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Investigating an Outbreak Principles of Epidemiology Lecture 8 Dona Schneider, PhD, MPH, FACE.

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Presentation on theme: "Investigating an Outbreak Principles of Epidemiology Lecture 8 Dona Schneider, PhD, MPH, FACE."— Presentation transcript:

1 Investigating an Outbreak Principles of Epidemiology Lecture 8 Dona Schneider, PhD, MPH, FACE

2 Epidemiology (Schneider) What is an outbreak? An epidemic or an outbreak exists when there are more cases of a particular disease than expected in a given area, or among a specific group of people, over a particular period of time.

3 Epidemiology (Schneider) Endemic vs. Epidemic Endemic Epidemic No. of Cases of a Disease Time

4 Epidemiology (Schneider) Why investigate outbreaks or epidemics? Control and prevention Severity and risk to others Research opportunities to gain additional knowledge Training opportunities Program considerations Public, political, or legal concerns

5 Epidemiology (Schneider) Step 1: Verify the outbreak Determine whether there is an outbreak – an excess number of cases from what would be expected Establish a case definition Non-ambiguous Clinical / diagnostic verification Person / place / time descriptions Identify and count cases of illness

6 Epidemiology (Schneider) Step 2: Plot an Epidemic Curve Graph of the number of cases (y-axis) by their date or time of onset (x-axis) Interpreting an epidemic curve Overall pattern: increase, peak, decrease Type of epidemic? Incubation period? Outliers: Unrelated? Early or late exposure? Index case? Secondary cases?

7 Starts slowly Time between the first case and the peak is comparable to the incubation period. Slow tail Vector-borne Disease

8 This is the most common form of transmission in food- borne disease, in which a large population is exposed for a short period of time. Point Source Transmission

9 In this case, there are several peaks, and the incubation period cannot be identified. Continuing Common Source or Intermittent Exposure

10 Salmonellosis in passengers on a flight from London to the United States, by time of onset, March 13--14, 1984 Source: Investigating an Outbreak, CDC

11 Legionnaires' Disease By date of onset, Philadelphia, July 1-August 18, 1976 Source: Investigating an Outbreak, CDC

12 Foodborne Outbreak (Propagated) Source: CDC, unpublished data, 1978

13 Epidemiology (Schneider) Step 3: Calculate attack rates Attack rate = (ill / ill + well) x 100 during a time period If there is an obvious commonality for the outbreak, calculate attack rates based on exposure status (a community picnic) If there is no obvious commonality for the outbreak, calculate attack rates based on specific demographic variables (hepatitis cases in a community)

14 Epidemiology (Schneider) Step 4: Determine the source of the epidemic If there is an obvious commonality for the outbreak, identify the most likely cause and investigate the source to prevent future outbreaks If there is no obvious commonality for the outbreak, plot the geographic distribution of cases by residence/ work/school/location and seek common exposures

15 Epidemiology (Schneider) Control of present outbreak Prevention of future similar outbreaks Step 5: Recommend control measures

16 The vast majority of outbreaks are food-borne

17 Foodborne Disease Outbreak An incident in which (1) two or more persons experience a similar illness after ingestion of a common food, and (2) epidemiologic analysis implicates the food as the source of the illness  Intoxication – ingestion of foods with Toxicants found in tissues of certain plants (Jimpson Weed) and animals (seal liver) Metabolic products (toxins) formed and excreted by microorganisms while they multiply (botulinum toxin) Poisonous substances introduced during production, processing, transportation or storage (chemicals, pesticides)

18 Foodborne Disease Outbreak (cont.)  Infections – Caused by the entrance of pathogenic microorganisms into the body and the reaction of the body tissues to their presence or to toxins they generate within the body  Rule of thumb – but not law Intoxicants are rapid onset, no fever Toxins in the stomach produce vomiting Toxins in the intestines produce diarrhea Infections produce fever

19 Epidemiology (Schneider) Types of Foodborne Contamination Physical Glass, metal fragments, tacks, dirt, bone, etc. Chemical Pesticides, cleaning compounds, poisonous metals, additives and preservatives Biological Bacteria, viruses, fungi, yeast, molds, parasites, poisonous fish and plants, insect and rodents

20 Epidemiology (Schneider) Bacterial Requirements Food: Most bacteria require what is known as potentially hazardous food Milk or milk products, eggs, meat, poultry, fish, shellfish, crustaceans, raw seed sprouts, heat treated vegetables and vegetable products (fruits?) Generally high protein, moist foods

21 Epidemiology (Schneider) Bacterial Requirements (cont.) Water: Bacteria require moisture to thrive The water activity (Aw) is the amount of water available in food The lowest Aw at which bacteria will grow is 0.85 Most potentially hazardous foods have a water activity of 0.97 to 0.99 pH: Best growth at neutral or slightly acidic pH Potentially hazardous foods have a pH of 4.6 – 7.0

22 Epidemiology (Schneider) Bacterial Requirements (cont.) Temperature: The danger zone for potentially hazardous foods is 45 to 140 degrees Fahrenheit This is the zone where most bacterial growth occurs Time: Potentially hazardous foods must not be allowed to remain in the danger zone for more than 4 hours Oxygen: Some bacteria require oxygen while others are anaerobic and others are facultative

23 Epidemiology (Schneider) Improper cooling of foods Improper cooking of foods Improper reheating of foods Improper holding temperature of foods Cross contamination Infected food handlers, poor employee hygiene Major Causes of Foodborne Disease

24 Temperature and Bacteria Control 250 240 Canning temperatures for low-acid vegetables, meat, and poultry in pressure canner 212 125 120 - 20 165 140 98.6 60 45 32 0 0 F Some bacterial growth; many bacteria survive Canning temperatures for fruits, tomatoes, and pickles in waterbath canner Water freezes Growth of bacteria is stopped, but bacteria level before freezing remains constant and not reduced Keep frozen foods in this range Water boils Most bacteria destroyed No growth, but survival of some bacteria Hottest temperature hands can stand Extreme DANGER ZONE. Rapid growth of bacteria and production of poisons by some bacteria Body temperature – ideal for bacterial growth 40 Slow growth of some bacteria that cause spoilage Some growth of food poisoning bacteria may occur DANGER ZONE Source: Keeping Food Safe to Eat, USDA

25 Epidemiology (Schneider) Bacterial Growth Curve Number of Cells Time Decline Phase Stationary Phase Log Phase Lag Phase

26 Epidemiology (Schneider) Effect of Temperature in Salmonella Growth Number of Salmonella per gram Days 21 453 95 o F (35 o C) 50 o F (10 o C) 44 o F (6.7 o C) 42 o F (5.5 o C)

27 Incubation Periods 2-4 hoursStaphylococcus aureus Cooked ham, meat, eggs, sauces and gravies 12 hoursClostridium perfringensCooked meats, gravy 12-36 hoursSalmonella*Meat, poultry, eggs 12-36 hoursClostridium botulinum Canned foods, smoked fish 12 hoursVibrio parahemolyticus*Raw fish, shellfish 24-48 hoursShigella*Contaminated by carrier, not foodborne * Fever

28 National Data on Etiology of Foodborne Illness Agent Bacteria (40 agents)68.7% Salmonella25.0% Staph. aureus12.7% Clostridium perfringens10.0% Clostridium botulinum9.5% Viral (11 agents)9.4% Parasites (31 agents)0.5% Fungal (16 agents)1.8% Plants (36 agents)- Fish (28 agents)12.3% Chemicals (28 agents)7.3%

29 On April 19, 1940, the local health officer in the village of Lycoming, Oswego County, New York, reported the occurrence of an outbreak of acute gastrointestinal illness to the District Health Officer in Syracuse. Dr. A. M. Rubin, epidemiologist-in-training, was assigned to conduct an investigation. When Dr. Rubin arrived in the field, he learned from the health officer that all persons known to be ill had attended a church supper the previous evening, April 18. Family members who had not attended the church supper had not become ill. Accordingly, the investigation was focused on the circumstances related to the supper. Source: CDC Investigating an Epidemic: Oswego, NY

30 Epidemiology (Schneider) Interviews regarding the presence of symptoms, including the day and hour of onset, and the food consumed at the church supper, were completed on 75 of the 80 persons known to have been present. A total of 46 persons who had experienced gastrointestinal illness were identified. Q: Is this an Epidemic? Endemic for the region? Due to seasonal variation? Due to random variation?

31 Select the correct case definition and find the error in the others: 1.All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. 2.Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper. 3.Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.

32 Select the correct case definition and find the error in the others: 1.All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. 2.Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper. 3.Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.

33 Select the correct case definition and find the error in the others: 1.All participants in the Oswego church supper held in the basement of the church in Lycoming, Oswego County, New York, on April 18, 1940, between 6:00 PM and 11:00 PM; whether they attended church or not; whether they participated in food preparation, transport, or distribution or not; whether they ate or not. Missing definition of sickness 2.Persons who developed acute gastrointestinal symptoms within 72 hours of eating supper on April 18, 1940, and who were among attendees of the Lycoming, Oswego Church supper. CORRECT 3.Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940. Did not specify that they went to the dinner

34 Incidence of Cases of Diarrhea Among People Attending Lycoming,Oswego Church Supper, June 1940

35 Epidemiology (Schneider) The supper was held in the basement of the village church. Foods were contributed by numerous members of the congregation. The supper began at 6:00 PM and continued until 11:00 PM. Food was spread out upon a table and consumed over a period of several hours.

36 Epidemiology (Schneider) Main Dishes Baked ham Spinach Mashed potatoes Cabbage salad Fruit Salad Side Dishes Jello Rolls Brown Bread Desserts Cakes Vanilla Ice Cream Chocolate Ice Cream BeveragesMilk Coffee Water Church Supper Menu

37 Epidemiology (Schneider) Which menu item(s) is the potential culprit? To find out, calculate attack rates. The foods that have the greatest difference in attack rates may be the foods that were responsible for the illness.

38 Epidemiology (Schneider) Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940 Number of persons who ate specified item Number of persons who did not eat specified item IllWellTotalAttack rate (%)IllWellTotalAttack rate % Baked ham291746171229 Spinach261743201232 Mashed potato231437231437 Cabbage salad181028 1947 Jello16723302252 Rolls211637251338 Brown bread18927282048 Milk224442771 Coffee191231271744 Water131124331851 Cakes271340191635 Ice cream (van)43115431821 Ice cream (choc)25224720727 Fruit salad426422769

39 Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940 Number of persons who ate specified item Number of persons who did not eat specified item IllWellTotalAttack rate (%)IllWellTotalAttack rate % Baked ham2917466317122959 Spinach2617436020123262 Mashed potato2314376223143762 Cabbage salad1810286428194760 Jello167237030225258 Rolls2116375725133866 Brown bread189276728204858 Milk2245044277162 Coffee1912316127174461 Water1311245433185165 Cakes2713406719163554 Ice cream (van)431154803182114 Ice cream (choc)252247532072774 Fruit salad4266742276961

40 Number of persons who ate specified item Number of persons who did not eat specified item IllWellTotalAttack rate (%)IllWellTotalAttack rate % Baked ham2917466317122959 Spinach2617436020123262 Mashed potato2314376223143762 Cabbage salad1810286428194760 Jello167237030225258 Rolls2116375725133866 Brown bread189276728204858 Milk2245044277162 Coffee1912316127174461 Water1311245433185165 Cakes2713406719163554 Ice cream (van)431154803182114 Ice cream (choc)252247532072774 Fruit salad4266742276961 Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940 Highlighted row indicates largest difference between attack rates

41 Attack Rate by Consumption of Vanilla Ice Cream, Oswego, New York; April 1940 IllWellTotal Attack Rate (%) Ate vanilla ice cream? Yes43115479.6 No3182114.3 Total46297561.3 The relative risk is calculated as 79.6/14.3 or 5.6 The relative risk indicates that persons who ate vanilla ice cream were 5.6 times more likely to become ill than those who did not eat vanilla ice cream

42 Epidemiology (Schneider) Conclusion An attack of gastroenteritis occurred following a church supper at Lycoming The cause of the outbreak was most likely contaminated vanilla ice cream

43 Epidemiology (Schneider) Surveillance Ongoing systematic collection, collation, analysis and interpretation of data; and the dissemination of information to those who need to know in order that action may be taken. World Health Organization

44 Epidemiology (Schneider) Purposes of Public Health Surveillance Estimate magnitude of the problem Determine geographic distribution of illnesses Portraying the natural history of disease Detect epidemic / Define a problem Generate hypotheses and stimulate research Evaluate control measures Monitor changes in infectious agents Detect changes in health practice Facilitate planning CDC

45 Epidemiology (Schneider) Passive Surveillance Physicians, laboratories, and hospitals are given forms to complete and submit with the expectation that they will report all of the cases of reportable disease that come to their attention Advantages: Inexpensive Disadvantages: Data are provided by busy health professionals. Thus, the data are more likely to be incomplete and underestimate the presence of disease in the population

46 Epidemiology (Schneider) Active Surveillance Involves regular periodic collection of case reports by telephone or personal visits to the reporting individuals to obtain the data Advantages: More accurate because it is conducted by individuals specifically employed to carry out the responsibility Disadvantages: Expensive

47 Epidemiology (Schneider) Sentinel Surveillance Monitoring of key health events, through sentinel sites, events, providers, vectors/animals Case report indicates a failure of the health care system or indicates that special problems are emerging Advantages: Very inexpensive Disadvantages: Applicable only for a select group of diseases

48 Epidemiology (Schneider) Some Surveillance Programs National Notifiable Diseases Surveillance System http://www.cdc.gov/epo/dphsi/nndsshis.htm Morbidity and Mortality Weekly Report (MMWR) http://www.cdc.gov Cancer Surveillance, Epidemiology and End Result (SEER) http://www.seer.cancer.gov/

49 Epidemiology (Schneider) “ Good surveillance does not necessarily ensure the making of right decisions, but it reduces the chances of wrong ones. ” Alexander D. Langmuir NEJM 1963;268:182-191


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