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Science Olympiad » Disease Detectives Event » Sample Problems and Answer Keys
Answer Key: E. coli and Salmonella

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  1. Primary case definition: A person with laboratory-confirmed E. coli O157:H7 or a clinically compatible illness who ate at the family reunion on February 26.

    Secondary case definition: A person with laboratory-confirmed E. coli O157:H7 or a clinically compatible illness who did not eat at the family reunion on February 26 and reported contact with a primary case-patient.

    A case definition should include common characteristics of the people who are ill and the time and place the illness occurred.


  2. Ranch dressing: 18/35 = 51.4%

    Iceberg lettuce: 57/86 = 66.3%

    The attack rate is calculated by [a/(a+b)].


  3. Iceberg lettuce


  4. Create a new variable in database for any ranch dressing. Combine the results of both food items using ranch dressing. Then do a 2 x 2 table on the combined variable and illness.


  5. Interview them on illness history (before and after February 26) and food history on February 26. Interview them on food preparation. Investigators need to determine where and when food was prepared, what ingredients were used, how food was stored, where food was purchased, how it was prepared, and determine if cross-contamination could have occurred. Collect stool samples on food handlers to see if any are carrying the organism. If any food handler is positive for E. coli O157:H7, they need to refrain from food handling duties until the infection is cleared (specifically two consecutive negative stool cultures collected at least 24 hours apart).

    Routine protocol for food-borne outbreak investigation included interviewing all food handlers to determine food handling procedures, items used in preparation, and illness history. Because E. coli O157:H7 infections can be very severe, stool cultures should be performed to determine if any are infected.


  6. B


  7. True

    John Snow is considered a pioneer in modern epidemiology. He studied a large cholera outbreak in London. He traced the source to one water pump. He convinced the authorities to remove the handle at the Broad Street Pump. The outbreak quickly abated.


  8. True


  9. False

    A cohort study is used with a defined population for which a complete list of participants is available. Examples where a cohort study would be used include wedding reception, family reunion, or a business luncheon. The relative risk is used to determine association between exposure and illness.


  10. False


  11. True
  12. A case-control study is used when the population is not well defined. Examples where a case-control study would be used include a restaurant-associated outbreak, an outbreak associated with town festival or public water supply. The odds ratio is used to determine association between exposure and illness.


  13. graph

    An epidemic curve is a histogram that provides a simple visual display of an outbreak’s magnitude and time course. Time is plotted along the X-axis and the absolute number of cases along the Y-axis.


  14. In the 1950s the number of reported cases was extremely high and very static. There was a decrease in the reported cases after the peak in 1958. The number of reported cases in the 1960s were generally lower than the 1950s. Shortly after the vaccine was licensed, there was a dramatic drop in the number of reported cases. Since 1967, there have been three peaks in incidence, two in the 1970s. The number of reported measles declined steadily in the 1980s until the latter part of the decade when another peak occurred. After the last peak, the number of reported cases has decreased to historical lows.


  15. People got lax in vaccinating so fewer people were vaccinated. The vaccine also wore off after a number of years, leaving adolescents susceptible to measles again.


  16. The biggest change was the recommendation for a booster dose of MMR before junior high. Many colleges and high schools required either two doses of vaccine before entry (or a signed waiver).


  17. There has been a steady increase in salmonellosis in the United States in this 30-year period. This may be due to an increase in disease, and increase in reporting, or an increase in physicians testing for Salmonella. A new risk factor may have emerged (e.g., an increase in the number of people owning exotic pets, especially reptiles; or an increase of S.enteriditis in eggs). Outbreaks of Salmonella in nationally distributes products can affect a large number of people.


  18. While the overall trend for salmonellosis shows a steady increase, the breakdown by serotypes shows some static activity. S.typhimurium had a large peak in 1985 due to an outbreak in unpastuerized milk. S.enteriditis had the largest increase. More labs identifying serotypes may have resulted in the number with unknown serotypes to decrease.


Tiebreaker #1

  1. D
  2. E
  3. G
  4. F
  5. B
  6. E
  7. I
  8. E
  9. D
  10. C

Tie breaker #2

[a/(a+b)]/[c/(c+d)] = [57/(57+29)]/[8/(8+40)] = 3.98 or 4

This page last reviewed April 24, 2007

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