B3

Learning Objectives and References

  • Learning Targets for Epidemic Disease Occurrence and Patterns:

    • Identify the different levels of disease.
    • Classify epidemic patterns according to their manner of spread through a population.
    • Define and explain the Chain of Infection.
    • Identify the components that make up the chain of infection.
    • Differentiate between direct and indirect modes of transmission.
    • Identify the different types of reservoirs.
    • Identify the factors that influence the outcome of an exposure.

  • Core References for Biostatistics and Epidemiology:

    • Ganir, Florence (2019). Biostatistics and Epidemiology. Sta. Cruz Laguna: EDRIC Publishing House.
    • Principles of Epidemiology in Public Health Practice: An Introduction to Applied Epidemiology and Biostatistics, Third Edition (October 2006). Centers for Disease Control and Prevention (CDC).

Baseline Levels of Disease Occurrence

  • Endemic Level: This refers to the amount of a particular disease that is usually present in a community. It is also known as the baseline level. This level is the observed level of disease and is not necessarily the desired level, which may ideally be zero. In the absence of intervention, and assuming the level does not deplete the pool of susceptible individuals, the disease may continue indefinitely at this level. Consequently, the baseline is often regarded as the expected level of the disease.

  • Variations in Investigation Thresholds: Some diseases are so rare in specific populations that even a single case triggers an epidemiologic investigation. Examples of such diseases include rabies, plague, and polio. For other, more common diseases, investigations are only warranted when there are significant deviations from the norm.

Categorization of Disease Distribution

  • Sporadic: Refers to a disease that occurs infrequently and irregularly.

  • Endemic: Refers to the constant presence and/or usual prevalence of a disease or infectious agent within a specific geographic area or population.

  • Hyperendemic: Refers to persistent, high levels of disease occurrence within a population.

  • Epidemic: Refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in a specific population in a particular area.

  • Outbreak: This term carries the same definition as an epidemic but is typically used to describe a more limited geographic area.

  • Cluster: Refers to an aggregation of cases grouped in place and time that are suspected to be greater than the expected number, even if the exact expected number is unknown.

  • Pandemic: Refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.

Etiology and Contributing Factors of Epidemics

  • Conditions for Epidemics: Epidemics occur when an agent and susceptible hosts are present in adequate numbers and the agent can be effectively conveyed from a source to the hosts.

  • Specific Causes of Epidemics:

    • A recent increase in the amount or virulence of the agent.
    • The recent introduction of the agent into a setting where it has not been previously present.
    • An enhanced mode of transmission that exposes more susceptible persons.
    • A change in the susceptibility of the host response to the agent.
    • Factors that increase host exposure or involve introduction through new portals of entry.

Patterns of Epidemic Spread

  • Common-Source Outbreak: Occurs when a group of persons are all exposed to an infectious agent or toxin from the same source.

    • Point-Source Outbreak: A sub-classification of common-source outbreaks where the group is exposed over a relatively brief period. Every person who becomes ill does so within one incubation period.
      • Examples: The epidemic of leukemia cases in Hiroshima following the atomic bomb blast; the epidemic of hepatitis A among patrons of a Pennsylvania restaurant who consumed contaminated green onions.
      • Epidemic Curve: When cases are plotted over time, the graph typically shows a steep upslope and a gradual downslope, described as a log-normal distribution.
    • Continuous Common-Source Outbreak: Case-patients are exposed over a period of days, weeks, or longer. The range of exposures and incubation periods tends to flatten and widen the peaks of the epidemic curve.
    • Intermittent Common-Source Outbreak: The epidemic curve shows a pattern reflecting the intermittent nature of the exposure.

  • Propagated Outbreak: Characterized by spread from person to person.

  • Mixed Epidemics: These have features of both common-source and propagated epidemics. They often begin with a common-source outbreak followed by secondary person-to-person spread.

    • Example: A shigellosis epidemic among 3,0003,000 women attending a national music festival. Many developed symptoms after returning home, leading to subsequent generations of Shigella cases across several states propagated by person-to-person transmission.

  • Other Outbreak Patterns: Some epidemics are neither traditionally common-source nor propagated.

    • Zoonotic or Vectorborne Diseases: Result from sufficient prevalence of infection in host species, the presence of vectors, and human-vector interaction.
    • Example 1 (Lyme Disease): Emerged in the northeastern United States in the late 1980s; spread from deer to humans by deer ticks.
    • Example 2 (West Nile Encephalitis): Outbreak in the Queens section of New York City in 1999; spread from birds to humans by mosquitoes.

The Chain of Infection: Conceptual Framework

  • Definition: The chain of infection is a series of sequential stages or events that must occur to cause an infection. If any link in this chain is missing, the infectious disease is unlikely to be transmitted. Identifying these components allows for the breaking of one or more links to stop transmission.

  • The Six Elements of the Chain:

    1. Infectious Agent (Pathogen)
    2. Reservoir
    3. Portal of Exit
    4. Mode of Transmission
    5. Portal of Entry
    6. Susceptible Host

Components of the Chain: Infectious Agents and Reservoirs

  • Infectious Agents (Pathogens): Microorganisms or "germs" such as bacteria, viruses, fungi, and protozoa that cause disease.

    • The word "pathogen" is derived from Greek, meaning "that which produces suffering."
    • To cause disease, an infectious dose is required, which varies based on the agent's infectivity and virulence.

  • Reservoir: The habitat in which the agent normally lives, grows, and multiplies. It may or may not be the source from which the agent is transferred to the host.

    • Human Reservoirs: Many common infectious diseases (STDs, measles, mumps, streptococcal infections, respiratory pathogens) have human reservoirs. Smallpox was eradicated because humans were its only reservoir.
    • Animal Reservoirs: Humans are incidental hosts for many diseases transmitted animal-to-animal. Zoonosis refers to infectious diseases transmissible under natural conditions from vertebrate animals to humans.
      • Bacterial/Parasitic Examples: Brucellosis (cows/pigs), anthrax (sheep), plague (rodents), trichinellosis/trichinosis (swine), tularemia (rabbits).
      • Viral Examples: Rabies (bats, raccoons, dogs), West Nile encephalitis (birds), monkeypox (prairie dogs).
      • Emergent zoonoses: HIV/AIDS, Ebola, and SARS are thought to have emerged from animal hosts.
    • Environmental Reservoirs: Includes plants, soil, and water.
      • Soil: Reservoir for fungal agents like those causing histoplasmosis, and for Clostridium botulinumClostridium\,botulinum.
      • Water: Legionella pneumophilaLegionella\,pneumophila often resides in cooling towers and evaporative condensers.

Components of the Chain: Portals of Exit and Entry

  • Portal of Exit: The path by which a pathogen leaves its host, usually corresponding to the site where the pathogen is localized.

    • Respiratory Tract: Influenza viruses, Mycobacterium tuberculosisMycobacterium\,tuberculosis.
    • Urine: Schistosomes.
    • Feces: Cholera vibrios.
    • Skin Lesions: Sarcoptes scabieiSarcoptes\,scabiei (scabies).
    • Conjunctival Secretions: Enterovirus 70.
    • Blood/Placenta: Rubella, syphilis, toxoplasmosis (transplacental); Hepatitis B (skin cuts/needles); Malaria (blood-sucking arthropods).

  • Portal of Entry: The manner in which a pathogen enters a susceptible host, providing access to tissues where the pathogen can multiply or a toxin can act. Agents often enter via the same portal they used to exit the previous host.

    • Fecal-Oral Route: Pathogens exit in feces and enter through the mouth via contaminated hands, food, water, or utensils.
    • Specific Entry Portals: Skin (hookworm), mucous membranes (syphilis), and blood (Hepatitis B, HIV).

Modes of Disease Transmission

  • Direct Transmission:

    • Direct Contact: Occurs via skin-to-skin contact, kissing, and sexual intercourse. Also includes contact with soil or vegetation (e.g., Infectious mononucleosis, gonorrhea, hookworm spread by contaminated soil).
    • Droplet Spread: Spray with relatively large, short-range aerosols from sneezing, coughing, or talking. Classified as direct because transmission occurs within a few feet before droplets fall. Examples include pertussis and meningococcal infection.

  • Indirect Transmission: Transfer via suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors).

    • Airborne Transmission: Carried by dust or droplet nuclei suspended in air. Droplet nuclei are dried residue less than 5 μm5\,\mu\text{m} in size. They can remain suspended long-term and be blown over great distances.
      • Example: Measles virus can remain suspended in a physician's office after an infected child has left.
    • Vehicles: Inanimate objects such as food, water, biological products (blood), and fomites (handkerchiefs, bedding, surgical scalpels). Vehicles can carry pathogens passively (Hepatitis A) or provide an environment for growth/toxin production (Clostridium botulinumClostridium\,botulinum in canned foods).
    • Vectors: Mosquitoes, fleas, and ticks.
      • Mechanical Transmission: Pathogen is carried physically (e.g., flies carrying ShigellaShigella on appendages; fleas carrying Yersinia pestisYersinia\,pestis in the gut).
      • Biologic Transmission: Agent undergoes maturation or changes in the host (e.g., malaria or guinea worm disease).

The Susceptible Host and Determinants of Exposure Outcomes

  • Host Susceptibility Factors: Individual resistance depends on genetic makeup, specific immunity, and nonspecific factors.

    • Genetic/Constitutional: For example, individuals with sickle cell trait have partial protection against a specific type of malaria.
    • Specific Immunity: Protective antibodies developed via infection, vaccines, toxoids (deactivated toxins), transplacental transfer, or injection of immune globulin.
    • Nonspecific Defenses: Skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune responses.

  • Factors Influencing Exposure Outcomes:

    • Host Factors: Extremes of age, underlying diseases (HIV/AIDS, malignancy), transplants, medications altering flora, surgical procedures, radiation therapy, and indwelling devices.
    • Agent Factors: Infectivity, pathogenicity, virulence, and size of inoculum.
    • Environmental Factors: Contamination of environment/equipment, availability of transmission means, temperature, and humidity.
    • Exposure Factors: Route of exposure, duration of exposure, and the ability of the pathogen to maintain infectivity over distance.

Questions and Discussion

  • Question 1: The following comprises the six sequential stages for an infection to happen except:

    • a. Pathogen
    • b. Susceptible host
    • c. Neither A nor B
    • d. Both A or B
    • Response: C. Pathogen and susceptible host are both essential parts of the chain of infection.

  • Question 2: True or false: The infection continues even without a reservoir which does not necessarily affect the chain.

    • Response: False. A reservoir is essential for the germ to survive; without it, the chain is broken.

  • Question 3: Which of the following statements is/are not true about Portal of Exit?

    • a. Portal of exit refers to the manner in which a pathogen enters a susceptible host.
    • b. Infectious agents use the same portal to enter a new host that they used to exit the source host.
    • c. Both A and B.
    • d. None of the above.
    • Response: A. Statement A describes the portal of entry rather than the portal of exit.

  • Question 4: What is the habitat in which the agent normally lives, grows, and multiplies?

    • a. Host
    • b. Reservoir
    • Response: B. Reservoir.