8.1 What is Epidemiology?
- Epi means 'on or upon'
- Demos means "people, "population"
- Epidemiology is the study of how diseases affect whole communities.
- This chapter expands upon the triad model of disease causation.
Epidemiology Definitions
- Endemic: diseases that persist at a moderate or steady state level within a given geographic area.
- Sporadic: disease outbreaks that have no pattern of occurance in a given time or location.
- Epidemic: an unusually high number of cases in excess of normal expectation of a similar illness in a population, community or region
- Pandemic: a worldwide epidemic
- Morbidity: refers to illness or disease state
- Mortality: numbers of deaths correlated with a particular disease
- Incidence: measurement of morbidity; the number of new cases of a disease that occurs in a specified perod of time in a susceptible population
- Prevalence: measurement of morbidity; number of cases in an existing population at a specified time.
- Carrier: an individual who harbors the virus but is not infected as measured by serologic methods or by evidence of clinical illness.
- Incubation period: time between infection with a virus and the onset of symptoms.
- Prodromal period: first appearance of mild or nonspecific signs and symptoms of an illness
- Mode of transmission: how an infectious disease is spread or passed on.
- Etiological agent/pathogen: disease causing agent.
- Reservoir: where the etiolgical agent lives, grows, and multiplies e.g. human, animal or arthropod.
- Case definition: a standard set of criteria that is used to identify who has the disease being studied.
- Communicable Period: time period when an infected individual or animal is contagious and he/she can directly or indirectly infect another person, animal or arthropod
- Convalescence: the recovery period after an illness.
- Zoonosis: any infection or infectious disease transmissible from animals to humans.
8.2 History of Epidemiology: From Observational Data to Preventive Action
- Pioneers of epidemiology
- Their important observations and actions led to disease prevention:
- Edward Jenner (1796, smallpox)
- John Snow (1854, cholera)
- Florence Nightingale (1855, mortality rates of wounded men in British army during the Crimean War)
Edward Jenner
- Observed that milkmaids were not badly scarred and disfigured from smallpox.
- They appeared to be"immune".
- Deduced that the pustules or lesions on the hands of milkmaids from cowpox would protect individuals from contracting smallpox.
John Snow
- Believed in the germ theory of disease during a time period when most people believed in the miasmatic theory.
- September, 1854, cholera epidemic, London, England
- Mapped locations of water intake and sick
- Observed that sick individuals drank from the same water source which was contaminated with Vibrio cholerae
- Broadstreet Pump
Florence Nightingale
- Credited mostly for her modern nursing practices of the times.
- She was also a statistician.
- Collected statistics and mapped mortality rates of British soldiers during the Crimean War.
- Observed unsanitary conditions in the army hospital.
- Soldiers were dying of typhus, cholera, and dysentery instead of battle wounds.
- Nightingale believed these infections were preventable.
8.3 The Complexities of Disease Transmission
- Factors associated with increased risk of disease transmission.
- Modes of transmission: direct or indirect.
- Chain of infection: term frequently used in hospitals with regard to the control and prevention of infectious diseases.
- How to break the chain of infection:
- Rapid ID of pathogen
- Proper sanitation
- Disinfect or sterilize fomites
- Barrier technique
- Handwashing
- Proper trash and waste disposal
- Proper food handling
- Aseptic technique
- Recognition of high-risk individuals
Concept of Herd Immunity
- Based on the premise that if the majority of population (herd) is mostly protected from a disease through immunization or genetic resistance, the chance of a major epidemic is unlikely.
Population Lacking Herd Immunity Spreads Disease
8.4 Epidemiology Today
- What an epidemiologist wants to know:
- Case definition (what)
- Person (who)
- Place (where)
- Time (when)
- Risk factors (how and why)
- Gathering data:
- Descriptive studies
- Performed right after the epidemic occurs
- Short study e.g. Snow's 1854 cholera study
- Published as case reports
- Analytical epidemiology
- Determines the "why and how" of the epidemic
- Hyothesis-driven studies
- May involve control group
- Cohort method
Descriptive Study Examples
Analytical Epidemiology
Cohort Method
Surveillance and Serological Epidemiology
- Several forms:
- Monitor data from mandated morbidity and mortality reports
- Active field surveillance
- Serological screening of populations (antibodies are the"footprints" of disease)
Communicable Disease Surveillance Organizations and Their Publications or Reporting Mechanisms
ProMED-mail: An Online Program Used to Monitor Disease Outbreaks
- Founder: Jack Woodall
- Modest support from the Federation of American Scientists and SatelLife (The Global Health Information Network)
- ProMED-mail is the CNN of internet reporting
- Contains queries relating to media, newspaper reports and local observations.
8.5 A Word About Prevention and Containment of Infectious Diseases: Quarantine
- Practice of quarantine is still used today to prevent person-to-person transmission of infectious diseases (e.g SARS outbreak, tuberculosis).
- Isolation
- Placards
8.6 Travel Medicine
Viral Diseases Spread by Arthropod Vectors (Insects)
Considerations When Traveling
- What arthropod vectors in the area?
- Contaminated food or water (food and waterborne illnesses)
- Medical facilities in remote areas
8.7 Tracking Diseases from Outer Space: Early Warning Systems
- Environmental factors play a role in outbreaks of viral illnesses.
- Use of satellites to monitor changes on land surfaces.
- Heavy rainfall (more mosquito eggs laid in standing water)
- Temperature changes
- Vegetation changes
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