Lecture on Communicable Diseases: Modes of Transmission

Introduction

Definition: Immediate transfer of infectious agents from a reservoir (infected person) to a new susceptible host.
Methods:
- Direct Contact:
  - Involves oral secretions, blood, or other infectious fluids (e.g., skin lesion drainage, semen, vaginal secretions).
  - Occurs through touching, biting, kissing, or sexual intercourse.
- Droplet Spread:
  - Contact with infected droplets from coughing or sneezing.
  - Requires close proximity (< 2 meters) between individuals.
  - Example: Pertussis (whooping cough).
Key Point: Both direct contact and droplet spread are forms of direct transmission.

Airborne Transmission:
- Infectious agents carried by dust or droplet nuclei suspended in air.
- No need for direct contact or close proximity.
- Droplet nuclei can remain airborne for long periods and travel distances.
- Example: Measles.
Vehicle-borne Transmission:
- Infectious agents transported within contaminated inanimate materials (fomites).
- Examples of fomites: toys, sharp objects, tissues, food, and water.
- Food & Water Contamination:
  - Infection: Pathogen directly ingested (e.g., E. coli, salmonella).
  - Intoxication: Toxin ingestion, a byproduct of pathogen (e.g., botulism).
- Historical Example: Cholera outbreak in London, traced to contaminated water by John Snow.
Vector Transmission:
- Infectious agent transmitted by living organisms (vectors) like mosquitoes, ticks, fleas, lice.
- Biological Vectors: Pathogens multiply within vectors (e.g., mosquitoes).
- Mechanical Vectors: Infectious agents picked up externally (e.g., flies).
- Complexity in prevention and control; involves community education and environmental measures.
- Example: Malaria prevention strategies (mosquito nets, vaccinations).

Modes of transmission include direct (direct contact, droplet spread) and indirect (airborne, vehicle-borne, vector) methods.