1. Overview of steps of OBI
Manish Chaudhary
MPH,

2. Steps in an OBI Verify the accuracy of disease reports
Confirm the diagnosis
Determine existence of an outbreak
Compare observed vs. expected in a preliminary investigation
Establish a case definition
May need to be modified as more information is available
When appropriate, classify by confirmed, probable or possible
Identify additional cases
Conduct descriptive epidemiology
Note: These steps do not necessarily occur sequentially, and not all the steps are involved in every investigation. Several steps can be implemented simultaneously and may be ongoing

3. Steps in an OBI
Generate and test hypothesis (eg. Disease causation, risk factors, transmission
Monitor course of the outbreak and reassess strategies
Carry out lab and environmental investigations
Implement disease control measures
Communicate findings

4. Summary Steps outlined are a basic guide. .
Remember that many steps occur simultaneously
Make good communication a priority
Be flexible; adapt to the actual event .

5. Example: Gastroenteritis( food poisoning)
The investigation was conducted in order to determine which of the food eaten was contaminated with microorganisms or chemicals responsible. If the outbreak is sudden, it is not difficult to locate the suspected meal, to which most of the sick persons had attended.

6. 1 Survey
Interview everybody who attended that particular meal.
1.1 types of food eaten within 48 hrs
1.2 time of eating
1.3 whether he/she has signs and symptoms of food poisoning
1.4 important clinical characteristics

7. 2 Data analysis: consolidate the data so obtained and analyzed for 2
2 Data analysis: consolidate the data so obtained and analyzed for 2.1 common signs and symptoms 2.2 duration of illness, severity and nature of the disease 2.3 incubation period 2.4 attack rate among those who ate and did not eat each kind of food.

8. 3. Hypothesis formulation: hypothesis on the causative food and pathogenic agent. 4. Testing of hypothesis: 4.1 Search for association between suspected food and incidence of gastroenteritis( using relative risk) 4.2 test for statistical significance using Chi square test.

9. 5 Investigation on the source of infection:
Laboratory examination of food proved by previous analysis to be causative.
Physical examination of food handlers and the food transportation.
Scrutinizing examination of place and utensils used for food preparation.

10. Example of questionnaire on food poisoning investigation
Name of the resondent Age Sex Occupation Address Do you have abnormal feeling after eating the meal? No( ) Yes ( ) If yes explain the feelings…………………………………….. Time of onset………………………………………………

11. Example of questionnaire on food poisoning investigation
List of food eaten and time
Food eaten
Time

12. Example of questionnaire on food poisoning investigation
Signs and symptoms
Nausea
Abdominal pain
Dizziness
Nunb
Vomit
Diarrohea
Blurred vision
Fever unsoncious
Others specify…………………

13. Example of questionnaire on food poisoning investigation
Did you go to see doctor? Yes ( ) No ( ) If yes, indicate the doctors name……………… Address…………… Tel………………… Date of food eaten………… Date of interview……………….

14. Analysis example
There are 95 persons on party, Investigators determined that of 64 cases with onset, all had eaten lunch in Arafat at 2pm on October 31. Fifteen members of the mission did not eat lunch; none became ill.
Q1: Calculate the attack rate for those who ate lunch and those who did not. What do you conclude?
Attack rate: no. of new cases of a specified disease during a specified time interval/ total population at risk during the same interval
For those who ate lunch = 64/95 × 100% = 67.37%
For those who did not eat lunch = 0/15=0
Attack rate was higher among those who ate lunch at Arafat as compared to those who did not.

15. Q 2. Using appropriate time periods, draw an epidemic curve

16. Q3: Are there any cases for which the time of onset seems inconsistent
Q3: Are there any cases for which the time of onset seems inconsistent? How might they be explained?
Maximum cases had incubation period between hrs whereas few cases had incubation period ranging from 0-5 hrs (minimum) to hrs (maximum). Probable reason could be food contamination with organism with shorter or longer incubation periods

17. Q4. Modify the graph you have drawn to illustrate the distribution of incubation periods.

18. Q5. Determine or calculate the minimum, maximum, mean, median, mode, range and standard deviation of the incubation periods.
Hours
Midpoint (х)
No. of People (f) fх
Cumulative frequency
0-5
2.5 2 5
5-10
7.5 9
67.5 11
10-15
12.5 37
462.5 48
15-20
17.5
192.5 59
20-25
22.5 3 62
25-30
27.5 1 63
30-35
32.5 64
855

19. Mean = 855/64 = hours
Median = 64/2 = 32nd observation
Applying formula for median in grouped series
2х (32-11)/37 = 1.13
Therefore, Median = =11.13 hours

20. Q6: Calculate the frequency of each clinical symptom among the cases
Diarrhoea : 62/64= 96.8 %
Cramps : 52/64 = %
Blood in stool : 8/64 = 12.5 %
Nausea = 2/64 = 3.1%
Vomiting = 2/64 = 3.1%

21. Qno.7 : How does the information on the symptoms and incubation periods help you to narrow the differential diagnosis?
Salmonella food poisoning: Incubation period: hours. Symptoms : chills, fever, nausea, vomiting and profuse watery diarrhoea for 2-3 days
Staphylococcal food poisoning : IP (1-6 hours). Symptoms: sudden onset vomiting, abdominal cramps and diarrhoea
Botulism :IP hours. Symptoms: dysphagia, diplopia, ptosis, dysarthria, blurring of vision, muscle weakness, quadriplegia. Frequently fatal
Cl. Perfringens food poisoning: IP (6 to 24 hours with peak at hours) Symptoms: diarrhoea, abdominal cramps 8-24hrs after consumption of food. Rapid recovery
B. cereus food poisoning : IP 1-6 hrs for emetic form and hrs for diarrhoeal form) Symptoms: vomitting, diarrhoea, abdominal pain, no vomiting or fever

22. Q8. Calculate the attack rate among the specific food consumer and non consumer, also prepare 2х2 table for specific food consumers.
Attack rate (consumed rice) = 62/93= 66.66%
Attack rate (not consumed rice) = 2/2 = 100%
Symptoms
Rice
Present
Absent
Ate 62 31 93
Did not eat 2 64 95

23. Attack rate (consumed meat)= 63/88=71.59%
Attack rate (not consumed meat)= 1/7= 14.28%
Symptoms
Meat
Present
Absent
Ate 63 25 88
Did not eat 1 6 7 64 31 95

24. Attack rate (consumed sauce)= 50/76= 65.78%
Attack rate (not consumed sauce) = 14/19= 73.68%
Symptoms
Tomato Sauce
Present
Absent
Ate 50 26 76
Did not eat 14 5 19 64 31 95

25. Q9. Do these calculations help you to determine which food(s) served at the lunch may have been responsible for the outbreak?
Yes, Consumption of meat may have been responsible for the outbreak as the attack rate among those who consumed meat is higher as compared to rice and sauce. Similarly, attack rate among those who did not consume meat is lower compared to rice and sauce.

26. Q 10. Outline further investigations which should be pursued
Q 10. Outline further investigations which should be pursued. List one or more factors that could have led to the contamination of the implicated food
Stool culture
Serological assay for fecal cytotoxin
Blood for antibodies
Environmental study : eating place, kitchen, questioning of food handlers regarding food preparation
Factors:
The lunch was prepared the night before at 10 pm
The food pots were placed in an open spot near the kitchen and allowed to stand overnight
On October31, the pots were kept in the truck only, till 2pm.
Temperature 35°C that day
Food not refrigerated from time of preparation to consumption
Lack of sanitary cooking facilities
Cooking in dusty open space

27. Personal hygiene among those handle, prepare and cook the food
Q11. In the context of this outbreak, what control measures would you recommend?
Personal hygiene among those handle, prepare and cook the food
Infected food handlers should be excluded from further handling
Sanitary improvements : food premises should be free from dust
Health education: thorough hand washing
The remaining food should be condemned

28. THANK YOU