Infectious Disease Transmission and Koch's Postulates

Koch's Postulates

• Koch's postulates are a set of criteria designed to establish a causal relationship between a microbe and a disease.

Transmission Chain

• The transmission chain describes the steps involved in the spread of an infectious agent.
• This chain includes:
  • Agent: The pathogen causing the disease.
  • Reservoir: The environment where the pathogen lives.
  • Exit: How the pathogen leaves the reservoir.
  • Mode of Transmission: How the pathogen travels to a new host.
  • Portal of Entry: How the pathogen enters the new host.
  • Susceptible Host: An individual who can be infected by the pathogen.

Classifying Transmission

• Horizontal Transmission: Disease spread between individuals of the same generation.
• Vertical Transmission: Disease spread from parent to offspring.
• Direct Transmission: Physical contact between infected and susceptible individual.
  • Kissing, sexual contact, body lesions.
• Indirect Transmission: Contact with contaminated objects or vectors.
  • Objects: Air, water, food, fomites (inanimate objects), blood, saliva.
  • Vectors: Insects, ticks, and other living organisms that transmit disease.

Airborne Transmission

• Diseases spread through droplets released from infected individuals during coughing, sneezing, or talking.
• Droplets can remain suspended in the air for long periods.
• Airborne transmission requires the pathogen to be resistant to drying.

Waterborne Transmission

• Spread of disease through contaminated water.
• Often caused by excreta from infected animals or humans.
• Results in infection during bathing, washing, drinking, or food preparation.

Vector-Borne Transmission

• Diseases transmitted through vectors like insects.
• Not all vector-borne diseases are caused by the insect itself, but by organisms they carry.
• Examples include malaria, West Nile virus, dengue fever, and yellow fever.

Soil Transmission

• Infections from contact with the soil.
• Some microorganisms, like tetanus spores, can survive in soil for long periods.
• Soil can also transmit parasites, like hookworms.

Food and Faecal-Oral Transmission

• Often associated with pathogens that infect the digestive system.
• Enter through ingestion of contaminated food or water.
• Shed from the body through faeces.

Host Defences

• The body has various defence mechanisms for preventing disease.
• These include:
  • Tears
  • Saliva
  • Stomach acid
  • Skin
  • Mucociliary escalator
  • Blood brain barrier
  • Fatty acids and salts
  • Gingival crevicular fluid
  • Blood

Mucociliary Escalator

• The respiratory tract is lined with mucus and cilia that trap and remove microbes.

Skin

• The skin's tough outer layer, keratin, provides a mechanical barrier to infection.
• Mucous membranes, such as the alimentary and respiratory tracts, eyes, and gingival sulcus, are susceptible to infection as they are not keratinized.

Dealing with Ingress

• The immune system is the body's primary defence against pathogens.
• Lymphocytes are specialized white blood cells responsible for immune responses.
• B lymphocytes produce antibodies, which bind and neutralize foreign antigens.
• T lymphocytes directly destroy infected or cancerous cells.
• Phagocytosis is the process of engulfing and digesting microbes by phagocytes.

Immunity

• Natural immunity is present from birth and is inherited.
• Acquired immunity is developed throughout life and is characterized by the production of antibodies.
• Active immunity is gained through exposure to an infectious agent.
• Passive immunity is obtained through the transfer of pre-formed antibodies.