Ch_19_lecture

Chapter 19 Epidemiology

• Scriptural Reference: Luke 4:23 - “Doubtless you will quote to me this proverb, ‘Physician, heal yourself.”

A Glimpse of History

• Puerperal Fever: A serious bacterial infection of the uterus.

  • By mid-19th century, 1 in 8 women died from this in Vienna hospitals post-childbirth.

  • Ignaz Semmelweis: Noted that the incidence of puerperal fever was significantly higher in a section managed by medical students compared to one by midwives and students.

  • Death of a Friend: Semmelweis's friend died from symptoms akin to puerperal fever after a scalpel wound during an autopsy, leading him to deduce that a "poison" from the students was being transferred to women.

  • He implemented a handwashing protocol with strong disinfectant, reducing incidence to one-third.

Epidemiology

• Definition: The study of disease dynamics in populations.

  • Epidemiologists: Often referred to as health detectives, they gather and analyze data regarding disease sources and risks.

  • Key Functions: Design infection control strategies and predict disease spread. Uses knowledge spanning ecology, microbiology, sociology, statistics, and psychology.

  • Many public health habits (e.g., handwashing, waste disposal) derive from epidemiological insights.

• Disease Types:

  • Communicable Diseases: Infectious and can spread from host to host (e.g., measles, colds, influenza).

    • Transmission is dependent on environment, pathogen, and host interactions.

    • Control measures include improved sanitation, antimicrobials, and vaccinations.

  • Non-Communicable Diseases: Do not spread from host to host (e.g., diseases from individual's microbiota like Clostridium tetani).

19.1 Principles of Epidemiology

• Rates of Disease:

  • Epidemiologists prioritize disease rates over raw case numbers (important to contextualize rates based on population size).

  • Attack Rate: Percentage of individuals who fall ill after exposure to a pathogen.

  • Incidence Rate: Number of new cases in a specified time frame against the population size, indicating individual risk of disease.

  • Prevalence: Total cases (old and new) present in a population at any point in time, reflecting the disease's social impact.

  • Rates are typically expressed per 100,000 people.

Morbidity and Mortality

• Morbidity: Incidence of disease in a population at risk.

  • Higher rates seen in contagious diseases (e.g., influenza).

• Mortality: Overall death rate in a population, often linked to non-communicable diseases in developed countries.

  • Case-Fatality Rate: Percentage that dies from a specific disease (e.g., high rates for plague and Ebola; decreasing for AIDS due to treatment advancements).

Disease Presence

• Endemic Diseases: Constantly present within a population (e.g., common colds, measles).

• Epidemic: Spikes in cases above normal expected levels, potentially from introduced or endemic diseases.

• Outbreak: Specific cases occurring in a defined time and place.

• Pandemic: Global spread of disease (e.g., AIDS).

Reservoirs of Infection

• Definition: Natural habitat of a pathogen, found in animals, humans, or environments (e.g., soil, water).

  • Importance of identification for effective disease control (e.g., controlling rodent populations to curb plague).

  • Human Reservoirs: May be exclusive to humans or share with other species.

  • Symptomatic vs. Asymptomatic: Symptomatic are direct sources of pathogens, asymptomatic can unknowingly spread (e.g., Neisseria gonorrhoeae).

  • Environmental Reservoirs: Difficult to eliminate pathogens found in non-human sources.

Portals of Exit and Entry

• Portals: Routes that pathogens take to exit or enter hosts.

• Common Exit Routes:

• Intestinal Tract: Pathogens shed in feces (e.g., Vibrio cholerae).

• Respiratory Tract: Exit in respiratory droplets (e.g., Mycobacterium tuberculosis).

• Skin: Pathogens can be shed in skin cells (e.g., Staphylococcus aureus).

• Genital Tract: Pathogens from secretions (e.g., Neisseria gonorrhoeae).

Disease Transmission

• Types:

  • Vertical Transmission: From pregnant woman to fetus or mother to infant.

  • Horizontal Transmission: Person-to-person via air, contact, food, or water.

• Contact Types:

  • Direct Contact: Immediate interactions (e.g., handshake, sexual intercourse).

  • Indirect Contact: Through contaminated objects (fomites) like doorknobs or glasses.

• Droplet Transmission: Respiratory droplets fall within one meter; minimized by covering mouth when sneezing.

• Food and Water Contamination: Can transmit pathogens, underscoring the importance of sanitation.

• Airborne Pathogens: Smaller particles can remain suspended and cause diseases; control via ventilation or filters.

• Vectors: Arthropods that carry pathogens (e.g., mosquitoes, ticks).

Factors Influencing Epidemiology

Pathogen Factors

• Virulence: Ability of a pathogen to cause disease, involving adhesion mechanisms and evasion of immune defenses (e.g., Neisseria gonorrhoeae).

• Dose: Minimum required quantity of pathogens to cause disease.

• Incubation Period: Duration before symptoms appear; longer periods can facilitate greater spread of disease.

Host Factors

• Immunity: Individual’s previous exposure and vaccinations.

  • Herd Immunity: Protects non-immune individuals if a significant portion of the population is immune.

• General Health: Conditions like malnutrition and overcrowding increase susceptibility.

• Age: Very young and elderly individuals are particularly vulnerable.

• Gender: Sexual and anatomical differences affect susceptibility (e.g., women at higher risk for UTIs).

• Cultural Practices: Influence exposure levels and immunity.

Epidemiological Studies

• Historical Context: John Snow (1849-1854) demonstrated vital principles by linking cholera to contaminated water, highlighting the importance of epidemiological studies.

Types of Studies

• Descriptive Studies: Analyze the who, where, and when regarding outbreaks to identify risk factors and epidemic patterns.

  • Common-source Epidemic: Rapid cases from a single exposure.

  • Propagated Epidemic: Slow increase in cases from person-to-person spread.

• Analytical Studies: Examine risk factors related to diseases and establish connections.

• Experimental Studies: Investigate cause/effect relationships between factors and disease incidence.

Learning Objectives for Chapter 19: Epidemiology

1. Define Epidemiology

Epidemiology is the study of how diseases affect the health and illness of populations. It involves the analysis of outbreak patterns, causes, and effects to inform public health strategies.

2. Definitions

• Portal of Exit: The way in which pathogens leave the host, such as through coughing, sneezing, or bodily fluids.

• Portal of Entry: The routes through which pathogens enter a new host, like the respiratory tract, skin, or mucous membranes.

• Communicable Diseases: Diseases that can be transmitted from one individual to another, such as the flu or measles.

• Non-communicable Diseases: Illnesses that cannot be spread between individuals, often caused by genetic or environmental factors, like diabetes or cancer.

3. Morbidity Rate vs. Mortality Rate

• Morbidity Rate: This measures the incidence of a disease in a population, showing how many individuals fall ill.

• Mortality Rate: This indicates the number of deaths in a population due to a specific disease, reflecting overall death rates.

4. Incidence vs. Prevalence

• Incidence: The number of new cases of a disease in a specific time period relative to the population at risk, indicating individual risk.

• Prevalence: The total number of cases (both new and existing) at a particular time, indicating the overall burden of disease in a population.

5. Definitions

• Endemic: A disease that is consistently present within a certain population or area (e.g., common colds).

• Epidemic: A sudden increase in the number of cases of a disease above what is normally expected in a specific area.

• Outbreak: A specific occurrence of disease cases in a localized area at a particular time.

• Pandemic: A global spread of a disease affecting a large number of people (e.g., COVID-19).

6. Reservoir

A reservoir is the natural habitat of a pathogen where it lives and multiplies. Examples include humans (for diseases like measles), animals (e.g., bats for rabies), or environmental sources (like water for cholera).

7. Symptomatic Infection vs. Asymptomatic Carriers

• Symptomatic Infection: Individuals who exhibit clear signs of an illness and can spread pathogens (e.g., a person with visible flu symptoms).

• Asymptomatic Carriers: Individuals who harbor and can transmit a pathogen without showing any symptoms (e.g., some carriers of hepatitis).

8. Zoonoses

Zoonoses are diseases that can be transmitted from animals to humans, such as rabies or the flu.

9. Horizontal vs. Vertical Transmission

• Horizontal Transmission: Spread of disease from person to person, or between different individuals, usually through direct or indirect contact.

• Vertical Transmission: Transfer of disease from a mother to her fetus during pregnancy or childbirth.

10. Mechanisms of Transmission

Specific transmission mechanisms include:

• Direct contact between individuals.

• Indirect contact via contaminated objects (fomites).

• Airborne transmission through droplets or aerosols.

• Vector-borne transmission through insects.

11. Definitions

• Fomites: Inanimate objects that can carry pathogens (e.g., doorknobs, utensils).

• Droplet Nuclei: Tiny droplets that can carry infectious agents and remain airborne for longer periods.

• Vectors: Living organisms that transmit pathogens, such as mosquitoes or ticks.

12. Mechanical vs. Biological Vectors

• Mechanical Vectors: Organisms that carry pathogens on their bodies without being infected themselves (e.g., flies carrying bacteria on their feet).

• Biological Vectors: Organisms that support the lifecycle of the pathogen and are infected themselves (e.g., mosquitoes that carry malaria).

13. Major Portals of Exit and Entry

Common portals of exit include:

• Respiratory tract (sneezes, coughs)

• Intestinal tract (feces)

• Skin (wounds)

• Genital tract (secretions) Common portals of entry are similar, primarily involving mucous membranes, skin, and respiratory systems.

14. Major Factors Influencing Epidemiology

Factors influencing disease epidemiology include:

• Pathogen virulence and dosage.

• Host immunity and health status.

• Age, gender, and cultural practices.

15. Types of Epidemiological Studies

• Descriptive Studies: Describe the epidemiology of disease by analyzing the who, where, and when.

• Analytical Studies: Investigate the relationship between risk factors and health outcomes.

• Experimental Studies: Test hypotheses by observing effects of controlled interventions.

16. Definitions

• Risk Factor: A characteristic that increases the likelihood of disease (e.g., smoking for lung cancer).

• Placebo: An inactive substance used as a control in clinical trials.

• Double-blind Study: A study where neither the participants nor the researchers know who receives the treatment or placebo, minimizing bias.

17. Epidemiological Approaches

• Cross-sectional Study: Observes a population at a single point in time to identify disease prevalence.

• Retrospective Case-Control Study: Compares past exposure of individuals with a disease to those without the disease.

• Prospective Study: Follows individuals over time to see who develops the disease based on exposures.

These objectives are structured to be clear and concise for easy understanding and memorization, facilitating exam preparation.