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Infectious Disease Epidemiology

Infectious Disease Epidemiology. Casey E. Barton, M.S. July 26, 2001. REMINDERS!!!!!!!!!!. TODAY by 5pm Submit all outstanding homeworks and redos July 31 st Teaching-Learning Exam Course Evaluations Tutorial after class August 2 nd FINAL EXAM Last day of class!!!!!!!.

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Infectious Disease Epidemiology

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  1. Infectious Disease Epidemiology Casey E. Barton, M.S. July 26, 2001

  2. REMINDERS!!!!!!!!!! • TODAY by 5pm • Submit all outstanding homeworks and redos • July 31st • Teaching-Learning Exam • Course Evaluations • Tutorial after class • August 2nd • FINAL EXAM • Last day of class!!!!!!!

  3. Top 10 Reported Infectious Diseases 1999

  4. Epidemiologic Triangle HOST AGENT ENVIRONMENT

  5. AGENTS • An Agent MUST be present for an infectious disease to develop • Epidemiologists describe: • Frequency of disease • Mechanisms of transmission • The causal agent

  6. AGENTS • Bacteria • Viruses and rickettsia • Mycoses (fungi) • Parasites • Protozoa • Helminths • Arthropods

  7. AGENTS: Bacteria • Leading killers in 19th century • Examples: • Tuberculosis • Salmonellosis • Streptococcal infections • Problem: emerging bacteria strains resistant to current antibiotics

  8. AGENTS: Viruses and Rickettsia • Viral infection include: • Hepatitis A • Herpes simplex • Influenza • HIV • Rickettsial infections include: • Q fever • Rocky Mountain Spotted Fever

  9. Mycoses (Fungi) • Examples: • Ringworm • Aspergillosis • Candidiasis

  10. AGENTS: Parasites • Protozoa • Malaria • Cyclosporiasis • Giardiosis • Chagas’ Disease • Helminths (worms) • Tapeworms • Roundworms • Schistosomiasis

  11. AGENTS: Arthropods • Insect vectors including: • Mosquitoes • Ticks • Flies • Mites • Other insects • Arthropod-borne diseases include: • RMSF • Malaria • Encephalitis • Lyme Disease

  12. Modes of Transmission • Airborne or Contact • Food- and Water-borne • Blood-borne • Sexually Transmitted • Arthropod-borne • Zoonotic

  13. Means of Transmission

  14. Means of Transmission • Direct: person to person • Indirect: through intermediate sources such as vehicles, fomites or vectors • Horizontal: transmission from one person to another (direct or indirect) • Vertical: mother to fetus either genetic or across placenta

  15. Characteristics of Infectious Disease Agents • Infectivity • Pathogenicity • Virulence • Toxigenicity • Resistance • Antigenicity

  16. Infectivity • Capacity of the agent to enter and multiply in a susceptible host and thus produce infection or disease • Examples: measles and polio • Measure of Infectivity: Secondary Attack Rate (SAR%)

  17. Pathogenicity • Capacity of the agent to cause disease in the infected host • Example: measles • Measure of pathogenicity: proportion of individuals with clinically apparent disease

  18. Virulence • Severity of the disease • i.e. whether clinical manifestations are produced • Example: Rabies • Measure of Virulence: Case Fatality Rate (CFR)

  19. Toxigenicity • Capacity of the agent to product a toxin or poison • Examples: botulism and shellfish poisoning • Disease is caused by toxins produced by the agent, rather than by the actual agent

  20. Resistance • Ability of the agent to survive adverse environmental conditions • Examples: parasite with spores or cysts, Hepatitis virus • Resistance is also applied to the HOST

  21. Antigenicity • Ability of the agent to induce antibody production in the host • Agents may or may not induce long-term immunity • Examples: Measles re-infection is rare • Immunogenicity • Related term • refers to an infection’s ability to produce specific immunity

  22. Host • Subclinical infection  Active Disease • Host’s ability to fight off infectious agent • Nonspecific defense mechanisms • Disease-specific defense mechanisms

  23. HOST: Non-specific defense mechanisms • Age, Sex, Race, Genetics • Religion, Customs, Occupation, Nutritional status • Skin, Mucosal surfaces, Saliva, • High pH of gastric juices • Immune system (phagocytes and macrophages)

  24. HOST: Disease-specific defense mechanisms • Immunity: resistance of host to an agent • Active: all or part of organism invokes an immunologic response • Natural: results from an infection • Artificial: results from immunization • Passive: preformed antibody invokes short term immunity • Natural: antibodies cross the placenta • Artificial: immuneglobulin after certain exposures

  25. ENVIRONMENT • Physical: weather, temperature, humidity, geologic formations, habitat • Biological: population density, flora, fauna, reservoirs • Socioeconomical: behavioral, personality, attitudinal, and cultural characteristics of a group of people

  26. Endemic vs.Epidemic • Endemic: constant presence of a disease or agent in a defined geographical area • Epidemic: excessive occurrence of disease (above endemic level) in a defined geographical area • 2 Types: • Common Source • Propogated Source

  27. Epidemic • Common Source Epidemic: event or exposure comes from a single source that all persons in the group had a chance to encounter • Propogated Epidemic: caused by either direct or indirect transmission of infectious disease from one individual to another and can have multiple sources from which disease can be transmitted

  28. Endemic versus Epidemic ENDEMIC Point EPIDEMIC Propagating EPIDEMIC NOTE: X axis = Time; Y axis = # new cases

  29. Reservoir (Niche) • Fosters survival of infectious disease agent • Types of reservoirs • Human • Plant • Animal • Arthropod • Physical environment

  30. Inapparent infection • Subclinical infection: absence of clinically obvious signs and symptoms • Asymptomatic individuals can transmit disease • Carrier status • Serologic evidence of infection

  31. Incubation Period • Time interval between exposure to an infectious agent and the appearance of the first signs of disease • During this time, the infectious agent replicates within the host • Can be hours, days, weeks, or years • Applies only to clinically apparent cases of disease

  32. Generation Time • Time between lodgment of an infectious agent in a host and the maximal communicability of the host • May or may not be equivalent to incubation period • Applies to both clinically apparent and inapparent cases of disease • Utilized for describing the spread of infectious agents that have a large proportion of subclinical cases

  33. Herd Immunity • A population may become immune to an infectious agent after a large proportion of individuals have become immune • i.e. through past infections or vaccination • Can occur when immune persons prevent the spread of disease to unimmunized individuals and confers protection to the population even though not every single individual has been immunized

  34. Effect of Herd Immunity on Spread of Infection (from Jekel et al. 1996, p. 12) - + - + + + - - - + + + + + - - - - + + + + - - - + + + + + Absence of Herd Immunity Presence of 50% Herd Immunity

  35. Iceberg Concept of Disease Death Clinical illness Visible Cases Invisible Cases Asymptomatic cases Carriers

  36. Zoonoses • World Health Organization (WHO): • diseases and infections which are naturally transmitted between vertebrate animals and man • Examples: Rabies, roundworms, Lyme disease, Brucellosis, Cryptosporidiosis • Epizootic and enzootic but refers to diseases in animals

  37. Emerging Infectious Diseases • Changes in climate – global warming • Human activities – farming, reforestation • Technologic changes – air travel, organ transplantation • Demographic changes – migration to cities

  38. Host Prevention • Immunity – immunizations • Behavioral change • Treatment of contact and carriers • screening

  39. Control • Environmental • Personal hygiene • Food protection and safety • Water supplies • Sanitation, regulation • Vectors • Mosquitoes, insecticides • Animal population control

  40. Measures of Disease Outbreaks • Attack Rate • Secondary Attack Rate • Case Fatality Rate

  41. Attack Rate • Type of incidence rate AR% = # Ill___ * 100 during a time period # Ill + # Well • Can calculate food or exposure specific AR%

  42. Secondary Attack Rate • Yields an index of the spread of disease within a circumscribed unit, i.e. household, dorm • Index case: case that first comes to the attention of public health authorities • SAR% = # new cases in group – initial cases * 100 # susceptible persons in group – initial cases

  43. Case-Fatality Rate • Refers to the number of deaths caused by a specific disease among those who have the disease • Provides index of deadliness of a particular disease within a specific population • CFR% = # deaths due to disease X * 100 # cases disease X • DIFFERENT than crude death rate

  44. Centers for Disease Control and Prevention (CDC) • National Center for HIV,STD, and TB Prevention • National Center for Infectious Diseases • AIDS, STD and TB Laboratory Research • Division of Bacterial and Mycotic Diseases • Division of Parasitic Diseases • Division of Vector-borne Infectious Diseases • Division of Viral and Rickettsial Diseases • National Immunization Program

  45. Infectious Disease Information • Emerging Infectious Diseases Journal • www.cdc.gov/ncidod/eid/ • Morbidity and Mortality Weekly Report (MMWR) • www.cdc.gov/mmwr/ • Program for monitoring Emerging Diseases (Promed) • www.promedmail.org/pls/promed/promed.home

  46. THE END!!!

  47. EXAMPLE CALCULATIONS Which food item appears to be the most probable vehicle for the food-borne infection assocaited with illness at this picnic?

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