Epidemiology Lecture
-
Upload
khalid-ali -
Category
Documents
-
view
28 -
download
8
description
Transcript of Epidemiology Lecture
Investigating an Outbreak
Principles of EpidemiologyLecture 8
Dona Schneider, PhD, MPH, FACE
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.
Epidemiology (Schneider)
Endemic vs. Epidemic
Endemic Epidemic
No.
of C
ases
of a
Dis
ease
Time
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
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
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?
• Starts slowly• Time between the first case and the peak is comparable
to the incubation period. • Slow tail
Vector-borne Disease
• 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
• In this case, there are several peaks, and the incubation period cannot be identified.
Continuing Common Source or Intermittent Exposure
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
Legionnaires' DiseaseBy date of onset, Philadelphia, July 1-August 18, 1976
Source: Investigating an Outbreak, CDC
Foodborne Outbreak (Propagated)
Source: CDC, unpublished data, 1978
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)
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
Epidemiology (Schneider)
■ Control of present outbreak
■ Prevention of future similar outbreaks
Step 5: Recommend control measures
The vast majority of outbreaks are food-borne
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)
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
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
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
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
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
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
Temperature and Bacteria Control250240
Canning temperatures for low-acid vegetables, meat, and poultry in pressure canner
212
125120
- 20
165
140
98.6
6045
32
0
0 F
Some bacterial growth; many bacteria survive
Canning temperatures for fruits, tomatoes, and pickles in waterbath canner
Water freezesGrowth 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
40Slow growth of some bacteria that cause spoilage
Some growth of food poisoning bacteria may occur
DA
NG
ER ZO
NE
Source: Keeping Food Safe to Eat, USDA
Epidemiology (Schneider)
Bacterial Growth Curve
Num
ber of Cells
Time
Decline Phase
Stationary Phase
Log Phase
Lag Phase
Epidemiology (Schneider)
Effect of Temperature in Salmonella Growth
Num
ber of Salm
onella per gram
Days21 4 53
95oF (35o C)
50oF (10o C)
44oF (6.7o C)
42oF (5.5o C)
Incubation Periods
Contaminated by carrier, not foodborne
Shigella*24-48 hours
Raw fish, shellfishVibrio parahemolyticus*12 hours
Canned foods, smoked fish
Clostridium botulinum12-36 hours
Meat, poultry, eggsSalmonella*12-36 hours
Cooked meats, gravy Clostridium perfringens12 hours
Cooked ham, meat, eggs, sauces and gravies
Staphylococcus aureus2-4 hours
* Fever
National Data on Etiology of Foodborne Illness
7.3%Chemicals (28 agents)
12.3%Fish (28 agents)
-Plants (36 agents)
1.8%Fungal (16 agents)
0.5%Parasites (31 agents)
9.4%Viral (11 agents)
9.5% Clostridium botulinum
10.0% Clostridium perfringens
12.7% Staph. aureus
25.0% Salmonella
68.7%Bacteria (40 agents)
Agent
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
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?
Select the correct case definition and find the error in the others:
• 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.
• 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.
• Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.
Select the correct case definition and find the error in the others:
• 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.
• 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.
• Church members who developed acute gastrointestinal symptoms within 72 hours of the church supper held in Lycoming, Oswego on April 18, 1940.
Select the correct case definition and find the error in the others:
• 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
• 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
• 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
Incidence of Cases of Diarrhea Among People Attending Lycoming,Oswego Church Supper, June 1940
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.
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
Beverages • Milk• Coffee• Water
Church Supper Menu
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.
Epidemiology (Schneider)
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
692742624Fruit salad27720472225Ice cream (choc)21183541143Ice cream (van)351619401327Cakes511833241113Water441727311219Coffee712744422Milk48202827918Brown bread381325371621Rolls52223023716Jello471928281018Cabbage salad371423371423Mashed potato
321220431726Spinach
291217461729Baked ham
Attack rate %TotalWellIllAttack rate (%)TotalWellIll
Number of persons who did not eat specified item
Number of persons who ate specified item
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
6169274267624Fruit salad742772053472225Ice cream (choc)142118380541143Ice cream (van)5435161967401327Cakes6551183354241113Water6144172761311219Coffee6271274450422Milk584820286727918Brown bread6638132557371621Rolls585222307023716Jello6047192864281018Cabbage salad6237142362371423Mashed potato
6232122060431726Spinach
5929121763461729Baked hamAttack rate %TotalWellIllAttack rate (%)TotalWellIll
Number of persons who did not eat specified item
Number of persons who ate specified item
6169274267624Fruit salad742772053472225Ice cream (choc)142118380541143Ice cream (van)5435161967401327Cakes6551183354241113Water6144172761311219Coffee6271274450422Milk584820286727918Brown bread6638132557371621Rolls585222307023716Jello6047192864281018Cabbage salad6237142362371423Mashed potato
6232122060431726Spinach
5929121763461729Baked ham
Attack rate %TotalWellIllAttack rate (%)TotalWellIll
Number of persons who did not eat specified item
Number of persons who ate specified item
Attack Rates by Items Served: Church Supper, Oswego, New York; April 1940
Highlighted row indicates largest difference between attack rates
Attack Rate by Consumption of Vanilla Ice Cream, Oswego, New York; April 1940
61.3752946Total
14.321183No
79.6541143YesAte vanilla ice cream?
Attack Rate (%)
TotalWellIll
• 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
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
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
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
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
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
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
Epidemiology (Schneider)
Some Surveillance Programs
■ National Notifiable Diseases Surveillance Systemhttp://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/
Epidemiology (Schneider)
“Good surveillance does not necessarily ensure
the making of right decisions, but it reduces the
chances of wrong ones.”
Alexander D. LangmuirNEJM 1963;268:182-191