Epidemiology and the Chain of Infection

Chain of Infection

Reservoir

• Human reservoirs:

  • Symptomatic infections provide clear sources of pathogens.

  • Disease can be transmitted before symptoms appear or after they have resolved.

  • Asymptomatic infections: Persons harboring a pathogen without exhibiting symptoms can act as carriers, facilitating the spread of disease.

• Non-human animal reservoirs:

  • Represent a source of many pathogens.

  • Control of pathogen spread is challenging in wild animal populations.

  • Zoonoses (zoonotic diseases): Exist primarily in animals but are transmissible to humans.

• Environmental reservoirs:

  • Examples include soil, which can be difficult or impossible to eliminate as a disease source.

Portals of Exit

• Body surface or orifice: All potential exit routes for pathogens.

• Intestinal tract: Pathogens can be shed in feces.

• Respiratory tract: Pathogens exit the body in droplets of saliva or mucus.

• Skin: Pathogens can be shed along with skin cells.

• Genital: Pathogens exit via secretions.

Disease Transmission

• Vertical transmission:

  • Involves transfer from a pregnant woman to a fetus or from mother to infant during childbirth or breastfeeding.

• Horizontal transmission:

  • Spread of disease from person to person via several methods including air, physical contact, ingestion, or by vectors.

  • Direct transmission: This method involves the immediate transfer of an infectious agent to a portal of entry:

  • Direct Contact: For example, handshake or sexual intercourse.

  • Droplet Transmission: Respiratory diseases can spread when pathogen-laden droplets are inhaled.

Disease Transmission (Continued)

• Indirect transmission: Occurs via several mechanisms:

  • Airborne transmission: Respiratory diseases are often transmitted by liquid droplets released while talking, sneezing, or coughing.

  • Droplet nuclei: Microbes attached to dried materials, such as dead skin cells or dust, can become airborne and inhaled, carrying pathogens to the lungs.

  • Control measures can include ventilation systems, negative pressure environments, and HEPA filters.

  • Vehicle-borne transmission: Involves pathogens being transmitted by objects, food, or water.

  • Cross-contamination: Transfer of pathogens between foods.

  • Fomites: Inanimate objects that can harbor and transmit pathogens.

Disease Transmission (Continued)

• Vector-borne transmission: A vector is a living organism that can carry a pathogen.- Mechanical vector: Carries the microbe on its body from one location to another without being part of the pathogen's life cycle.

  • Biological vector: Participates in the life cycle of the pathogen, contributing to its transmission.

Portal of Entry

• Portal of entry: The route through which pathogens enter a host, typically through body surfaces or orifices.- Respiratory pathogens usually require inhalation via the nose to cause disease.

  • Intestinal pathogens require ingestion via the mouth for disease to develop.

  • Fecal-oral transmission: Occurs when fecal organisms are transferred to the mouth and ingested.

R0 (Reproduction Number)

• R0 (Basic Reproduction Number): Represents the hypothetical contagiousness of an infection within a completely naïve unvaccinated population.- Takes into account the following factors:

  • Infectious period: The duration an infected individual can spread the disease.

  • Mode of transmission: How the pathogen is transmitted (e.g., droplet, contact).

  • Contact rate: The frequency at which individuals encounter one another.

  • This number typically remains unchanged for a given pathogen.

Site of Infection

• Localized Infection:

  • Infections isolated to one specific area of the body, examples include:

  • Local infections: Can occur in areas such as the intestinal tract, respiratory tract, conjunctiva, skin (e.g., HPV), genitals, and liver.

• Systemic Infection:

  • Involves the presence of pathogens in the bloodstream, referred to as viremia.

Factors That Influence the Epidemiology of Disease

• The Host Population:- Immunity to Pathogen: Previous exposure to a pathogen or immunization can affect susceptibility.

  • Herd Immunity: Protects non-immune individuals in a population; typically, greater than 90% immunity is sufficient to maintain this.

  • Antigenic variation can overcome herd immunity, allowing pathogens to evade the immune system.

  • General Health: Factors such as malnutrition, overcrowding, and fatigue influence susceptibility to infections.

  • Genetic Background: Natural immunity varies significantly among individuals due to genetic differences, influencing susceptibility.

  • Certain individuals may possess specific receptors that alter their vulnerability to certain pathogens.

  • Age Factors: Very young children and the elderly are generally more susceptible to infections due to weaker or underdeveloped immune systems.

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