Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Introduction to epidemiology

47 views

Published on

HSCI 330 SFU

Published in: Education
  • Be the first to comment

  • Be the first to like this

Introduction to epidemiology

  1. 1. A (re)Introduction to ˌepəˌdēmēˈäləjē HSCI330
  2. 2. LECTURE CONTENT • Descriptive and Analytic Epidemiology • Cases, Case Definitions, and Diagnostic Tests • Course of Disease • Epidemic Curves • Measures of Morbidity and Mortality • Chain of Infection • Compartmental Disease Modeling
  3. 3. Epi  demos  logos “the study of” “upon, on, or over” “the people” The study of what affects people.
  4. 4. “Epidemiology is the study of the distribution and determinants of health-related states or events (HRSE) in human populations, and the application of this study to prevent and control health problems.” - , 2001 -
  5. 5. • Endemic – A state of persistent disease that behaves predictably with regards to the population impacted, chronicity, and recurrence. • Outbreaks • Epidemic – An outbreak with a greater than expected number of cases for a given disease in a given population over a given period of time. • Pandemic – An epidemic affecting a large number of people over multiple countries, continents, or regions. • Syndemic – Multiple reinforcing epidemics that synergistically work together to cause poor health outcomes in a population. Classifying HRSEs
  6. 6. • Give an example for each of the following classes of HRSEs: • Endemic • Epidemic • Pandemic • Syndemic • Describe how you might detect the presence or emergence of each class of HRSE. Small Group Exercise
  7. 7. • Make a list describing what steps would you take to prevent a disease after identifying it. • Share your list with the person next to you. • Work to refine your lists into a single list. • We will combine your lists as a class. Small Group Exercise
  8. 8. 1. Describe the health related state or event (HRSE) of interest. 2. Identify the causes and risk factors associated with this HRSE. 3. Assess which modifiable risk factors could be targeted to produce better outcomes. 4. Develop and pilot an intervention that addresses these modifiable risk factors. 5. Estimate the efficacy and cost-beneficence of scaling-up your intervention. 6. Monitor the effectiveness of your intervention in remediating the HRSE. Epidemiology and Public Health
  9. 9. “Epidemiology is the study of the distribution and determinants of health-related states or events (HRSE) in human populations, and the application of this study to prevent and control health problems.” - , 2001 - Analytic epidemiology • Attempts to identify the determinants of a HRSE. • How?, Why?, What? Descriptive epidemiology • Attempts to describe the distribution of a HRSE. • Who?, Where?, When? Clinical epidemiology • Attempts to define and describe the biology and clinical markers of a HRSE.
  10. 10. • Before you can describe a HRSE, you must first define it. • A case is a person who has been diagnosed as having a disease, disorder, injury, or condition • The first disease case in the population is the primary case • The first disease case brought to the attention of the epidemiologist is the index case • The index case is not always the primary case • Secondary/tertiary/etc. cases are those persons who become infected and ill after a disease has been introduced into a population and who become infected from contact with the primary case. • A contact is someone possible infected through contact with a case. Classifying HRSEs
  11. 11. PRIMARY CASE
  12. 12. • Suspected Case • An individual who has all of the signs and symptoms of a disease or condition, yet not diagnosed • Confirmed Case • All criteria met • As more information (e.g., laboratory results) becomes available to the physician, he or she generally upgrades the diagnosis. When all criteria are met and they meet the case definition, the case is classified as a confirmed case. Cases & Non-Cases
  13. 13. Sensitivity and Specificity Specificity is the fraction of those without disease who will have a negative test result. Sensitivity is the fraction of those with disease who will have a positive test result. Sensitivity and specificity are characteristics of the test.
  14. 14. Positive / Negative Predictive Value PPV is the fraction of true positives who will have a positive test result. NPV is the fraction of true negatives who will have a negative test result. Positive and negative predictive values are influenced by the prevalence of disease
  15. 15. Calculating the Efficacy of Diagnostic Tests Specificity A/(A+C) × 100 Sensitivity D/(D+B) × 100 PPV A/(A+B) × 100 NPV D/(D+C) × 100
  16. 16. Natural Course INFECTIOUS PERIOD Exposure Moment Onset of Pathogenic Infectiousness Onset of Disease & Symptomology Recovery or Death LATENT PERIOD INCUBATION PERIOD Stage of Susceptibility Stage of Subclinical Disease Stage of Clinical Disease Stage of Recovery or Death 1 2 3 4 Diagnosis & Clinical Follow-up Primordial and Primary Prevention Secondary Prevention Tertiary Prevention
  17. 17. Identify each of the following individuals as a suspected case or a confirmed case. If a confirmed case, were they a primary case, index case, or secondary case in their respective epidemics? • Gaetan Dugas – HIV • Ardouin Antonio – HIV • Robert Rayford – HIV • Arne Vidar Røed – HIV • Margrethe P. Rask – HIV • Ken Horne – HIV Group Activity • Mary Mallon – Typhoid • Mabalo Lokela – Ebola • Liu Jianlun – SARS • Edgar Enrique Hernandez – Flu 2009 • Emile Ouamouno – Ebola 2014
  18. 18. • Who does the HRSE impact? • Where and when does the HRSE occurs? Descriptive Epidemiology
  19. 19. Imagine you have just been tasked by the Ministry of Health to begin studying an emerging disease that was identified by the Coroner’s Office. Nothing is known about this new disease, except for it has been observed in an alarming number of blood samples collected at autopsy. • What descriptive factors would you like to request from the administrative and health records of the individuals diagnosed? • What do you expect to learn about the disease from each requested variable? • What additional information might you request from the coroner about these blood samples? Group Activity
  20. 20. male white cisgender european straight able-bodied credentialed young attractive upper class anglophones light or pale gentile fertile female person of color trans/ queer non-european LGBTQ disability nonliterate old unattractive working class ESL dark jewish infertile Describing Cases
  21. 21. • What additional information might you request from the coroner about these blood samples? Group Activity
  22. 22. • Spaces. Municipalities, Jurisdictions, Health Districts or Catchment Areas. • Places. Neighbourhoods and venues such as bathhouses, schools, workplaces. • The physical “where,” but also the contextual “where.”
  23. 23. • An epidemic curve gives a graphical display of the numbers of incident cases in an outbreak or epidemic, plotted over time. The form of the resulting distribution of cases can be used to propose hypotheses on the nature of the disease and its mode of transmission. The Epidemic Curve Common Source Propagated Mixed
  24. 24. Exposure Incubation max min Units of Time NumberofCases Point Source Outbreak with No Propagation Cases within one incubation
  25. 25. Exposure Units of Time NumberofCases Intermittent Source Outbreak Exposure Exposure
  26. 26. Exposure Units of Time NumberofCases Continuing Common Source Outbreak Intervention
  27. 27. Exposure Units of Time NumberofCases Propagated with Limited Spread Index Case Endemic Levels?
  28. 28. Exposure Units of Time NumberofCases Propagated with Index Case and Epidemic Spread
  29. 29. Morbidity Incidence Measures Incidence Proportion, Attack rate, or Risk Number of new cases during specified time interval Population at start of time interval Incidence Rate or Person-time Rate Number of new cases during specified time interval Summed person-years of observation or average population during time interval Secondary Attack Rate Number of new cases among contacts Total number of contacts
  30. 30. Incidence Proportion 1. In a study of diabetics, 100 of the 189 diabetic men died during the 13-year follow- up period. Calculate the risk of death for these men. 2. In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad.
  31. 31. Incidence Rate Investigators enrolled 2,100 women in a study and followed them annually for four years to determine the incidence rate of heart disease. After one year, none had a new diagnosis of heart disease, but 100 had been lost to follow-up. After two years, one had a new diagnosis of heart disease, and another 99 had been lost to follow-up. After three years, another seven had new diagnoses of heart disease, and 793 had been lost to follow-up. After four years, another 8 had new diagnoses with heart disease, and 392 more had been lost to follow-up. The study results could also be described as follows: No heart disease was diagnosed at the first year. Heart disease was diagnosed in one woman at the second year, in seven women at the third year, and in eight women at the fourth year of follow-up. One hundred women were lost to follow-up by the first year, another 99 were lost to follow-up after two years, another 793 were lost to follow-up after three years, and another 392 women were lost to follow-up after 4 years, leaving 700 women who were followed for four years and remained disease free. Calculate the incidence rate of heart disease among this cohort. Assume that persons with new diagnoses of heart disease and those lost to follow-up were disease-free for half the year, and thus contribute ½ year to the denominator.
  32. 32. Incidence Rate Calculate the incidence proportion for the last problem.
  33. 33. Secondary Attack Rate Consider an outbreak of shigellosis in which 18 persons in 18 different households all became ill. If the population of the community was 1,000, then the overall attack rate was 18 ⁄ 1,000 × 100% = 1.8%. One incubation period later, 17 persons in the same households as these "primary" cases developed shigellosis. If the 18 households included 86 persons, calculate the secondary attack rate.
  34. 34. Morbidity Prevalence Measures Point Prevalence Number of cases during at a specific time point Population at the same time point Period Prevalence Number of current cases during time interval Average population during time interval or mid- interval ppopulation
  35. 35. Point Prevalence In a survey of 1,150 women who gave birth in Maine in 2000, a total of 468 reported taking a multivitamin at least 4 times a week during the month before becoming pregnant. Calculate the prevalence of frequent multivitamin use in this group.
  36. 36. Prevalence & Incidence The figure to the left represents 10 new cases of illness over about 15 months in a population of 20 persons. Each horizontal line represents one person. The solid line represents the duration of illness. The down arrow indicates the date of onset of illness. The up arrow and the cross represent the date of recovery and date of death, respectively. 1. Calculate the point prevalence (as %) on Apr. 1, 2005. 2. Calculate the period prevalence (as %) from Oct. 1, 2004 to Sept. 30. 2005 3. Calculate the Incidence Rate (per 100) from Oct. 1, 2004 to Sept. 30, 2005. Onset of Illness Death Recovery Oct. 1, 2004 April 1, 2005 Sept. 30, 2005
  37. 37. Period Prevalence Period Prevalence = ( 10 ⁄ 20) × 100 = 50.0% Oct. 1, 2004 Sept. 30, 2005 1 2 3 4 5 6 7 8 9 10 10
  38. 38. Point Prevelance Point Prevalence ( / (20 - )) × 100 = 38.89% April 1, 2005 1 2 3 4 5 6 7 7 1 2 2
  39. 39. Incidence Rate Incidence rate numerator = number of new cases between October 1 and September 30 = 4 (the other 6 all had onsets before October 1, and are not included) Incidence rate denominator = April 1 population = 18 ( died before April 1) Incidence rate = ( ⁄ 20 - ) × 100 = 22 new cases per 100 population Onset of Illness Death Oct. 1, 2004 April 1, 2005 Sept. 30, 2005 1 2 3 4 4 1 2 2 4 2
  40. 40. Mortality Frequency Measures Case Fatality Rate Number of deaths among cases Total number of cases Crude Death Rate Total number of deaths during a given time interval Mid-interval population Cause-specific death rate Number of deaths assigned to a cause over a given interval Mid-interval population
  41. 41. Case Fatality Rate In an epidemic of hepatitis A traced to green onions from a restaurant, 555 cases were identified. Three of the case-patients died as a result of their infections. Calculate the case-fatality rate.
  42. 42. Crude Death Rate In the United States in 2003, a total of 2,419,921 deaths occurred. The estimated population was 290,809,777. Calculate the crude death rate.
  43. 43. Cause-Specific Death Rate In 2003, 2,419,921 people died out of 290,809,777. Of these deaths, 108,256 were due to accidental injury. Calculate the cause-specific death rate attributable to accidental injruies.
  44. 44. Other Mortality Measures Proportionate Mortality Number of deaths assigned to a cause over a given interval Total number of deaths from all causes during time interval Death-to-case-ratio Number of deaths assigned to a cause over a given interval Number of cases for cause reported over a given interval
  45. 45. Proportionate Mortality In 2003, 2,443,930 people died in the United States. Of these, 128,924 were between the ages of 25 and 44. Among those in this age group, 16,283 people died of heart disease. Calculate the proportionate mortality (as %) for heart disease in this age group.
  46. 46. Death-to-Case Ratio Between 1940 and 1949, a total of 143,497 incident cases of diphtheria were reported. During the same decade, 11,228 deaths were attributed to diphtheria. Calculate the death-to-case ratio (deaths per 100 cases).
  47. 47. • How does the HRSE spread? • Why is the HRSE present in some individuals and not others? • What causes the HRSE? Analytic Epidemiology THIS WEEK NEXT WEEK NEXT WEEK
  48. 48. Chain of Infection
  49. 49. Reservoir The habitat (living or nonliving) on which an infectious agent lives, grows, multiplies, and depends on for its survival in nature • Humans, animals, food, feces, decaying organic matter. • Plants, soil, and water in the environment are also reservoirs for some infectious agents. 1
  50. 50. • The path by which a pathogen leaves its host. • This usually corresponds to the site where the pathogen is localized. Portal of Exit2
  51. 51. •Direct transmission - pathogens or agents are transferred through direct physical contact. •Indirect transmission - pathogens or agents are transferred or carried by some intermediate item, organism, means, or process to a susceptible host. Modes of Transmission3
  52. 52. Directions of Transmission • Vertical transmission transmission from an individual to its offspring through sperm, placenta, milk, or vaginal • Horizontal transmission transmission of infectious agents from an infected individual to a susceptible contemporary
  53. 53. Carriers • A carrier contains, spreads, or harbors an infectious organism. • Five types of carriers • Active – Individual has been exposed and is symptomatic. • Convalescent – Individual is recovered from disease but is still infectious. • Asymptomatic –Individual that has been exposed and harbors disease-causing pathogen but never shows any symptoms. • Incubatory – Individual who has been exposed and is harboring disease-causing pathogen but has not yet displayed symptoms. • Intermittent – Individual is recovered from disease but remains infectious in different places or time intervals.
  54. 54. Droplet Spread • Some infections are spread when an infected person talks, coughs or sneezes small droplets containing infectious agents into the air. • Due to their size, these droplets in the air travel only a short distance (around a meter) from the infected person before falling. • The droplets in the air may be breathed in by those nearby. • Spread can also occur by touching the nose or mouth with droplet contaminated hands.
  55. 55. Aerosol (“Airborne”) Spread • Some infections are spread when an infected person talks, breathes, coughs or sneezes tiny particles containing infectious agents into the air. • These are called small particle aerosols. • Due to their tiny size, small particle aerosols can travel long distances on air currents and remain suspended in the air for minutes to hours. • These small particle aerosols may be breathed in by another person. • Examples of airborne spread diseases: • chickenpox • measles • tuberculosis (TB)
  56. 56. A nonliving intermediary, also called a fomite, such as food, water, biologic product, or fomite (inanimate object) that conveys the infectious agent from its reservoir (habitat where an infectious agent lives, grows, and multiplies) to a susceptible host. Vehicles
  57. 57. An invertebrate living intermediary, most often an insect or arthropod (e.g., mosquito, flea, or tick), that conveys the infectious agent from its reservoir to a susceptible host • mechanical (agent does not undergo physiologic changes within the vector) or • biological (agent undergoes a part of its life cycle within the vector before transmission to new host) Vectors
  58. 58. Portals of Entry •The manner in which a pathogen enters a susceptible host. •The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act. •Often, infectious agents use the same portal to enter a new host that they used to exit the source host. 4
  59. 59. Mucosal Membranes • Some infections are spread directly when skin or mucous membrane (the thin moist lining of many parts of the body such as the nose, mouth, throat and genitals) comes into contact with the skin or mucous membrane of another person. Infections are spread indirectly when skin or mucous membrane comes in contact with contaminated objects or surfaces. • Examples of diseases spread by skin or mucous membrane contact: • chickenpox • cold sores (herpes simplex infection) • conjunctivitis • hand, foot and mouth disease • head lice • molluscum contagiosum • ringworm • scabies • school sores (impetigo) • Staphylococcus aureus infection • warts.
  60. 60. Faeco-Oral spread • Some infections are spread when microscopic amounts of faeces (poo) from an infected person with symptoms or an infected person without symptoms (a carrier) are taken in by another person by mouth. The faeces may be passed: • directly from soiled hands to the mouth • indirectly by way of objects, surfaces, food or water soiled with faeces. • Examples of diseases spread from faeces: • Campylobacter infection • Cryptosporidium infection • Giardia infection • hand, foot and mouth disease • hepatitis A • meningitis (viral) • rotavirus infection • Salmonella infection • Shigella infection • thrush • viral gastroenteritis • worms • Yersinia infection.
  61. 61. Host •The final link in the chain of infection is a susceptible host. •Susceptibility of a host depends on genetic or constitutional ability to resist or to exhibit immunity to a given pathogen. 5
  62. 62. • Innate immunity refers to specific and nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. • The innate immune response is activated by chemical properties of the antigen. • Non-specific mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. • Specific mechanisms include antibodies inferred via transplacental transfer. Immunity
  63. 63. • Acquired or adaptive immunity is the body's third line of defense. • This is protection against specific types of pathogens. • Acquired immunity may be either natural or artificial in nature, both of which have passive and active components. • Active immunity results from an infection or an immunization • Passive immunity comes from naturally or artificially gaining antibodies. Immunity
  64. 64. Herd Immunity The resistance to the spread of a contagious disease within a population that results if a sufficiently high proportion of individuals are immune to the disease, especially through vaccination.
  65. 65. Learning Activity In groups of two, identify a HRSE that follows the chain of infection and discuss how can each link in the chain could be addressed to prevent disease.
  66. 66. Controlling an Acute Outbreak • Reduce Host Susceptibility • Immunization • General Health Promotion • Pre-exposure Chemoprophylaxis • Interrupt Transmission of Agent • Isolation • Quarantine • Family and community measures • Hygiene measures • Inactivate Agent • Chemoprophylaxis • Hygiene measures
  67. 67. Vaccines – Neutralizes pathogens by instigating an immune response through the creation of antibodies which bind to antigen receptors on pathogens and disrupt their function. Antimicrobials – Non-selective disinfectants (e.g., bleach) are applied to surfaces to kill a wide range of organisms. Antiseptics are like disinfectants but are applied to living tissue. Broad-spectrum medications refer to the impact that they may have on many different viruses by interfering in general processes. • Antifungals – interfere with the life-cycle of fungus, often by attacking cell membranes. • Antiparasitic – target parasitic agents to destroy them or inhibit their growth. • Antibiotics – Interfere with the reproduction of bacteria by creating a toxic environment. • Antivirals – Inhibit the reproduction of virus by disrupting specific steps in their replication cycle.
  68. 68. Group Activity What are the fundamental factors are in shaping whether an outbreak becomes an epidemic, endemic, or pandemic?
  69. 69. Communicability • The ability of a disease to be transmitted from one person to another or to spread through the population is called communicability • Are all infectious diseases communicable? • What is an example of a chronic communicable disease? • What is an infectious disease that is chronic?
  70. 70. R0 The Basic Reproductive Number of an infection is the number of cases one case generates on average over the course of its infectious person in an otherwise uninfected population.
  71. 71. Other Terms related to Communicability • Invasiveness – the ability to get into a susceptible host and cause disease is termed. • Infectivity – the ability to of a pathogen to reproduce and cause infection once inside a host. • Pathogenicity – the disease evoking power of a pathogen. • Virulence – the ability of a pathogen to cause severe disease. • Immunogenicity – the ability of an agent to produce a specific immune response against an agent.
  72. 72. Susceptible Exposed Infectious Recovered S E I R In the SI model, individuals are born into the simulation with no immunity (susceptible). Once infected and with no treatment, individuals stay infected and are infectious throughout their life, and remain in contact with the susceptible population. If contacts persist, eventually the entire population will become infected.
  73. 73. Susceptible Exposed Infectious Recovered S E I R In the SIS model, individuals are born into the simulation with no immunity (susceptible). Infected individuals stay infected for only a limited time period and thereafter become susceptible (no innate immunity). Overtime, a steady proportion of people will be infected.
  74. 74. Susceptible Exposed Infectious Recovered S E I R In the SIR model, individuals are born into the simulation with no immunity (susceptible). Infected individuals stay infected for only a limited time period and thereafter become immune. Overtime, these epidemics will burn out as the number of susceptible hosts declines and the number of immune hosts increases.
  75. 75. Susceptible Exposed Infectious Recovered S E I R Waning ImmunityIn the SIRS model, individuals are born into the simulation with no immunity (susceptible). Infected individuals stay infected for only a limited time period and thereafter become immune. After a waning period, these individuals become susceptible again. These epidemics will oscillate, perhaps seasonally, as recovered people become susceptible.
  76. 76. Susceptible Exposed Infectious Recovered S E I R In the SEIR model, individuals are born into the simulation with no immunity (susceptible) and undergo a period of latency in which they are infected but not infectious. Once they are infectious, they will eventually recover with immunity. These epidemics will eventually burn out, if a population is stable, as the number of susceptible non-immune hosts decreases.
  77. 77. Susceptible Exposed Infectious Recovered S E I R Waning Immunity In the SEIRS model, individuals are born into the simulation with no immunity (susceptible) and undergo a period of latency in which they are infected but not infectious. Once they are infectious, they will eventually recover with temporary immunity. These epidemics will oscillate until they burn out as there are usually not enough susceptible cases at one time.
  78. 78. Susceptible Exposed Infectious Recovered S E I R The behavior of compartmental models changes when you account for vital dynamics and interventions! • Births add susceptible hosts, unless immunity is conferred by birth. • Deaths make recovery and thus re-infection impossible. • Vaccinations make people immune to infection, removing them from the susceptible pool. • Hygiene and other interventions can reduce the probability of exposure and infectiousness.
  79. 79. Next Week • Continue Discussing Analytic Epidemiology • Why is the HRSE present in some individuals and not others? • What causes the HRSE? • Make sure you’re keeping up on the podcasts. • Midterm will be in two weeks (no class). • In three weeks, you will have time to work with your groups to bring together your presentations (no class).

×