ch 14

Pathology, Infection, and Disease

  • Pathology: The study of disease. This field encompasses various areas:
    • Etiology: The cause of disease.
    • Pathogenesis: The manner in which a disease develops.
    • Effect of Disease: The impact of disease on the body.
  • Infection vs Disease:
    • Infection: The presence of a pathogenic microbe in the body.
    • Disease: Occurs when tissue damage results from infection.

Normal Microbiota and the Host

  • At birth, infants become colonized by microorganisms.
  • Colonization occurs in several body areas, notably:
    • Lower intestine
    • Upper respiratory tract
  • Normal Flora:
    • Permanent residents: Always present, generally non-pathogenic.
    • Transient flora: Temporarily present.
  • Symbiosis: Interactions between normal microbiota and the host.
    • Types of symbiosis include:
    • Commensalism: One benefits without affecting the other (e.g., Staphylococcus epidermidis).
    • Mutualism: Both benefit (e.g., E. coli producing vitamins in intestines).
    • Parasitism: One benefits at the other's expense (e.g., pathogens).

Opportunistic Microorganisms

  • Pathogens that only cause disease under favorable conditions.
  • Example: Staphylococcus aureus causing toxic shock syndrome.

Normal Flora Interactions

  • Synergism: Combined effects of organisms that enhance the outcome (e.g., Trichomonas vaginalis increasing susceptibility to HIV).
  • Cooperative interactions in microbial communities.

Classifying Infectious Diseases

  • Symptoms: Subjective changes felt by patients (e.g., pain).
  • Signs: Objective changes that can be measured (e.g., fever).
  • Syndrome: A group of signs and symptoms associated with a specific disease.
  • Types of Diseases:
    • Communicable: Spread from host to host (e.g., influenza).
    • Noncommunicable: Not transmitted between hosts (e.g., tetanus).

Occurrence of a Disease

  • Incidence: Fraction of the population contracting a disease during a time period.
  • Prevalence: Fraction of a population with a specific condition at a time.
  • Disease classifications include:
    • Sporadic Disease: Rare occurrences.
    • Endemic Disease: Constant presence (e.g., Neisseria gonorrhoeae).
    • Epidemic Disease: Rapid increase in incidence across a region.
    • Pandemic Disease: Worldwide spread (e.g., COVID-19).

Severity or Duration of a Disease

  • Acute Disease: Rapid onset and short duration (e.g., flu).
  • Chronic Disease: Develops slowly and lasts long (e.g., TB).
  • Asymptomatic Disease: No noticeable symptoms; person may be a carrier.
  • Latent Disease: Inactive period where symptoms are absent (e.g., shingles).

Extent of Host Involvement

  • Local Infection: Limited to a small body area (e.g., a boil).
  • Systemic Infection: Throughout the body (e.g., chicken pox).
  • Focal Infection: Multiple but defined areas impacted (e.g., tuberculosis).
  • Sepsis/Septicemia: Infection and inflammation due to microbes in the blood.

Stages of Disease Development

  • Incubation Period: Time between infection and symptom onset.
  • Prodromal Period: Mild symptoms appear after incubation.
  • Invasive Phase (Illness): Major symptoms evident, possibly fulminating infection.
  • Period of Decline: Symptoms lessen, increased risk of secondary infection.
  • Convalescent Period: Recovery and strong immune response, though at risk of secondary infection.

Spread of Infection

  • Infectious Agent Sources: Need a source to maintain infectivity, often found in hosts.
  • Entry and Exit Portals:
    • Entrances: Mucous membranes (respiratory, GI, urinal), skin, parenteral routes.
    • Exits: Respiratory, GI, and genitourinary tracts, skin, blood.

Reservoirs of Infection

  • Reservoir: Continuous source of pathogens, which can be living (human/animal) or nonliving (water, soil).

Transmission of Disease

  • Methods of Transmission:
    • Contact: Direct (close association) or indirect (via fomites).
    • Droplet: Via airborne droplets.
    • Vehicle: Inanimate reservoir transmission (e.g., water/food).
    • Vectors: Arthropods transmitting via mechanical or biological means.

Nosocomial Infections

  • Infections acquired in healthcare settings, affecting 5-15% of patients.
  • Common types include MRSA, CRE, and C-DIF.
  • Involves patient-to-patient spread, and requires strict infection controls.

Emerging Infectious Diseases

  • New or increasing diseases influenced by:
    • Genetic changes
    • Antibiotic misuse
    • Climate change
    • Human expansion and transportation.

Epidemiology

  • Study of disease occurrence and transmission patterns.
  • Key metrics:
    • Morbidity: Disease incidence.
    • Mortality: Disease-related deaths.

Historical Figures in Epidemiology

  • John Snow: Mapped cholera occurrence in London, demonstrated the water pump's role in transmission.
  • Ignaz Semmelweis: Advocated handwashing to decrease infection rates.
  • Florence Nightingale: Improved sanitation practices affecting health.

Cholera and Its Outbreaks

  • Caused by Vibrio cholerae, transmitted through contaminated water.
  • Severe dehydration can occur, leading to high mortality without treatment.
  • Historical outbreaks highlight sanitation's critical role in disease control (e.g., Goma Refuge Camp, 1994).
  • Cholera leads to characteristic "rice water" stool in patients.

The Broad Street Pump Incident

  • John Snow's intervention in 1854 involving the pump handle removed due to contamination concerns.
  • The true cause was later traced to a cesspit leaking into the water source, illustrating the importance of sanitary practices.