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.

The Dynamics of Disease Transmission

1,270 views

Published on

Presentation at Medical Sciences Seminar IV: Epidemiology and Biostatistics.
University of Tsukuba
Master of Public Health program.

Published in: Health & Medicine
  • Be the first to comment

The Dynamics of Disease Transmission

  1. 1. Nguyen Thi Ngoc Mai April 18, 2017
  2. 2. Contents  Modes of disease transmission  Natural history of disease  Endemic, Epidemic, Pandemic  Attack rate – cross-tabulation  Herd Immunity  Exercises 2
  3. 3. Modes of transmission  Direct  Skin-skin  Mucous- mucous  Vertical transmission  Indirect  Common vehicle: Food Water Air Fomites  Vector 3
  4. 4. Direct transmission Through person-to-person contact like kissing, touching, biting, sexual intercourse.  Skin-skin: Herpes type 1  Mucous-mucous: STI  Vertical transmission: HIV, Zika 4
  5. 5. 5
  6. 6. Indirect transmission Common vehicle: Food: Staphylococci, salmonella Water: Cholera, hepatitis A Air: Chickenpox, influenza Fomites: Cooking utensils, bedding, clothing, surgical, medical instrument… 6
  7. 7. Vector: An animal (normally invertebrate) that carries and spreads infectious agent. Fleas (plague) Aedes (zika, dengue fever) Anopheles (malaria) 7
  8. 8. Natural history of disease9
  9. 9. 10
  10. 10. Subclinical infection  Unapparent, covert, missed or abortive cases  Disease agent multiply in the host but dose not manifest to clinical disease  Contaminates the environment!!  Carriers 11
  11. 11. Carriers Harbors the organism but is not infected as measured by serologic study or evidence of clinical illness Inadequate treatment Inadequate immune response 12
  12. 12. 13
  13. 13. Latent disease14
  14. 14. Endemic – Epidemic - Pandemic  Endemic: The habitual presence of a disease within a geographic area, or usual occurrence of a given of a given disease.  Epidemic: The number of cases excess of normal expectancy  disease outbreak  Pandemic: When epidemics occur at several continents – global epidemic 15
  15. 15. Disease outbreaks  “The occurrence, in a defined community or region, of cases of an illness with a frequency clearly in excess of normal expectancy” – Heymann, D. (2004)  Primary case: The case that bring infection into the population  Attack rate  Secondary case: Persons who were infected by a primary case  Secondary attack rate P S S T Susceptible Immune Sub-clinical Clinical S T 16
  16. 16. Attack rate Useful for comparing the risk of disease in groups with different exposures. 17
  17. 17. Sick Total Attack Rate Ate egg salad 135 175 77.1 Did not eat egg salad 85 250 34.0 Attack rate: • people who ate egg salad: 135/175 x 100 = 77.1% • People who didn’t: 85/250 = 34.0% 18
  18. 18. Cross-tabulation  When confronted with several possible causes  Help determining which of the possible agents is likely to be the cause. 19
  19. 19. Ate tuna Did not eat tuna Sick Total Attack rate Sick Total Attack rate Ate egg salad 60 75 80.0 75 100 75.0 Did not eat egg salad 70 200 35.0 15 50 30.0 20
  20. 20. Herd immunity  “A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community” - CDC 21
  21. 21. 22
  22. 22. Epidemic refers to a. A disease that has a low rate of occurrence but that is constantly present in a community or region b. An attack rate in excess of 10 per 1,000 population c. The occurrence of illness of similar nature clearly in excess of the normal expectation for that population at that time d. Diseases of the respiratory system that occur seasonally e. The annual case rate per 100,000 population 23
  23. 23. Table 2. Number of sick people who ate each of the various specified combinations of the food items Ate Pheasant Did not Eat Pheasant Ate caviar 100 100 Did not eat caviar 100 100 Table 1. Total number of people who ate specified combination of food items Ate Pheasant Did not Eat Pheasant Ate caviar 50 20 Did not eat caviar 50 25 24
  24. 24. What is the sore throat attack rate in people who ate both pheasant and caviar? a. 50/50 b. 50/70 c. 50/75 d. 50/100 e. 50/200 25
  25. 25. According to the results shown in Tables 1 and 2, which of the following food items (or combination of food items) is (are) most likely to be the infective item(s)? a. Pheasant only b. Caviar only c. Neither pheasant nor caviar d. Both pheasant and caviar e. Cannot be calculated from the data given 26
  26. 26. 27

×