Epidemiology Notes

Chapter 19: Epidemiology

• Quote: "Physician, heal yourself." – Luke 4:23

A Glimpse of History

• Puerperal fever: A bacterial infection of the uterus.
• Statistics: In the mid-19th century, 1 in 8 women died from this infection in Vienna hospitals following childbirth.
• Ignaz Semmelweis' Observations:
  • Noted that incidence in a section handled by medical students was 4 times higher than in a section run by midwives.
  • A friend’s death from a scalpel wound echoed similar symptoms of puerperal fever.
  • Hypothesized that a "poison" was transferred by medical students to women in childbirth.
• Germ Theory Foundations:
  • This insight occurred before Pasteur and Koch's germ theory was widely accepted; the pathogen was likely Streptococcus pyogenes.
  • Implemented handwashing with disinfectants, leading to a drop in incidence by 1/3.

Epidemiology Overview

• Definition: Epidemiology is the study of disease patterns in populations.
• Epidemiologists: Health detectives who gather data on disease sources and risk factors.
• Objectives:
  • Design strategies for infection control.
  • Predict and prevent disease spread.
• Disciplines Involved:
  • Ecology, microbiology, sociology, statistics, psychology.
• Daily Habits Informed by Epidemiology:
  • Handwashing and waste disposal practices.

Disease Categories

• Communicable Diseases:

  • Transmitted from one host to another (e.g., measles, influenza).
  • Transmission involves interactions between the environment, pathogen, and host.
  • Control measures: Improve sanitation, vaccination, antimicrobial medications.

• Non-communicable Diseases:

  • Not spread between hosts; often arise from an individual's microbiota or environment (e.g., Clostridium tetani).

Principles of Epidemiology

• Rates of Disease:
  • Focus on rates over absolute numbers; understand context (e.g., small vs large city).
  • Attack Rate: Percentage becoming ill after exposure; reflects infectious dose and immunity.
  • Incidence Rate: New cases/time/population; measures individual risk.
  • Prevalence: Total cases at a specific time/period; indicates disease impact on society.
  • Morbidity: Incidence of disease for at-risk populations (e.g., high morbidity for influenza).
  • Mortality: Overall death rate; often linked with non-communicable diseases in developed countries.
  • Case-Fatality Rate: Percentage of deaths from a specific disease.

Disease Definition and Trends

• Endemic: Constant presence of a disease (e.g., common cold).
• Epidemic: Unusually high cases, may originate from introduced or endemic sources.
• Outbreak: Cluster of cases in a specific timeframe.
• Pandemic: Global spread of a disease (e.g., AIDS).

Reservoirs of Infection

• Definition: Natural habitats where pathogens live (in humans, animals, or the environment).
• Human Reservoirs:
  • Can be symptomatic/asymptomatic; easier to control (e.g., smallpox).
  • Some individuals can transmit diseases without symptoms (e.g., Neisseria gonorrhoeae).
• Animal Reservoirs:
  • Diseases (e.g., rabies) primarily exist in animals but can infect humans.
• Environmental Reservoirs:
  • Often challenging to eliminate (e.g., Clostridium).

Portals of Exit and Entry

• Portals of Exit: Routes for pathogens in the body (e.g., feces, saliva, skin).
• Portals of Entry: Routes through which pathogens infect hosts (e.g., respiratory tract, broken skin).

Disease Transmission

• Vertical Transmission: From mother to fetus/infant.
• Horizontal Transmission: Person-to-person.
  • Direct Contact: Handshakes, intimacy.
  • Indirect Contact: Fomites (inanimate objects).
  • Droplet Transmission: Respiratory droplets; controlled through hygiene.
  • Food and Water Contamination: Hazards exist in contaminated food and municipal water.
  • Airborne Transmission: Smaller particles can carry pathogens longer distances.
  • Vectors: Living organisms that carry pathogens (e.g., mosquitoes, ticks).

Pathogen Factors Influencing Epidemiology

• Virulence: Ability to cause disease; determined by adherence and immune evasion capabilities.
• Dose: Minimum quantity of pathogens; extremely low doses may not cause disease.
• Incubation Period: Time taken for symptoms to appear; influences spread potential (e.g., typhoid fever spread).

Host Factors Influencing Disease

• Immunity: Previous exposure/immunization status.
  • Herd Immunity: Protects non-immune individuals; often needs >90% immunity.
• General Health: Malnutrition, overcrowding influence susceptibility.
• Age and Gender: Very young and elderly are more susceptible; specific diseases target certain genders.
• Cultural Practices: Dietary habits can affect exposure; breastfeeding protects infants.
• Genetic Background: Variation in natural immunity due to genetic differences.

Epidemiological Studies

• Historical Context: John Snow's cholera investigations laid foundations for modern epidemiology.
• Study Types:
  • Descriptive Studies: Data collection post-outbreak (focus on person, place, time).
  • Analytical Studies: Investigate relevancy of risk factors; includes cross-sectional, retrospective, and prospective studies.
  • Experimental Studies: Assess cause/effect relationships, often using control groups.