genitourinary tract - STIs
• Skin (intact or broken) - some agents penetrate intact skin (hookworm, schistosomes); most require a breach
• Conjunctiva - gonorrhea, adenovirus
• Placenta - vertical transmission agents

Link 6: Susceptible Host

• Immunity status: natural infection, vaccination, maternal antibodies
• Age: extremes of age are more vulnerable
• Nutritional status: malnutrition impairs immune response
• Underlying disease: HIV, diabetes, malignancy, immunosuppressant therapy
• Genetic factors: certain HLA types confer resistance or susceptibility
• Behavioral factors: hygiene, sexual behavior, occupational exposure

1. Enter via a portal of entry
2. Reach the "site of election" (target tissue) in the host
3. Overcome host defenses and multiply
4. Find a way out (portal of exit) to propagate further

Breaking the Chain - Public Health Interventions

Key principle: An infectious disease can only sustain itself in a population if the chain remains intact. The basic reproduction number (R₀) - the average number of secondary cases from one primary case in a fully susceptible population - reflects the overall efficiency of the chain. When R₀ > 1, the chain sustains; when < 1, the outbreak dies out.

• Park's Textbook of Preventive and Social Medicine (Block 2, pp. 107-111)
• Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed. (Block 4, p. 524)
• CDC NIOSH - Chain of Infection Components

Breaking the Chain of Infection - Public Health Interventions

• "Disinfection/sterilization" belongs under Infectious Agent, not just under mode of transmission.
• "Respiratory precautions (masks)" more precisely act at the portal of exit (source masking) AND at the portal of entry (recipient masking) - they were incorrectly listed only under portal of exit before.
• "Chemoprophylaxis" belongs under Susceptible Host, not under portal of entry.
• "Herd immunity" is an outcome of vaccinating the host population, so it sits under Susceptible Host, not as a standalone concept.
• Vector control belongs under Mode of Transmission, not reservoir, since the vector is the mechanism of transfer (though reservoir control may also reduce vector burden).

Detailed Classification of Vector-Borne Diseases

Part I: Classification by Type of Transmission

1. Direct Contact (Arthropod-to-Human)

2. Mechanical Transmission

Key point: The agent does NOT need an extrinsic incubation period in mechanical transmission. The vector can transmit immediately.

3. Biological Transmission

a) Propagative

b) Cyclo-propagative

c) Cyclo-developmental (Developmental only)

4. Special Modes within Biological Transmission

Transovarial Transmission

• Examples: Rickettsiae in ticks, arbovirus (e.g., dengue) in Aedes mosquitoes, Borrelia in soft ticks

Transstadial Transmission

• Examples: Borrelia burgdorferi (Lyme disease) in Ixodes ticks; spotted fever Rickettsiae in hard ticks

Transovarial + transstadial transmission together allow ticks to act as both reservoir AND vector, making them especially dangerous epidemiologically.

Part II: Classification by Type of Vector (Arthropod)

Insects (Class Insecta)

Arachnids (Class Arachnida)

Crustacea

Part III: Factors That Determine Vector Capacity

1. Host feeding preferences - anthropophilic (prefers humans) vs. zoophilic vectors
2. Infectivity - ability to acquire and transmit the disease agent
3. Susceptibility - ability to become infected by the agent
4. Survival rate in the environment (the vector must survive long enough to complete the EIP)
5. Domesticity - degree of association with human habitation (e.g., Aedes aegypti breeds in peridomestic water containers)
6. Environmental factors - temperature, humidity, and rainfall influence vector density and EIP length. Seasonal peaks of vector-borne diseases (e.g., malaria in monsoon season) reflect these factors.

Summary Diagram of Biological Transmission Types

BIOLOGICAL TRANSMISSION
├── Propagative          → Agent multiplies only (no form change)
│                           e.g., Plague in rat flea
├── Cyclo-propagative    → Agent multiplies + changes form
│                           e.g., Malaria in Anopheles
└── Cyclo-developmental  → Agent changes form only (no multiplication)
                            e.g., Filaria in Culex mosquito

• Park's Textbook of Preventive and Social Medicine, pp. 110-111 & 871
• Henry's Clinical Diagnosis and Management by Laboratory Methods, Block 32