Epidemic Investigation 2

Subject: Comm Med 2 Topic: Epidemic Investigation 2 Lecturer: Dr. Carnate Date of Lecture: February 20, 2012 Transcriptionist: Sybarite Editor: Muttley Pages: 4

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On the evening of April 28, a total of 100 people went to the Emergency Room Departments of 3 local hospitals in Greenport, a suburban neighborhood. All of the patients complained of headache, headache, fever, nausea, vomiting, and diarrhea. Stool samples were collected from all 100 patients; stools stools form 90 patients were positive for bacteria. They were all (90) diagnosed diagnosed as cases of gastroenteritis. gastroenteritis. Questions: 1. What the disease being considered?  Answer: Gastroenteritis. This is step 1 of epidemic epidemic investigation, to verify the diagnosis. 2. What was the case definition used?  Answer: Case definition is the standard set of criteria for deciding if a person is to to be classified as having the disease or not. In this case it is a patient with a (+) stool sample and signs signs and symptoms of headache, fever, nausea, vomiting, and diarrhea. The reported cases that comply with this definition are termed true cases. 3. Was there an outbreak of o f gastroenteritis?  Answer: This is step 2 of epidemic investigation, to determine if an epidemic epidemic actually exists. Normally, Normally, what would be done is the number of t rue cases would be counted and compared to the usual occurrence of cases. If it is in excess, then it is considered considered an epidemic. In this case, no actual actual figures were given, but Dr. Carnate said to assume that the number of t rue cases > than the number of usual cases and that this is an epidemic. Gastroenteritis outbreaks like this are usually caused by the consumption of a contaminated or poisoned meal. Food contamination may be caused by by pathogenic viruses or bacteria. However, acute outbreaks are more often produced by bacterial toxins from Staphylococcus Staphylococcus species, Clostridium perfringens, Salmonella species, and  Vibrio cholera. The local health department was notified of the potential food – borne outbreak of gastroenteritis. An epidemic team was sent to t he area to gather data on the distribution of the disease by person – place – time. The team came up with the following data (Tables 1, 2, and 3) Table 1. Distribution of cases cases by time of onset of symptoms Time of onset of symptoms April 28 8 pm 2 9 pm 18 10 pm 30 11 pm 24 12 am 12 April 29 1 am 4 Total 90 Table 2. Distribution of cases cases by age and gender Age Females 0 – 5 1 6 – 10 38 >11 10 Total 49

# of cases

Males

1 38 2 41

Total

2 76 12 90

Questions: 4. Draw the epidemic curve. Answer: see back page. This is is the 3rd step of epidemic investigation, which is characterizing the epidemic according to person, place, and time.

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What is the shape of the curve?  Answer: Bell What type of outbreak does the shape of the curve suggest?  Answer: The bell curve is explosive, ie, it takes place over a matter of hours versus days. When was the most probably time of exposure? The usual incubation period of gastroenteritis is 12 –  36 hours, with a mean of 24 hours. Determine the most probably time time of exposure using the mean incubation period method and the minimum – maximum incubation period method. Answers: th Mean Incubation Period: Total cases = 90. Median of total cases = 45. 45 patient was seen at 10 pm. The incubation period is 12 – 36 hours, the mean therefore is 24. Count back 24 hours hours to April 27 at 10 pm and this is the mean incubation period. st Minimum – maximum incubation period: Look at the 1 case and count back the minimum incubation period, which is 12 hours. The minimum incubation period is then April 28 at 8 am. Then look at the last case and count back the maximum maximum incubation period, which is 36 hours. The maximum incubation  period is then April 27 at 1 pm. Which population was most affected? affected? (Describe according to age and gender) Answer: Refer to Table Table 2 above. above. Age group most affected was 6 – 10. Gender most affected affected was the females, marginally. In what other way(s) can the affected population be described. Answer: By place. place. See notes to follow. follow. What hypotheses were generated by findings of the descriptive epidemiology?  Answer: Read the following paragraph paragraph to answer this particular particular question. Eating at the hotel and school school cafeteria may be related to developing gastroenteritis. Also, females and those those 6 – 10 years old are at risk. And finally, exposure occurred most likely between between April 27, 1pm to April 28, 8am.

The epidemic team conducted an analytic study and investigated the places where the affected per sons, their relatives, and neighbors ate on April 27  – 28. The following were their findings. findings. Table 3. Distribution of the population by places places where they ate and development of symptoms. Place # who ATE # with Attack # who DID # with Attack Relative at the symptoms Rate NOT EAT symptoms Rate Risk place at the place Local hotel 207 61 29.47 157 47 29.94 0.98 Local 246 25 10.16 122 13 10.66 0.95 restaurant 68 14.32 189 29 15.34 0.93 Lion’s Club 475 Lunch School 289 67 23.18 495 22 4.44 5.22 cafeteria Questions: 9. What research design did the epidemic team use?  Answer: Cohort. This is a retrospective study (looking (looking back in time). And they are comparing the population that were exposed vs. those who were not exposed. 10. Compute for the attack rate rate (per hundred) by place. Which places had the highest attack rate?  rate?  Answer: Attack rates were calculated for for those who ate and for for those who did not not eat at said place. It is calculated by: (Note: there is enough information to fill in the blanks blanks if that information information were necessary but it isn’t in this case) # with symptoms # without symptoms Total 61 207 Ate at place Did not eat at place 47 157 Total A = Attack rate for for those those who are symptomatic symptomatic and ate at a particular particular place A+B C = Attack rate for those who are symptomatic symptomatic and did not eat at a particular place C+D So those places with the highest attack rate s were the local hotel and the school cafeteria. 11. Compute for the relative risk (compare attack rates for those who ate and did not eat at a particular   place). Answer: Relative risk is calculated by: by: Attack rate for those who ate at a particular place (exposed) = Relative risk

Attack rate for those who did not e at at a particular place (non – exposed) Another way to write the above equation is: A/(A + B) = Relative Risk C/(C + D) In what place was the contaminated food most likely served?  Values above show the highest attack rate for the school cafeteria. Therefore, the location of the source of infection is the school cafeteria. After the place of exposure was identified, the team searched for the food that was most likely contaminated. The team discovered the following: Table 4. Distribution of the population by food they ate. Food item ATE the food item DID NOT EAT the food item # # with ATTACK # # with ATTACK symptoms RATE symptoms RATE Burritos 276 28 10.14 266 27 10.15 Cheeseburger 218 21 9.63 131 14 10.69 Salad 105 49 46.67 297 15 5.05 Baked potato 139 11 7.91 213 31 14.55 Fruit cocktail 88 48 54.55 279 25 8.96 Ice cream 175 18 10.29 203 49 24.14

Relative Risk

0.99 0.90 9.24 0.54 6.09 0.43

Questions: 12. What research design did the epidemic team use?  th Answer: This is is the 4 step of epidemic investigation: investigation: identifying and/or pinpointing pinpointing the source of  infection. The researchers used a cohort retrospective retrospective study to perform perform this step: those who ate a particular food item vs. those who did not. 13. Compute for attack rate rate (per hundred) by food eaten. Which food had the highest attack rate?  rate?  Answer: Attack rates are shown shown above. Salad and fruit cocktail had the highest attack rates. 14. Compute for the relative risk (compare attack rates for those who ate and did not eat a particular   food). What was the food food item implicated as sources sources of the outbreak?  Answer: Relative risk values are shown shown above. Salad and fruit fruit cocktail are the suspected suspected sources of the outbreak. She made note that eating potatoes potatoes and ice cream were preventive preventive for developing gastroenteritis because RR