Principles of Disease and Epidemiology

Principles of Disease and Epidemiology

Pathology, Infection, and Disease

  • Pathology: The study of disease.
  • Etiology: The cause of a disease.
  • Pathogenesis: The manner in which a disease develops.
  • Infection: Invasion or colonization of the body by pathogens.
  • Infectious disease: Occurs when an infection results in any change in the state of health.
    • An infection may exist in the absence of a detectable disease.
    • Disease may result when a particular type of microorganism locates in a part of the body where it is not normally found.

Koch’s Postulates

  • Koch’s postulates are used to prove the cause of an infectious disease.
  • The same pathogen must be present in every case of the disease.
  • The pathogen must be isolated from the diseased host and grown in pure culture.
  • The pathogen from the pure culture must cause the disease when it’s inoculated into a healthy, susceptible laboratory animal.
  • The pathogen must be isolated from the inoculated animal and must be shown to be the original organism.

Exceptions to Koch’s Postulates

  • Some pathogens can cause several disease conditions.
  • Some pathogens cause disease only in humans.
  • Some microbes have never been cultured.
  • Several different pathogens may cause the same signs and symptoms.

Classifying Infectious Diseases

  • Symptoms: Subjective changes in body function that are felt by a patient as a result of disease.
    • Not apparent to an observer.
  • Signs: Objective changes in a body that can be measured or observed as a result of disease.
  • Syndrome: A specific group of signs and symptoms that accompany a disease.

Communicable vs. Noncommunicable Diseases

  • Communicable disease: A disease that is spread from one host to another.
    • Examples: COVID-19, chicken pox, measles, influenza, genital herpes, tuberculosis.
  • Contagious diseases: Diseases that are easily and rapidly spread from one host to another.
  • Noncommunicable disease: A disease that is not spread from one host to another.
    • Example: tetanus.

Occurrence of a Disease

  • Incidence: Number of people who develop a disease during a particular time period.
  • Prevalence: Number of people who have a disease at a specified time, regardless of when it first appeared.
    • Takes into account both old and new cases.
  • Sporadic disease: Disease that occurs only occasionally.
  • Endemic disease: Disease constantly present in a population.
  • Epidemic disease: Disease acquired by many people in a given area in a short time.
  • Pandemic disease: Worldwide epidemic.

Severity or Duration of a Disease

  • Duration: Average time that individuals have a disease from diagnosis until they are either cured or die.
  • Acute disease: Symptoms develop rapidly but have a short duration.
  • Chronic disease: Symptoms develop slowly, likely to last for a long period.
  • Subacute disease: Intermediate between acute and chronic.
  • Latent disease: Causative agent is inactive for a time but then activates and produces symptoms.
  • Herd immunity: Immunity in most of a population.
  • Severity: The presence and extensiveness of a disease in the body and its ability to cause death. *Examples of COVID-19 disease severity:
    • Asymptomatic: No signs or symptoms.
    • Mild: Fever, dry cough, tired, muscle pain, sore throat.
    • Moderate: Breathlessness, tachycardia, persistent cough, higher fever.
    • Severe: Pneumonia, extreme breathlessness, chest pain, high temperature, bluish lips/face.
    • Critical: Severe acute respiratory syndrome (SARS), inflamed alveoli, may require ventilator
      • Sepsis: Extreme inflammatory syndrome in response to severe infection.

Infection Fatality Ratio (IFR) and Case Fatality Ratio (CFR)

  • Infection fatality ratio (IFR): Divide the number of deaths attributed to a disease by the total number of infected individuals within a specific time period
    • IFR=Number of deathsTotal number of infected individualsIFR = \frac{\text{Number of deaths}}{\text{Total number of infected individuals}}
    • The lower the IFR, the lower the number of fatalities.
  • Case fatality ratio (CFR): Proportion of individuals diagnosed with a disease who die from that disease within a certain period of time.
    • CFR=Number of deaths from diagnosed casesTotal number of diagnosed casesCFR = \frac{\text{Number of deaths from diagnosed cases}}{\text{Total number of diagnosed cases}}

Extent of Host Involvement

  • Local infection: Pathogens are limited to a small area of the body.
  • Systemic (generalized) infection: An infection spread throughout the body by the blood and lymph.
  • Focal infection: Systemic infection that began as a local infection.
  • Sepsis: Toxic inflammatory condition arising from the spread of microbes, especially bacteria or their toxins, from a focus of infection.
  • Bacteremia: Bacteria in the blood.
  • Septicemia: Also known as blood poisoning; growth of bacteria in the blood; bacteria are proliferating in the blood.
  • Toxemia: Toxins in the blood.
  • Viremia: Viruses in the blood.
  • Primary infection: Acute infection that causes the initial illness.
  • Secondary infection: Opportunistic infection after a primary (predisposing) infection.
  • Subclinical infection: No noticeable signs or symptoms (inapparent infection, asymptomatic infection).

Predisposing Factors

  • Variables that make the body more susceptible to disease or may alter the course of a disease
    • Nutrition
    • Sex
    • Genetic inheritance
    • Climate
    • Environment
    • Vaccination
    • Age
    • Lifestyle/behaviors
    • Compromised host

Development of Disease

  • Incubation period: Interval between initial infection and first signs and symptoms.
  • Prodromal period: Short period after incubation; early, mild nonspecific symptoms.
  • Period of illness: Disease is most severe.
  • Period of decline: Signs and symptoms subside.
  • Period of convalescence: Body returns to its prediseased state; recovery.

Reservoirs of Infection

  • Continual sources of infection
    • Human reservoirs
      • People with signs and symptoms Or
      • Carriers may have inapparent infections or latent diseases
        • Asymptomatic carriers
        • Incubating carriers
        • Convalescent carriers
        • Chronic carriers
        • Passive carriers
    • Animal reservoirs
      • Zoonoses are diseases primarily in wild and domestic animals that can be transmitted to humans
    • Nonliving reservoirs
      • Soil and water
      • Foods

Contact Transmission

  • Direct contact transmission: Requires close association between the infected and a susceptible host.
  • Congenital transmission: Transmission from mother to fetus or newborn at birth.
  • Indirect contact transmission: Spreads to a host by a nonliving object called a fomite.
  • Droplet transmission: Transmission via airborne droplets less than 1 meter.

Vehicle Transmission

  • Transmission by an inanimate reservoir
    • Airborne
    • Waterborne
    • Foodborne
      • May involve cross-contamination (transfer of pathogens from one food to another)

Vectors

  • Arthropods, especially fleas, ticks, and mosquitoes
  • Transmit disease by two general methods
    • Mechanical transmission: Arthropod carries pathogen on its feet
    • Biological transmission: Pathogen reproduces in the vector; transmitted via bites or feces

Healthcare-Associated Infections (HAIs)

  • Acquired while receiving treatment in a health care facility
    • Also known as nosocomial infections
  • Affect 1 in 31 hospital patients in the United States
    • 700,000 per year infected; over 70,000 deaths

Causes of Healthcare-Associated Infections

  • HAIs result from:
    • Microorganisms in the hospital environment
    • Weakened status of the host
    • Chain of transmission in a hospital
  • Compromised host: An individual whose resistance to infection is impaired by disease, therapy, or burns

Microorganisms in the Hospital

  • Hospitals are a major reservoir for a variety of pathogens
  • Certain normal microbiota are opportunistic and pose a risk
  • Antimicrobial resistance is high among HAIs
  • Clostridium difficile is a leading cause of HAIs

Compromised Host

  • Individual whose resistance to infection is impaired by disease, therapy, or burns
    • Broken skin or mucous membranes
    • Suppressed immune system
    • Invasive procedures and devices

Control of Healthcare-Associated Infections

  • Universal precautions
    • Designed to reduce the transmission of microbes in health care and long-term care settings
    • Protect patients, residents, staff, visitors from contact with pathogens
  • Standard precautions: Basic, minimum practices
    • Applied to every person, every time, all levels of heathcare
    • Include hand hygiene, personal protective equipment, respiratory hygiene, cough etiquette, disinfection of equipment, environmental cleaning and disinfection, safe infection practices, patient placement
  • Transmission-based precautions: Supplemental to standard precautions; designed for known or suspected infections that are highly transmissible or involve epidemiologically important pathogens
    • Three categories:
      • Contact precautions
      • Droplet precautions
      • Airborne precautions
  • Reduce number of pathogens
    • Handwashing
    • Disinfecting tubs used to bathe patients
    • Cleaning instruments scrupulously
    • Using disposable bandages and intubation equipment
  • Improve patients’ resistance to infection
    • Prescribe antibiotics only when necessary
    • Avoid invasive procedures
    • Minimize use of immunosuppressive drugs
  • Infection control committees in hospitals oversee and monitor infection control

Emerging Infectious Diseases

  • Diseases that are new or changing, increasing in incidence, or showing a potential to increase in the near future
  • Most (75%) are zoonotic, primarily of viral origin, and likely to be vector-borne

Contributing Factors to Emerging Infectious Diseases

  • Genetic recombination between organisms
    • Escherichia coli O157:H7 and avian influenza (H5N1)
  • Evolution of existing organisms
    • Vibrio cholerae O139
  • Widespread use of antibiotics and pesticides
    • Antibiotic-resistant strains
  • Inherent genetic instability of some microbes
    • Characteristic of RNA viruses
    • Antigenic shift and drift
  • Changes global climate and weather patterns
    • Hantavirus
  • Modern transportation
    • Zika virus, Chikungunya, and West Nile encephalitis
  • Insect vectors transported to new areas where they become established
    • Aedes aegypti, A. albopictus
  • Ecological disaster, war, and expanding human settlement
    • Coccidioidomycosis
  • Animal control measures
    • Lyme disease
    • Rat lungworm in invasive tree frogs in Florida
  • Public health failure
    • Diphtheria
  • Bioterrorism

Epidemiology

  • The study of where and when diseases occur and how they are transmitted in populations

Evolution of Epidemiology

  • John Snow (1848–1849): Mapped the occurrence of cholera in London
  • Ignaz Semmelweis (1846–1848): Showed that handwashing decreased the incidence of puerperal sepsis
  • Florence Nightingale (1858): Showed that improved sanitation decreased the incidence of epidemic typhus

Roles of Epidemiology

  • Determine etiology of a disease
  • Identify other important factors and patterns concerning the spread of disease
  • Assemble data and graphs to outline incidence of disease
  • Predict the probability than an infection will spread through a population
    • Reproductive number: Average number of people who will contract a disease from one infected individual
  • Explore various methods for controlling a disease

Types of Epidemiology

  • Descriptive epidemiology: Collection and analysis of data
    • Snow’s search for the source of the cholera outbreak
  • Analytical epidemiology: Analyzes a particular disease to determine its probable cause or risk factors
    • Nightingale’s work
  • Experimental epidemiology: Involves a hypothesis and controlled experiments
    • Semmelweis
    • Clinical trial: test and control group

Case Reporting

  • Enables researchers to establish the chain of transmission
  • Requires health care workers to report specified diseases to local, state, and national health officials
  • Notifiable Infection Diseases list
  • Provide early warning of possible outbreaks

The Centers for Disease Control and Prevention (CDC)

  • Collects and analyzes epidemiological information in the United States
  • Publishes Morbidity and Mortality Weekly Report (MMWR)
    • Morbidity: Incidence of a specific notifiable disease
    • Mortality: Deaths from notifiable diseases
  • Articles include reports of disease outbreaks, case histories, and summaries of the status of particular disease
  • Notifiable infectious diseases: Diseases in which physicians are required to report occurrence
  • Morbidity rate: Number of people affected in relation to the total population in a given time period
  • Mortality rate: Number of deaths from a disease in relation to the total population in a given time period