The Dynamics of Disease Transmission
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Presentation at Medical Sciences Seminar IV: Epidemiology and Biostatistics. University of Tsukuba Master of Public Health program.
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The Dynamics of Disease Transmission
- 1. Nguyen Thi Ngoc Mai April 18, 2017
- 2. Contents Modes of disease transmission Natural history of disease Endemic, Epidemic, Pandemic Attack rate – cross-tabulation Herd Immunity Exercises 2
- 3. Modes of transmission Direct Skin-skin Mucous- mucous Vertical transmission Indirect Common vehicle: Food Water Air Fomites Vector 3
- 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
- 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. Vector: An animal (normally invertebrate) that carries and spreads infectious agent. Fleas (plague) Aedes (zika, dengue fever) Anopheles (malaria) 7
- 8. Natural history of disease9
- 9. 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. 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. 13
- 13. Latent disease14
- 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. 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. Attack rate Useful for comparing the risk of disease in groups with different exposures. 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. Cross-tabulation When confronted with several possible causes Help determining which of the possible agents is likely to be the cause. 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. 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. 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. 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. 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. 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. 27
Editor's Notes
- Hopefully after the seminar today, we will have a better and more profound understanding about epidemiology terminology learned in Wagatsuma prof class this afternoon.
- Vertical transmission: Passage of a disease-causing agent (pathogen) from mother to baby during the period immediately before and after birth. Droplet spread: Droplets from the cough or sneeze of an infected person move through the air and get into the mouth or nose of people nearby. Do not travel far, normally within 1 meter.
- Trans placental transmission: pathogens can cross the placenta and cause infection in the fetus. HBV, HIV
- Fomites: Contaminated nonliving objects Common vehicle: Inanimate object, agent may(not) multiply/develop in/on the vehicle
- Mechanically: Fleas (plague), aedes (zika, dengue fever) Biologically: The vector play host on the agent. The agent need the host to develop and mature and/or to reproduce. Only after this phase is over does the vector becomes infective: Anopheles (malaria)
- Latent periods: The time interval from infection to development of infectiousness. Incubation periods: The interval from receipt of infection to the time of onset of clinical illness. Factors: Dose of inoculum, site of multiplication, rate of multiplication, host defense mechanism
- Incubation period varies from disease to disease. As you can see in the previous slide, at least part of the incubation period, the he or she can transmit the disease to other. For example, in HIV/AIDS, the infectious period occurs many years before the onset of symptoms, meanwhile in SARS incubation period only lasts for 2 weeks. In this case, isolation can be very valuable. Among us, who still remember SARS outbreak in 2003? According to the WHO, 8437 people became ill, of those, 813 died. A major contributor to control of the epidemic was early isolating policy, in order to reduce interpersonal contact of travelers with a history of travel to highly affected areas.
- Not destined to become clinical apparent Potential source of infection to others Could someone tell us what a carrier is?
- - escape recognition - continuing to live a normal life among population or community - readily infect the susceptible individuals - over a wider area and longer period of time under favorable conditions.
- Mary Mallon was the first person in the United States identified as an asymptomatic carrier of the pathogen associated with typhoid fever. From 1900 to 1907, Mallon worked as a cook in the New York City area for seven families. She was presumed to have infected 22 people, three of whom died, over the course of her career as a cook. She was twice forcibly isolated by public health authorities, and died after three decades of isolations at the age of 69, due to pneumonia.
- Virus that can incorporate their genetic material into the genetic material of the infected host as provirus. Latent tuberculosis -> reactivation.
- Chicken pox every year around Jan to May Epidemic: H1N1 flu pandemic in 2009: 14,286 confirmed death 2002-2003: SARS epidemic in Asia and Canada with 775 death. 2011: Hand, foot, mouth disease in Vietnam, 170 death.
- For example: The first table shows the total number of persons who ate each of two specified food items (tuna and egg salad) that were possibly infective with group A Streptococci. The second table shows the number of sick person.
- Looking at the data by row, among who ate egg salad and did not, eating tuna do not increase did not increase much the incidence of Streptococci illness. But looking at the data vertically, we could see eating the egg salad increase the attack rate of illness significantly, both in those who ate tuna (80% vs 35%) and who didn’t eat (75% vs 30%) => Thus the egg salad is clearly implicated.
- The responsibility of getting vaccinated. It's not only about you, it's about vulnerable people that can't get vaccinated, like newborn babies, or people going through chemo. Proportion of immunity that will provide herd immunity depends on the potential for an infectious agent to spread
- In the animation above, red is an invading pathogen, and you can see that the more people vaccinated against it in a community, the harder it is for it to spread. But as soon as vaccination rates drop below around 90 percent, the pathogen runs wild. Medium levels slow down the progression of the illness, but they don't offer robust protection to the unvaccinated. But once you read a high enough level of vaccination, the disease gets effectively road-blocked. It can't spread fast enough because it encounters too many vaccinated individuals, and so the majority of the population (even the unvaccinated people) are protected.
- Table 1 shows the total number of people who ate each of the two specified food items possibly infective with group A streptococci. Table 2 shows the number of sick people (e.g., those with acute sore throats) who ate each of the various specified combinations of the food items. Use these data for questions 2 and 3.