chapter14

Principles of Disease and Epidemiology

Chapter 14: Clostridium Difficile

Learning Objectives

  • 14‐1 Define pathology, etiology, infection, and disease.

  • 14‐2 Define how the human microbiome is acquired.

  • 14‐3 Compare types of symbiosis: commensalism, mutualism, and parasitism.

  • 14‐4 Contrast normal and transient microbiota with opportunistic microorganisms.

  • 14‐5 List and explain Koch’s postulates and exceptions.

  • 14‐6 Differentiate communicable from noncommunicable diseases.

  • 14‐7 Categorize diseases by frequency of occurrence.

  • 14‐8 Categorize diseases by severity.

  • 14‐9 Define herd immunity.

  • 14‐10 Identify four predisposing factors for disease.

  • 14‐11 Sequence disease patterns: period of decline, convalescence, illness, prodromal period, incubation period.

  • 14‐12 Define the reservoir of infection.

Definitions

  • Pathology: The study of disease.

  • Etiology: The study of the cause of disease.

  • Pathogenesis: The development of disease.

  • Infection: Colonization of the body by pathogens.

  • Disease: An abnormal state where the body is not functioning normally.

Human Microbiome

  • Acquired in utero and from food, people, and pets throughout life.

  • Established through the Human Microbiome Project (2007) which analyzes microbial communities and their relationship to human health.

  • Normal microbiota: Permanently colonize the host without causing disease under normal conditions.

  • Transient microbiota: Present temporarily for days, weeks, or months.

Composition Influences

  • Factors determining composition of normal microbiota include:

    • Nutrients

    • Physical and chemical factors

    • Host defenses

    • Mechanical factors

Microbial Relationships

  • Microbial antagonism: Competition between microbes for resources.

  • Symbiosis: Relationship where different organisms interact:

    • Commensalism: One benefits, the other is unaffected.

    • Mutualism: Both benefit.

    • Parasitism: One benefits at the other's expense.

Opportunistic Pathogens

  • Normal microbiota can become opportunistic pathogens.

    • Example: E. coli in the intestines can cause infections in other body sites.

Koch's Postulates

  • Koch's postulates are criteria for establishing a causative relationship between a microbe and a disease:

    1. Pathogen must be present in every case.

    2. Isolate pathogen from diseased host and grow in pure culture.

    3. Pathogen must cause disease in a healthy host.

    4. Re-isolate pathogen from the inoculated host.

  • Exceptions include pathogens causing multiple diseases, diseases exclusive to humans, and unculturable microbes.

Classifying Infectious Diseases

  • Symptoms: Patient-reported changes due to disease.

  • Signs: Measurable changes.

  • Syndrome: A recognizable complex of signs and symptoms.

Types of Diseases

  • Communicable diseases: Spread from host to host.

  • Contagious diseases: Easily spread.

  • Noncommunicable diseases: Not transmitted between hosts.

Disease Occurrence

  • Incidence: Fraction of population contracting disease over time.

  • Prevalence: Fraction of population affected at a given time.

  • Sporadic disease: Occurs occasionally.

  • Endemic disease: Constantly present.

  • Epidemic disease: Rapid spread in a locality.

  • Pandemic disease: Global epidemic.

  • Herd immunity: Immunity within a population.

Disease Severity

  • Acute: Rapid onset symptoms.

  • Chronic: Develops slowly.

  • Subacute: Symptoms between acute and chronic.

  • Latent: Period of inactivity in disease progression.

Host Involvement

  • Local infection: Confined to a small area.

  • Systemic infection: Throughout the body.

  • Focal infection: Local infection that spreads systemically.

Stages of Disease Development

  1. Incubation period: Time from infection to first symptoms.

  2. Prodromal period: Early signs appear.

  3. Period of illness: Most severe symptoms.

  4. Period of decline: Symptoms subside.

  5. Period of convalescence: Recovery to pre-diseased state.

Transmission of Disease

  • Reservoirs of Infection: Sources of infectious agents.

    • Human reservoirs: Carriers of diseases like AIDS.

    • Animal reservoirs: Zoonotic infections like rabies.

    • Nonliving reservoirs: Environmental sources such as soil.

Modes of Transmission

  • Contact Transmission: Direct, indirect, and droplet methods.

  • Vehicle Transmission: Through water, air, or food.

  • Vector Transmission: By arthropods (mechanical and biological).

Healthcare-Associated Infections (HAIs)

  • Infections acquired during hospital treatment, significant in healthcare settings.

  • Result from in-hospital microorganisms, host susceptibility, and transmission chains.

  • Compromised host: At risk due to weakened immune response.

Prevention of HAIs

  • Implement universal precautions; ensure hygiene and cleanliness practices.

  • Establish infection control protocols in healthcare facilities.

Emerging Infectious Diseases

  • New or increasing incidences of diseases (e.g., AIDS, SARS).

  • Contributing factors include genetic changes, environmental shifts, and human behaviour.

Epidemiology

  • Study of disease spread, determining causes, and developing control methods.

  • Three types are descriptive, analytical, and experimental epidemiology.

  • Centers for Disease Control and Prevention (CDC): Monitors public health and publishes disease data.