Chain of Infections

Chain of Infection

The chain of infection consists of several components that must be present for an infection to occur.

Causative Organism

  • Definition: The organism that causes the disease.
  • Examples:
    • Bacteria: Escherichia coli.
    • Viruses: Herpes simplex virus (replicates only inside living cells).
    • Fungi: Yeasts, filamentous fungi, and dermatophytes.
    • Protozoa: Unicellular organisms like Plasmodium.
  • Infectivity depends on:
    • Number of microorganisms.
    • Virulence of infecting microorganism.

Reservoir

  • Definition: The place where the organism lives and reproduces.
  • Sources:
    • Human source.
    • Animal source.
    • Inanimate source.
Human Source
  • Normal flora residing on the skin's surface and within body cavities, fluids, and discharges.
    1. Auto-infection:
      • Occurs when organisms from one body part transfer to another.
      • Example: UTI.
    2. Cross-infection:
      • Occurs when organisms from one person transfer to another person or environment.
      • Example: Transfer from a nurse's hand to a patient's wound.
    3. Incubation:
      • Organisms multiply without the person showing signs or symptoms (manifestations).
    4. Carrier:
      • A person harbors pathogenic microorganisms after an attack or without ever experiencing the disease.
  • Illustrative diagram:
    • Shows the relationship between Infection, Incubation period, Latent period, Disease, and Period of communicability.
Animal Source
  • Examples of diseases transmitted from animals:
    • Tuberculosis: from cattle.
    • Anthrax: from sheep, cattle, pigs.
    • Ringworm: from dogs, cats, cattle.
    • Toxoplasmosis: from cattle, goats.
    • Rabies: from dogs.
    • Cholera: from contaminated shellfish.
    • Malaria: from Anopheles mosquitoes.
Inanimate Source
  • Examples: Soil, seawater, food, water, and milk.
  • Diseases:
    • Tetanus.
    • Gas gangrene.

Portal of Exit

  • Definition: The escape route of the organism from the reservoir.
  • Examples:
    • Mucus from the respiratory tract.
    • Vomitus.
    • Discharges from infected wounds, eyes, ear, nose, vagina.
    • Infected blood.
    • Urine and feces.

Mode of Transmission

  • Definition: Ways in which microorganisms move from a reservoir to a new host.
  • Methods:
    • Airborne (inhalation).
    • Contact (touching).
    • Alimentary (ingestion).
    • Vector (insects/animals).
Airborne Transmission
  • Occurs via:
    1. Airborne Droplets:
      • Organisms carried or expelled through coughing, sneezing, or blowing the nose.
      • Examples: Tuberculosis (TB), Severe Acute Respiratory Syndrome (SARS).
    2. Dust particles:
      • From dead skin flakes, environmental dust particles, fluff from linen, which settle on surfaces.
Contact Transmission
1.  Direct contact:
    *   Physical transfer between an infected person and a host.
    *   Examples: Syphilis, Gonorrhoea, Skin infections.
2.  Indirect contact:
    *   Through contaminated inanimate objects.
    *   Example: Use of contaminated nursing or medical equipment.
Transmission via Ingestion
  • Microorganisms enter the gastrointestinal tract via:
    • Infected food or water.
    • Contaminated utensils or hands.
    • Examples: Creutzfeldt-Jakob disease (CJD), Cholera.
Vector Transmission
  • Via mosquitoes, flies, rats, and other animals.
    • Examples: Malaria (Plasmodium falciparum), Dengue (Aedes Aegypti).

Portal of Entry

  • Definition: How the organism enters the body.
  • Routes:
    1. Integumentary system:
      • Through a break in the skin or mucous membrane.
    2. Respiratory tract:
      • By inhaling contaminated droplets.
    3. Genitourinary tract:
      • Through contamination with infected vaginal secretions, semen, or catheter access sites.
    4. Gastrointestinal tract:
      • By ingesting contaminated food/water.
    5. Circulatory system:
      • Through the bite of insects/rodents.
    6. Transplacental:
      • Through transfer of a microorganism from mother to fetus via the placenta and umbilical cord.

Susceptible Host

  • Definition: The degree of resistance an individual has to pathogens.
    • Lower resistance means a higher risk of infection.
Factors Increasing Susceptibility:
1.  Age: very young or old.
2.  Concurrent disease: existence of other diseases.
3.  Immunization/Vaccination status: individuals not fully immunized.
4.  Stress: compromised emotional state lowers defense mechanisms.
5.  Weakened immune system: due to certain medications, e.g., chemotherapy drugs.
6.  Lifestyles: practices like multiple sex partners, sharing needles, or tobacco/drug use can alter defenses.
7.  Career: jobs involving increased exposure to pathogenic sources, e.g., with needles or chemical agents.
8.  Nutritional status: acceptable height, weight, and body frame reduce the risk of illness.
9.  Heredity: genetically more susceptible to infections than others.

Breaking the Chain of Infection

  • Infection control measures are directed towards breaking the chain of infection to prevent the spread of diseases.