1. Define case and conduct case finding
Dr. Christina Rundi
Ministry of Health, Malaysia
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

2. Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009
Introduction
Careful description and characterization of the outbreak is an important first step in any epidemiological investigation.
Descriptive epidemiology provides a picture of the outbreak in terms of the three standard epidemiological parameters – time, place and person.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

3. What is case definition?
A set of criteria to determine whether a person should be classified as being affected by the disease under investigation.
It is an epidemiological tool for counting cases , not used to guide clinical practice.
A case definition should be simple and practical.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

4. Components of Case Definition
FOUR components:
 clinical and laboratory criteria; the clinical features should be significant or hallmark signs of the illness;
 a defined period of time;
 restriction by “place”;
 restriction by “person” characteristics – limiting the group to, for example, persons over one year of age, persons with no recent diarrhoeal disease, etc.
clinical and laboratory criteria to assess whether a person has the illness under investigation; the clinical features should be significant or hallmark signs of the illness;
 a defined period of time during which cases of illness are considered to be associated with the outbreak;
 restriction by “place” – for example, limiting the group to patrons of a particular restaurant, employees of a particular factory or residents of a particular town;
 restriction by “person” characteristics – limiting the group to, for example, persons over one year of age, persons with no recent diarrhoeal disease, etc.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

5. Common components and examples of an outbreak case definition
Element* Descriptive features Examples
Person Age group “children under the age of 5 years”
Sex “males”
Occupation “health care workers at hospital X” Race
Exclusion criteria “persons with no previous history of chronic cough or asthma”
Place Geographic location “resident of Y county or state”
Facility “living in X nursing home”; “student at A high school”
Time Illness onset “onset of illness between May and August 31, 2007”
Clinical features Pneumonia “clinical or radiographically confirmed pneumonia” “shortness of breath and fever”
Laboratory Cultures; serology Pneumococcus isolated
criteria from blood; rapid influenza test positive
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

6. Examples of a case definition:
"Student attending X High School who has onset of fever and cough between January 4 and 24, 2007."
“A resident of, or visitor to, Rapid City, South Dakota who was diagnosed by a physician, either clinically or radiographically, with community-acquired pneumonia (CAP) with symptom onset after May 1, 2005 and who had laboratory confirmation of Legionnaires’ disease by culture of Legionella, by urinary antigen test for Legionella pneumophila serogroup 1 (Lp1), by a four-fold or greater rise in serum antibody titer to Lp1, or detection of specific Legionella antigen by direct fluorescent antibody staining.”
Gregg, M.B. Field Epidemiology. New York: Oxford University Press, 2002.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

7. Concept of Sensitivity and Specificity
The sensitivity measures the proportion of actual positives which are correctly identified.
Example: the percentage of sick people who are identified as having the condition.
The specificity measures the proportion of negatives which are correctly identified.
Example: the percentage of well
people who are identified as not
having the condition.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

8. Sensitivity and specificity
Ideally, a case definition will include all cases (high sensitivity) but exclude any person who does not have the illness (high specificity).
A sensitive case definition will detect many cases but may also count as cases individuals who do not have the disease.
A more specific case definition is more likely to include only persons who truly have the disease under investigation but also more likely to miss some cases.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

9. How to develop a Case Definition
Measles: possible case definitions
Fever and runny nose?
Fever and rash and Koplik’s spots and conjunctivitis?
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

10. How to develop a Case Definition
Measles: definitions
Fever and runny nose
Too sensitive
Too many other illnesses produce same symptoms
Call many illnesses “measles”
Fever and rash and Koplik’s spots and conjunctivitis
Too specific
Many cases of measles do not have all these signs
Miss many real cases of measles
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

11. How to develop a Case Definition
CDC case definition:
generalized maculopapular rash > 3 days and fever and at least one of the following: cough, coryza or conjunctivitis
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

12. Issues with sensitivity and specificity
No rules about how sensitive or specific a case definition should be.
Early stage of an outbreak investigation – to detect as many cases as possible requires a sensitive case definition. (e.g. a person with three or more loose stools in a 24-hour period).
Later stage, the clinical picture is often clearer and the diagnosis is laboratory-confirmed; can use a more specific case definition (e.g. laboratory-confirmed Salmonella infection).
A single case definition that suits all needs is rare.
Quite common for case definitions to change during an investigation.
A case definition can emphasize getting all possible cases (sensitivity), or can emphasize having only the exact illness you are investigating as cases (specificity). Generally, you start with a “loose” definition early in the investigation, which lends itself to identifying anyone who might possibly be a case (this is a sensitive case definition). It is better to gather too much information than too little. For instance, getting information about patients that later end up not being true cases is better than having to go back and find cases that you mistakenly ruled out early on. And gathering information from potential cases about many exposures that turn out not to be related to the outbreak is easier than going back, trying to contact cases again, and asking more questions about exposures you did not include in your early investigation because of a narrow case definition. As you become more sure of the symptoms and the agent, the place of exposure, and the time frame, you can safely narrow the case definition.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

13. Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009
Case definitions
Confirmed cases – positive laboratory result (isolation of the causative agent or positive serological test). This case definition has high specificity. Patients with epidemiological link with patients. Probable cases – have the typical clinical features of the illness but without laboratory confirmation. Possible cases – have fewer or atypical clinical features. This case definition has high sensitivity.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

14. Example: Salmonellosis
Clinical description: An illness of variable severity commonly manifested by diarrhea, abdominal pain, nausea, and sometimes vomiting. Asymptomatic infections may occur and the organism may cause extraintestinal infections.
Laboratory criteria for diagnosis: Isolation of Salmonella from a clinical specimen
Case classification :
Confirmed: a case that is laboratory confirmed
Probable: a clinically compatible case that is epidemiologically linked to a confirmed case
Case Definitions for Infectious Conditions Under Public Health Surveillance
MMWR 46(RR10);1-55 Publication date: 05/02/1997
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

15. Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009
Case definitions
􀂃 Example of case definition used in the investigation of an Escherichia coli O157 outbreak.
A case – any resident with gastrointestinal illness in Area A within five days of attending the Area A Fair in June 2003.
Cases may be further categorized as:
Confirmed case: gastrointestinal illness with microbiological confirmation of E. coli O157
Probable case: bloody diarrhoea or haemolytic uraemia syndrome without microbiological confirmation
Possible case: non-bloody diarrhoea without microbiological confirmation
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

16. Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009
Identifying cases
The cases that prompt an outbreak investigation often represent only a small fraction of the total number of people affected.
To determine the full extent of the problem and the population at risk of illness, an active search for additional cases should be undertaken.
Methods for finding additional cases will vary from outbreak to outbreak.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

17. Methods for identifying cases
Easy to identify cases when involve clearly identifiable groups; for example, persons attending the same wedding party.
If an outbreak affects a restricted population (e.g. students in a school or factory workers) and if a high proportion of cases are unlikely to be diagnosed, a survey of the entire population can be conducted.
Cases may know other people with the same condition , particularly among household members, work colleagues, classmates, friends or neighbours.
Some methods:
Directly alert the public. For example, in outbreaks caused by a contaminated commercial food product, announcements in the media can alert the public to avoid the implicated product and to see a medical practitioner if they have symptoms typical of the disease in question.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

18. Methods for identifying cases
Directly contacting physicians, hospitals, laboratories, schools or other populations at risk.
Directly alert the public. For example, in outbreaks caused by a contaminated commercial food product, announcements in the media can alert the public.
If the cause of outbreak is known, review of laboratory surveillance data (unique subtype or biochemical or molecular feature) can help to find people with similar infections.
Some methods:
Directly alert the public. For example, in outbreaks caused by a contaminated commercial food product, announcements in the media can alert the public to avoid the implicated product and to see a medical practitioner if they have symptoms typical of the disease in question.
Foodborne Outbreak Investigation, Hanoi, 1-5 June 2009

19. References
Foodborne Disease Outbreaks: Guidelines for Investigation and Control. WHO 2008.
Centers for Disease Control and Prevention (CDC):
Gregg, M.B. Field Epidemiology. New York: Oxford University Press, 2002
Case Definitions for Infectious Conditions Under Public Health Surveillance
MMWR 46(RR10);1-55 Publication date: 05/02/1997