Principles of Disease and Epidemiology

Chapter 14: Principles of Disease and Epidemiology

Learning Objectives

  • Define Epidemiology & Function of the CDC

    • Epidemiology is the study of where and when diseases occur and how they are transmitted in populations.
    • The Centers for Disease Control and Prevention (CDC) collects and analyzes epidemiological information in the United States, including local, state, and national data.
    • The CDC publishes the Morbidity and Mortality Weekly Report (MMWR).

  • List Reasons for Emerging Infectious Diseases (EIDs)

    • Diseases that are new, increasing in incidence, or showing a potential to increase in the near future.
    • Contributing factors include genetic recombination, evolution of new strains, widespread use of antibiotics and pesticides, changes in weather, modern transportation, insect vectors, ecological disasters, war, expanding human settlements, public health failures, and bioterrorism.

  • List Different Reservoirs of Infection and Routes for Disease Transmission

    • Reservoirs of Infection:
    1. Human Reservoirs: Carriers may have inactive infections or latent diseases.
    2. Animal Reservoirs: Zoonoses, diseases transmitted from animals to humans.
    3. Nonliving Reservoirs: Soil and water.
    • Modes of Transmission:
    1. Contact Transmission: Direct or indirect contact.
    2. Vehicle Transmission: Transmission by inanimate reservoirs (water, food).
    3. Vector-borne Transmission: Transmission by vectors (arthropods).

  • List Predisposing Factors for Diseases

    • Predisposing factors include mental health (stress and exhaustion), gender, inherited traits, climate and weather, lack of vaccination, fatigue, age, lifestyle, nutrition (inadequate), and chemotherapy (immune compromise).

  • Define Terms for Classifying Occurrence of Diseases

    • Incidence: Number of new cases during a particular time period.
    • Prevalence: Total number of existing cases (both old and new).
    • Sporadic Disease: Occurs occasionally.
    • Endemic Disease: Constantly present in a population.
    • Epidemic Disease: Acquired by many in a given area in a short time.
    • Pandemic Disease: Worldwide epidemic affecting multiple continents.

  • List Koch's Postulates and Exceptions to the Postulates

    • Koch's Postulates:
    1. Same pathogen present in every case of disease, absent from healthy hosts.
    2. Pathogen must be isolated and grown in pure culture.
    3. Pathogen from pure culture must cause disease when inoculated into susceptible lab animal.
    4. Pathogen must be isolated from inoculated animal, shown to be original organism.
    • Exceptions:
    • Some pathogens can cause several disease conditions.
    • Some diseases may not show symptoms.
    • Some pathogens cause disease only in humans and have never been cultured.

  • Define Pathology, Etiology, Infection, and Disease

    • Pathology: The study of disease, including etiology (the cause) and pathogenesis (development of disease).
    • Infection: Invasion and colonization of pathogens in the body or host.
    • Disease: An abnormal state in which the body is not performing normal functions; pathogens that cause diseases are called pathogens.
    • Human Microbiome: Includes both normal (permanently colonizing) and transient (temporary) microbiota; contributes to health and disease.

Human Microbiome

Overview
  • Microbiome Definitions:
    • The microbiome consists of the collection of microorganisms living in and on the body.
    • Generally includes a diverse array of microbial species which can be classified as good (beneficial) or bad (pathogenic).
Colonization
  • Microbes begin to colonize the body immediately after birth.
  • Sources include food, other people, and pets.
  • Colonization can be temporary or permanent.
    • Normal Microbiota: Permanently colonize the host without causing disease under normal conditions.
    • Transient Microbiota: Present for days, weeks, or months before disappearing.
Fun Facts
  • The human body is home to approximately 100 trillion microbes.
  • Each person's microbiome is unique, akin to a fingerprint.
  • Microbial genes outnumber human genes by a ratio of 150:1.
Relationships Between Normal Microbiota and Host
  • Symbiosis Types:
    1. Commensalism: One organism benefits while the other is unaffected.
    • Example: Staphylococcus epidermidis on the skin.
    1. Mutualism: Both organisms benefit.
    • Example: E. coli in the large intestine.
    1. Parasitism: One organism benefits at the expense of the other.
    • Example: Influenza viruses infecting a host cell.
  • Opportunistic Pathogens: Normal microbiota that do not cause disease under normal conditions but can cause disease under special circumstances.
Microbial Antagonism
  • Microbial Antagonism (Competitive Exclusion): The competition between microbes for resources.
  • Normal microbiota help protect the host by:
    1. Competing with other microbes for nutrients.
    2. Producing substances harmful to invading microbes.
    3. Modifying environmental conditions such as pH and available oxygen.

Pathology, Infection & Disease

Pathology Definitions
  • Pathology: The field studying disease, which includes:
    • Etiology: The cause of diseases.
    • Pathogenesis: The manner of development of diseases.
  • Infection: Involves the invasion and colonization of the body by pathogens leading to growth.
  • Disease: An abnormal state wherein the body does not perform normal functions.
  • Pathogens are microorganisms causing disease and include viruses, bacteria, fungi, and parasites.
Components of Disease Dynamics
  • Infection in Host (Factors Involved):
    • Pathogen: Various types like viruses, bacteria, fungi, and parasites.
    • Environment: Settings that facilitate infection (clinics, homes, travel, etc.).

Etiology of Infectious Diseases

Koch’s Postulates
  • Koch's Postulates developed by Robert Koch are a framework to demonstrate that a specific germ causes a specific disease:
  1. The same pathogen must be present in every case of the disease and absent from any healthy organism.
  2. The pathogen must be isolated from the diseased host and grown in pure culture.
  3. The cultured pathogen must cause the disease when introduced into a healthy, susceptible lab animal.
  4. The pathogen must be re-isolated from the inoculated animal and shown to be the same as the original organism.
Exceptions to Koch's Postulates
  • Not all pathogens fit Koch's criteria:
    • Some can lead to multiple disease conditions.
    • Certain diseases do not show clear symptoms.
    • Some pathogens uniquely affect human hosts.
    • Some pathogens cannot be cultured outside of the host.

Classifying Infectious Diseases

Terminology and Definitions
  • Symptoms: Changes felt by the patient due to disease (e.g., body pain).
  • Signs: Observable or measurable changes (e.g., elevated liver enzymes).
  • Syndrome: A specific group of signs and symptoms occurring together (e.g., Acquired Immunodeficiency Syndrome (AIDS)).
  • Communicable Disease: Spreadable from one host to another.
  • Contagious Diseases: Easily spreadable from host to host.
  • Noncommunicable Disease: Not spreadable from one host to another (e.g., tetanus).
Occurrence of Disease
  • Incidence: Number of new disease cases during a specified period.
  • Prevalence: The total number of existing disease cases.
    • Sporadic Disease: Occurs occasionally.
    • Endemic Disease: Constantly present in a population at a stable rate.
    • Epidemic Disease: Many individuals acquire the disease in a short period.
    • Pandemic Disease: Worldwide epidemic spanning many continents.
Patterns of Disease: Predisposing Factors
  • Factors that increase susceptibility:
  1. Mental Health (stress, exhaustion)
  2. Gender
  3. Inherited traits (e.g., sickle cell gene)
  4. Climate and weather
  5. Lack of vaccinations
  6. Fatigue
  7. Age
  8. Lifestyle choices
  9. Nutrition
  10. Chemotherapy leading to immune compromise.
Development of Disease Phases
  • Incubation Period: The time between infection and first signs/symptoms.
  • Prodromal Period: Early stage after incubation showing mild symptoms.
  • Period of Illness: Most severe stage of the disease.
  • Period of Decline: Symptoms begin to subside.
  • Period of Convalescence: Recovery as the body returns to its normal state.

The Spread of Infection

Reservoirs of Infection
  • Definition: Continual sources of infection.
  1. Human Reservoirs: Carriers may harbor inactive infections.
  2. Animal Reservoirs: Zoonotic diseases that cross over from animals to humans.
  3. Nonliving Reservoirs: Environments like soil and water harbor pathogens.
Modes of Transmission
  1. Contact Transmission: Involves direct or indirect contact between hosts.
    • Types include:
      • Direct Contact: Close association between infected and susceptible hosts.
      • Congenital Transmission: From mother to fetus/newborn.
      • Indirect Contact: Through nonliving objects (fomites).
      • Droplet Transmission: Mucus/saliva spread through coughing/sneezing at close range.
      • Airborne Transmission: Particles in the air traveling over distances greater than 1 meter.
  2. Vehicle Transmission: Spreading pathogens via inanimate materials (e.g., water, food).
  3. Vector-borne Transmission: Involves arthropods that transmit diseases either mechanically or biologically.
Vector-borne Diseases: Transmission Methods
  1. Mechanical Transmission: Pathogen carried on vector's exoskeleton.
  2. Biological Transmission: Pathogen undergoes a life cycle in the vector and is transmitted via bites or feces.

Healthcare-Associated Infections (HAIs)

Overview
  • Definition: Infections acquired while receiving treatment in health care facilities (e.g., hospitals, clinics).
  • Impact: Affects roughly 1 in 25 hospital patients, leading to approximately 2 million infections and over 70,000 deaths per year.
Causes of HAIs
  1. Presence of microorganisms in the hospital environment.
  2. Weakened hosts due to treatments or conditions.
  3. Chain of transmission via direct contact or fomites.
Control Measures for HAIs
  • Aseptic Techniques:
    • Handwashing, disinfecting equipment, cleaning instruments, and using disposable items also contribute to infection control.
  • Infection Control Committees: Hospital staff oversee the necessary precautions to prevent HAIs, ensuring proper cleaning, storage, and use of medical equipment.

Emerging Infectious Diseases (EIDs)

Definition
  • EIDs: Diseases that are either new, increasing in number, or have the potential to increase in the near future.
  • Most are zoonotic and viral in origin, often transmitted by vectors.
Contributing Factors for EIDs
  • Genetic Recombination: Such as in Escherichia coli O157, causing virulence changes.
  • Evolution of New Strains: For example, Vibrio cholerae O139.
  • Antibiotic Resistance: Resulting from overuse of antibiotics and pesticides.
  • Ecological Changes: Changes in weather patterns leading to diseases like hantavirus.
  • Modern Transportation: Rapid spread of diseases like Zika virus and West Nile virus.
  • Public Health Fails: Lack of response to health threats can allow for the resurgence of diseases.
Summary
  • Emerging Infectious Diseases are a significant public health concern due to evolving pathogens and changing environmental factors that contribute to their emergence and spread.

Epidemiology

Definition
  • Epidemiology: Branch of medicine that studies how diseases occur, are spread, and how they can be controlled in populations.
Role of Epidemiologists
  1. Determine the etiology and important factors affecting the spread of diseases.
  2. Develop methods for controlling diseases.
  3. Collect and analyze data regarding disease incidence and prevalence.