Điều Tra Vụ bùng phát
bệnh từ thực phẩm
GS, Ts Lê Hoàng Ninh
Bùng phát là gì ?(outbreak)
Dịch ( epidemic) hay còn gọi là bùng phát ( outbreak)khi
số ca bệnh cao hơn số dự kiến xảy ra ( trị số bình
thường) tại một địa phương, khu vực nào đó, hay trên
một nhóm dân số nào đó trong một thời khoảng nhất
định
Epidemiology (Schneider)
Số ca bệnh
Bệnh lưu hành địa phương
(endemic)và dịch ( epidemic)
Lưu hành
Thời
gian
Epidemiology (Schneider)
Dịch
Tại sao phải điều tra bùng phát/ dịch?

Kiểm soát và phòng ngừa

Sự ác tính và nguy cơ lây truyền cho người khác

Cơ hội nghiên cứu để hiểu biết tốt hơn

Cơ hội đào tạo

Xem xét chương trình y tế

Cab quan ngại khác: luật, chính trị, công cộng…
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


Epidemiology (Schneider)
Overall pattern: increase, peak, decrease

Type of epidemic?

Incubation period?
Outliers:

Unrelated?

Early or late exposure?

Index case? Secondary cases?
Vector-borne Disease
• Starts slowly
• Time between the first case and the peak is comparable
to the incubation period.
• Slow tail
Point Source Transmission
• This is the most common form of transmission in foodborne disease, in which a large population is exposed for
a short period of time.
Continuing Common Source or Intermittent Exposure
• In this case, there are several peaks, and the incubation
period cannot be identified.
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' Disease
By date of onset, Philadelphia, July 1-August 18, 1976
Source: Investigating an Outbreak, CDC
Foodborne Outbreak (Propagated)
Source: CDC, unpublished data, 1978
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)
Step 5: Recommend control measures

Control of present outbreak

Prevention of future similar outbreaks
Epidemiology (Schneider)
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
Types of Foodborne Contamination

Physical


Chemical


Glass, metal fragments, tacks, dirt, bone, etc.
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)
Major Causes of Foodborne Disease

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
Epidemiology (Schneider)
0F
250
240
Temperature and Bacteria Control
Canning temperatures for low-acid vegetables, meat, and poultry in pressure canner
Canning temperatures for fruits, tomatoes, and pickles in waterbath canner
212
Water boils
Most bacteria destroyed
165
No growth, but survival of some bacteria
DANGER ZONE
140
Some bacterial growth; many bacteria survive
125
120
Hottest temperature hands can stand
Extreme DANGER ZONE. Rapid growth of bacteria and production
of poisons by some bacteria
98.6
60
45
40
32
Body temperature – ideal for bacterial growth
Some growth of food poisoning bacteria may occur
Slow growth of some bacteria that cause spoilage
Water freezes
Growth of bacteria is stopped, but bacteria level before freezing
remains constant and not reduced
0
- 20
Keep frozen foods in this range
Source: Keeping Food Safe to Eat, USDA
Bacterial Growth Curve
Stationary Phase
Number of Cells
Log Phase
Lag Phase
Time
Epidemiology (Schneider)
Decline Phase
Number of Salmonella per gram
Effect of Temperature in
Salmonella Growth
50oF (10o C)
95oF (35o C)
44oF (6.7o C)
42oF (5.5o C)
1
2
3
Days
Epidemiology (Schneider)
4
5
Incubation Periods
2-4 hours
Staphylococcus aureus
12 hours
Clostridium perfringens
Cooked meats, gravy
12-36 hours
Salmonella*
Meat, poultry, eggs
12-36 hours
Clostridium botulinum
12 hours
Vibrio parahemolyticus*
24-48 hours
Shigella*
* Fever
Cooked ham, meat,
eggs, sauces and gravies
Canned foods, smoked
fish
Raw fish, shellfish
Contaminated by carrier,
not foodborne
National Data on Etiology of Foodborne Illness
Agent
Bacteria (40 agents)
68.7%
Salmonella
25.0%
Staph. aureus
12.7%
Clostridium perfringens
10.0%
Clostridium botulinum
9.5%
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%
Investigating an Epidemic: Oswego, NY
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
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?
Epidemiology (Schneider)
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.
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.
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
Incidence of Cases of Diarrhea Among People Attending Lycoming,Oswego Church Supper, June 1940
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)
Church Supper Menu
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
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
Number of persons who ate
specified item
Well
Total
Attack rate (%)
17
46
Number of persons who did not eat
specified item
Baked ham
Ill
29
Spinach
26
17
43
20
12
32
Mashed potato
23
14
37
23
14
37
Cabbage salad
18
10
28
28
19
47
Jello
16
7
23
30
22
52
Rolls
21
16
37
25
13
38
Brown bread
18
9
27
28
20
48
Milk
2
2
4
44
27
71
Coffee
19
12
31
27
17
44
Water
13
11
24
33
18
51
Cakes
27
13
40
19
16
35
Ice cream (van)
43
11
54
3
18
21
Ice cream (choc)
25
22
47
20
7
27
Fruit salad
4
2
6
42
27
69
Epidemiology (Schneider)
Ill
17
Well
12
Total
29
Attack rate %
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
Baked ham
Ill
29
Well
17
Total
46
Attack rate (%)
63
Ill
17
Well
12
Total
29
Attack rate %
59
Spinach
26
17
43
60
20
12
32
62
Mashed potato
23
14
37
62
23
14
37
62
Cabbage salad
18
10
28
64
28
19
47
60
Jello
16
7
23
70
30
22
52
58
Rolls
21
16
37
57
25
13
38
66
Brown bread
18
9
27
67
28
20
48
58
Milk
2
2
4
50
44
27
71
62
Coffee
19
12
31
61
27
17
44
61
Water
13
11
24
54
33
18
51
65
Cakes
27
13
40
67
19
16
35
54
Ice cream (van)
43
11
54
80
3
18
21
14
Ice cream (choc)
25
22
47
53
20
7
27
74
Fruit salad
4
2
6
67
42
27
69
61
Attack Rates by Items Served: Church Supper, Oswego, New York;
April 1940
Number of persons who ate
Ill
specified item
Well
Total
Attack rate (%)
Number of persons who did not eat
specified item
Ill
Well
Total
Attack rate %
Baked ham
29
17
46
63
17
12
29
59
Spinach
26
17
43
60
20
12
32
62
Mashed potato
23
14
37
62
23
14
37
62
Cabbage salad
18
10
28
64
28
19
47
60
Jello
16
7
23
70
30
22
52
58
Rolls
21
16
37
57
25
13
38
66
Brown bread
18
9
27
67
28
20
48
58
Milk
2
2
4
50
44
27
71
62
Coffee
19
12
31
61
27
17
44
61
Water
13
11
24
54
33
18
51
65
Cakes
27
13
40
67
19
16
35
54
Ice cream (van)
43
11
54
80
3
18
21
14
Ice cream (choc)
25
22
47
53
20
7
27
74
Fruit salad
4
2
6
67
42
27
69
61
Highlighted row indicates largest difference between attack rates
Attack Rate by Consumption of Vanilla Ice
Cream, Oswego, New York; April 1940
Ate vanilla
ice cream?
Ill
Well
Total
Attack Rate
(%)
Yes
43
11
54
79.6
No
3
18
21
14.3
Total
46
29
75
61.3
•
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
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 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/
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
Epidemiology (Schneider)