Transmission of Disease (BMS Learning Outcomes)

Koch's Postulates

• Koch's postulates are criteria used to establish a causal link between a microorganism and a specific disease.
• Postulate 1: The microorganism must be present in every case of the disease but absent in healthy individuals.
• Postulate 2: The microorganism must be isolated from the diseased host and grown in pure culture.
• Postulate 3: Introducing the pure culture into a healthy susceptible host must reproduce the disease.
• Postulate 4: The identical microorganism must be reisolated from the experimentally infected host.

Chain of Transmission

• The spread of infection requires several linked elements:
  • Agent: The infectious microorganism
  • Reservoir: The source of the agent (e.g., human, animal, environment).
  • Exit: The method by which the agent leaves the reservoir (e.g., coughing, sneezing, feces).
  • Mode of transmission: How the agent moves from the reservoir to a new host (e.g., direct contact, airborne).
  • Portal of entry: The route by which the infectious agent enters the new host (e.g., respiratory tract, skin wound).
  • Susceptible host: A host who is vulnerable to the agent's effects.

Transmission Mechanisms

• Horizontal transmission: Person-to-person spread
• Vertical transmission: Mother-to-child spread
• Direct transmission: Immediate contact with infectious substance (e.g., sexual contact, kissing, touching).
• Indirect transmission: Transmission through an intermediary like a contaminated surface or vector (e.g., vehicle-borne pathogens via food, water, or contaminated objects, vector-borne diseases via insects, animals).

Host Defense Mechanisms

• Mucociliary escalator: The respiratory tract's lining with mucus and cilia (tiny hairs) removes inhaled particles, trapping and clearing them out.
• Skin: The tough keratinized outer layer provides a mechanical barrier. Unkeratinized areas (mouth, eyes, respiratory tract) are more susceptible.
• Fatty acids and salts: Skin glands produce antibacterial agents like fatty acids and lysozyme.
• Saliva: Contains protective antibacterial agents and a cleansing effect.
• Gingival crevicular fluid: Antibacterial exudate in the gum area, containing immune defenses.
• Blood: Blood flow flushes out pathogens, and clotting mechanisms trap them.
• Blood-brain barrier: This filters out harmful substances, preventing infection entry into the brain and spinal cord.

Host Immune Response

• Immune system: Recognizes and destroys foreign invaders (antigens). Differentiates between body tissues and antigens.

• Lymphocytes: White blood cells, developing in bone marrow and circulating in lymphatic system.

  • B lymphocytes: Mature into plasma cells, producing antibodies targeted at specific antigens. Memory B cells provide long-term immunity.
  • T lymphocytes: Assist with immune response and directly destroy pathogens:
    • Killer T cells: Destroy virus-infected or cancerous cells.
    • Helper T cells: Activate B cells for antibody production.
    • Suppressor T cells: Control immune response, stopping reactions as needed.

• Phagocytosis: ("cell eating") is carried out by polymorphonuclear leucocytes (a type of white blood cell), which engulf invading micro-organisms.

Immunity

• Natural Immunity: Existing at birth; inherited from mother.
• Acquired Immunity: Develops throughout life, responding and creating immunity to a disease. Can be short-term (e.g., influenza) or long-term (e.g., measles).
• Active immunity (natural or artificial): The body produces its own antibodies in response to exposure to antigens through illness or vaccination. Long-lasting immunity.
• Passive immunity (artificial): Receiving antibodies from an external source (e.g., antibodies given after exposure to a disease, mothers antibodies given to the child). Temporary protection.