Overview of steps of OBI Manish Chaudhary MPH

Overview of steps of OBI Manish Chaudhary MPH,

Steps in an OBI 1. Verify the accuracy of disease reports • Confirm the diagnosis 2. Determine existence of an outbreak • Compare observed vs. expected in a preliminary investigation 3. Establish a case definition • • May need to be modified as more information is available When appropriate, classify by confirmed, probable or possible 4. Identify additional cases 5. 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

Steps in an OBI 6. Generate and test hypothesis (eg. Disease causation, risk factors, transmission 7. Monitor course of the outbreak and reassess strategies 8. Carry out lab and environmental investigations 9. Implement disease control measures 10. Communicate findings

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 .

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.

• 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

2 Data analysis: consolidate the data so obtained 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 each kind of food.

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.

5 Investigation on the source of infection: 1. Laboratory examination of food proved by previous analysis to be causative. 2. Physical examination of food handlers and the food transportation. 3. Scrutinizing examination of place and utensils used for food preparation.

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………………………

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

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

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……………….

Analysis example • There are 95 persons on party, Investigators determined that of 64 cases with onset, all had eaten lunch in Arafat at 2 pm on October 31. Fifteen members of the mission did not eat lunch; none became ill. • Q 1: 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.

Q 2. Using appropriate time periods, draw an epidemic curve 40 37 Epidemic Curve 35 30 25 20 No. of people 15 11 9 10 5 3 2 1 1 25 -30 hr 30 -35 hr 0 0 -5 hr 5 -10 hr 10 -15 hr 15 -20 hr 20 -25 hr

• Q 3: Are there any cases for which the time of onset seems inconsistent? How might they be explained? • Maximum cases had incubation period between 10 -15 hrs whereas few cases had incubation period ranging from 0 -5 hrs (minimum) to 30 -35 hrs (maximum). Probable reason could be food contamination with organism with shorter or longer incubation periods

Q 4. Modify the graph you have drawn to illustrate the distribution of incubation periods.

Q 5. 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 -10 7. 5 9 67. 5 11 10 -15 12. 5 37 462. 5 48 15 -20 17. 5 11 192. 5 59 20 -25 22. 5 3 67. 5 62 25 -30 27. 5 1 27. 5 63 30 -35 32. 5 1 32. 5 64 855

• Mean = 855/64 = 13. 35 hours • Median = 64/2 = 32 nd observation • Applying formula for median in grouped series 2 х (32 -11)/37 = 1. 13 Therefore, Median = 1. 13+ 10 =11. 13 hours

• Q 6: Calculate the frequency of each clinical symptom among the cases • • • Diarrhoea : 62/64= 96. 8 % Cramps : 52/64 = 81. 25 % Blood in stool : 8/64 = 12. 5 % Nausea = 2/64 = 3. 1% Vomiting = 2/64 = 3. 1%

• Qno. 7 : How does the information on the symptoms and incubation periods help you to narrow the differential diagnosis? Salmonella food poisoning: Incubation period: 12 -24 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 12 -36 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 10 -14 hours) Symptoms: diarrhoea, abdominal cramps 8 -24 hrs after consumption of food. Rapid recovery B. cereus food poisoning : IP 1 -6 hrs for emetic form and 12 -24 hrs for diarrhoeal form) Symptoms: vomitting, diarrhoea, abdominal pain, no vomiting or fever

Q 8. Calculate the attack rate among the specific food consumer and non consumer, also prepare 2 х2 table for specific food consumers. Symptoms Rice Present Absent Ate 62 31 93 Did not eat 2 0 2 64 31 95 Attack rate (consumed rice) = 62/93= 66. 66% Attack rate (not consumed rice) = 2/2 = 100%

Symptoms Meat Present Absent Ate 63 25 88 Did not eat 1 6 7 64 31 95 • Attack rate (consumed meat)= 63/88=71. 59% • Attack rate (not consumed meat)= 1/7= 14. 28%

Symptoms Tomato Sauce Present Absent Ate 50 26 76 Did not eat 14 5 19 64 31 95 • Attack rate (consumed sauce)= 50/76= 65. 78% • Attack rate (not consumed sauce) = 14/19= 73. 68%

• Q 9. 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.

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: Ø The lunch was prepared the night before at 10 pm Ø The food pots were placed in an open spot near the kitchen and allowed to stand overnight Ø On October 31, the pots were kept in the truck only, till 2 pm. Ø Temperature 35°C that day Ø Food not refrigerated from time of preparation to consumption Ø Lack of sanitary cooking facilities Ø Cooking in dusty open space

Q 11. 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

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