Disease Detectives

• Contact tracing interviews people who have been infected to identify others who may have been exposed.
• Participants use investigative skills in the scientific study of disease, injury, health, and disability in populations or groups of people
• Epidemiology:

  • Study of distribution and determinants of health-related states in specified populations
• Two Basic Types of Epidemiology:

  • Classical Epidemiology: population-oriented, studies community origins of health problems
  • Clinical Epidemiology: studies patients in healthcare settings to improve diagnosis and treatment
  • Further divided into Infectious Disease Epidemiology and Chronic Disease Epidemiology
• Basic Epidemiology Terms:

  • Asymptomatic: Showing no signs or symptoms, can be a carrier of disease
  • Cluster: An aggregation of cases closely grouped in time and space
  • Elimination: Reduction to zero of a disease in a defined geographical area
  • Endemic Disease: Present at a continuous level throughout a population/geographic area
  • Epidemic: Large numbers of people over a wide geographical area are affected
  • Etiology: Study of the cause of a disease
  • Fomite: A physical object that transmits an infectious agent
  • Iatrogenic: An illness caused by medication or a physician
• • Incubation Period: Time between contact with a pathogen and showing symptoms
  • Index Case: First patient in an epidemiological study
  • Morbidity: Rate of disease in a population
  • Mortality: Rate of death in a population
  • Nosocomial Disease: An infection acquired in a hospital
  • Outbreak: More cases of a disease than expected in a given area or among a specialized group
  • Pandemic: An epidemic occurring over several countries or continents affecting a large proportion of the population
  • Plague: A serious infectious disease transmitted by rodent fleas
• • Risk: The probability that an individual will be affected by an illness or injury
  • Surveillance: Systematic collection, analysis, interpretation, and dissemination of health data
• Incidence, Prevalence, and Duration:

  • Incidence: Number of new instances of disease in a population over a given time period
  • Prevalence: Number of affected persons in the population at any given point in time
  • Period prevalence and point prevalence are major ways to measure prevalence
  • Incidence includes time, prevalence does not
  • Incidence * Duration = Prevalence
• Thirteen Steps to Investigating an Outbreak:

  • Prepare for field work
  • Establish the Existence of an Outbreak
  • Verify the Diagnosis
  • Construct a Working Case Definition
  • Find Cases Systematically and Record Information
  • Describe and Orient the Data in Terms of Person, Place, and Time
  • Develop Hypotheses (Agent/Host/Environment Triad)
  • Evaluate Hypotheses
  • Refine Hypotheses if Necessary
  • Compare and Reconcile with Laboratory and/or Environmental Studies
  • Implement Control and Prevention Measures
  • Initiate or Maintain Surveillance
  • Communicate Findings
• Purpose of Surveillance:

  • Gain knowledge of patterns of disease, injury, or health problems in a community for prevention and control purposes
  • Influence public health decisions and evaluate control measures
• Types of Surveillance:

  • Passive: Diseases reported by healthcare providers
  • Active: Health agencies contact health providers seeking reports
  • Syndromic: Monitoring the syndrome of the disease as a proxy
  • Sentinel: Professionals report health events to health agencies representing a specific geographic area or group
• Hill's Criteria for Causation:

  • Nine criteria to establish a cause-and-effect relationship
  • Criteria include Strength of Association, Consistency, Specificity, Temporality, Dose-Response Relationship, Biological Plausibility, Experimental Evidence, Coherence
• Hill's Criteria for Causation:

  • Criteria designed to establish a causal relationship between a causative microbe and a disease
  • The microbe must be present in abundance in all cases of the disease, but not in healthy organisms
  • The microbe must be isolated from the diseased organism and grown in pure culture
  • The cultured microorganism should cause disease when introduced into a healthy organism
  • The microbe must be reisolated from the inoculated, diseased experimental host and identified as identical to the original specific causative agent
• Evan's Postulates:

  • The prevalence of the disease should be significantly higher in those exposed to the risk factor than those not
  • Exposure to the risk factor should be more frequent among those with the disease
  • In prospective studies, the incidence of the disease should be higher in those exposed to the risk factor
  • The disease should follow exposure to the risk factor with a normal or log-normal distribution of incubation periods
  • A spectrum of host responses along a logical biological gradient from mild to severe should follow exposure to the risk factor
• • A measurable host response should follow exposure to the risk factor in those lacking a response before the exposure or increase the response in those with a response before exposure
  • A host response should be infrequent in those not exposed to the risk factor
  • In experiments, the disease should occur more frequently in those exposed to the risk factor than in the control group
  • Reduction or elimination of the risk factor should reduce the risk of disease
  • Modifying or preventing host response should eliminate or decrease disease
• • All findings should make biological and epidemiological sense
• Types of Carriers/Vectors:

  • Convalescent: Humans capable of spreading disease after a period of illness
  • Incubatory: Transmit pathogens immediately following infection but prior to developing symptoms
  • Chronic: Can transmit a disease for a long period
  • Genetic: Inherited disease trait but shows no symptoms
  • Transient/Temporary: Can transmit an infectious disease for a short time
• Epidemiological Triads:

  • Foundation for descriptive epidemiology: person, place, and time
  • Chain of Transmission Triad:
  • An external agent
  • A vector or fomite that transmits the disease
  • A susceptible host for the disease
• Epidemiological Studies:

  • Basic Studies
  • Ecological: Comparisons of geographical locations
  • Cross-Sectional: Survey, health questionnaire, "snapshot in time"
  • Case-Control: Compare people with and without disease to find common exposures
  • Cohort: Compare people with and without exposures to see what happens to each
  • Randomized Controlled Trial: Human experiment that randomly assigns participants to an experimental or control group
• • Quasi Experiments: Research similarities with traditional experimental design or RCT, but lack element of random assignment to treatment/control; participants are assigned a group based on non-random criteria
• Advantages and Disadvantages to Study Designs:

  • Trial:
  • Most scientifically sound
  • Best measure of exposure
  • Time-consuming
  • Unethical for harmful exposures
  • Most expensive
  • Cohort Study:
  • Most accurate observational study
  • Good measure of exposure
  • Correct time sequence
  • Good for rare exposures
  • Easy risk calculation
  • Time-consuming
  • Expensive
  • Bad for rare diseases
  • Possible loss of follow-up
  • Case-Control Study:
  • Can study rare diseases
  • Relatively less expensive and relatively fast
  • Good for rare diseases
  • Good for long latency periods
  • Possible time-order confusion
• • Error in recalling exposure
  • Only 1 outcome
  • Cross-Sectional Study:
  • Fastest
  • Least expensive
  • Good for more than 1 outcome
  • Possible time-order confusion
  • Least confidence in findings
• 2x2 Table:

  • Used to calculate odds ratio and relative risk
  • Allows comparisons between case and control groups
  • P-value is the measure of confidence in findings
• Using Epi-Curves:

  • Histogram showing the course of an outbreak by plotting the number of cases according to time of onset
  • Point source epidemics
  • Continuous common source epidemics
  • Propagated (progressive source) epidemics
  • Intermittent source epidemics
• Validity of Study Results: Error and Bias:

  • Random error: Result of fluctuations around a true value due to sample population
  • Precision is inversely related to random error
  • Systematic error: Any error other than random error, consistent and repeatable
  • Selection bias: Participants selection affected by an unknown variable associated with exposure and outcome
  • Information bias: Bias introduced through error in measurement or observation
  • Confounding bias: Bias resulting from mixing effects of several factors
• Disease and Disease Transmission:

  • Natural History and Spectrum of Disease
  • Incubation period for infectious diseases, latency period for chronic diseases
  • Subclinical disease stage
  • Clinical disease stage
  • Spectrum of disease
  • Chain of Infection:
  • Agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host
• Portals of Exit:

  • Infected animals being butchered or infected persons undergoing surgery
  • Skin, gastrointestinal tract, and respiratory tract
• Modes of Disease Transmission:

  • Contact, vehicle, and vector
  • Contact transmission includes direct, indirect, and droplet
  • Direct contact: touching, kissing, dancing
  • Indirect contact: via fomites, which can be almost anything an infected individual touches
  • Droplet transmission: coughed, sneezed, or spit on
  • Vehicle transmission: via food, air, and liquid
  • Airborne transmission: via droplets or dust particles
  • Food-borne transmission: pathogens found in food not killed during processing
  • Water-borne transmission: via fecal contaminated water
• • Vector transmission: via arthropods like mosquitoes, flies, lice, ticks
• Portals of Entry:

  • Mucous membrane of the respiratory tract
  • Preferred portals of entry for many pathogens
  • Many pathogens unable to cause disease if their usual portal of entry is bypassed
• Susceptible Host:

  • A person who cannot resist a microorganism invading the body, multiplying, and resulting in infection
• Chain of Infection:

  • Characteristics of Agents:
  • Infectivity: capacity to cause infection in a susceptible host
  • Pathogenicity: capacity to cause disease in a host
  • Virulence: severity of disease that the agent causes to host
• Immunity:

  • Active Immunity: occurs when exposed to a live pathogen, develops the disease, and becomes immune
  • Passive Immunity: short-term immunization by injection of antibodies not produced by recipient's cells
  • Herd Immunity: protecting a whole community from disease by immunizing a critical mass of its populace
• Prevention:

  • Primordial prevention: intervention at the very beginning to avoid the development of risk factors
  • Primary prevention: early intervention to avoid initial exposure to agent of disease
  • Secondary prevention: during latent stage, screening and treatment may prevent progression to symptomatic disease
  • Tertiary prevention: during symptomatic stage, intervention may arrest, slow, or reverse progression of disease
  • Quaternary prevention: set of health activities to mitigate or avoid consequences of unnecessary/excessive intervention of the health system
• Foodborne Illness:

  • FAT TOM: food, acidity, time, temperature, oxygen, and moisture
  • Prevention tactics include cooking meat, poultry, and eggs thoroughly, avoiding cross-contamination, chilling leftovers promptly, washing produce, and reporting suspected foodborne illnesses
• Population is the entire set under study
• Statistics use sampling to take a subset of the population called a sample and make inferences about the population as a whole
• Population parameter is a characteristic of a population, while sample statistic is an attribute of a sample
• Distributions are characterized by center, shape, and spread
• Central tendency is a "typical" or "middle" value for a distribution