When an increase in the numbers of cases of disease are reported, a speedy response is critical. At the same time, it is also of utmost importantance to end up with an answer that will appropriately protect public health and safety. A systematic approach to outbreak investigation helps assure timely and accurate answers:
- Prepare for field work
- Establish the existence of an outbreak
- Verify the diagnosis
- Define and identify cases
- Measure frequency of adverse outcome and describe the data in terms of time, place, and person
- Develop hypotheses
- Evaluate hypotheses
- Refine hypotheses and carry out additional studies
- Implement control and prevention measures
- Communicate findings
Preparing for a possible outbreak investigation Section
Upon receiving initial reports of a possible outbreak, investigators should review the epidemiology, risk factors, clinical signs and symptoms and prevention and control of adverse health outcomes similar to the reported cases. Lines of communication between investigators and health care providers, policy makers and the press should be reviewed and understood by each involved party. Communication must be clear, open, and productive.
Read this example of an outbreak investigation and consider the steps of the investigation:
Multistate outbreak of Salmonella infections associated with frozen pot pies in the United States 2007. JAMA. 2009;301(3):264-266 (doi:10.1001/jama.301.3.264) http://jama.ama-assn.org/cgi/content/full/301/3/264
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In the Salmonella example, what steps were followed in the initial evaluation of the 4 reported cases of Salmonella?
The PA (state) Department of Health initiated an investigation after 4 cases were reported, utilizing PulseNet laboratory capabilities. PulseNet was created to assist epidemiologists in separating outbreak-associated cases from sporadic cases, to rapidly identify sources of outbreaks and to ensure timely and effective communication among public health laboratories. Over several months, state and local health departments collaborated with the CDC in an unsuccessful attempt to locate the source of the Salmonella. Initially, steps 1-5 were covered.
Confirming the possibility of an outbreak Section
Once prepared for investigation, the existence of the outbreak should be confirmed. Iterative examination of the evidence may be required to support or refute the existance of an outbreak.
Determination of an outbreak requires that more cases of a specific adverse health outcome have occurred than would be expected for the surveilled population in a specific geographic area or time period. First, the investigator must determine whether the reported initial cases are worthy of further investigation. A question to be answered is whether the cases could share a common cause. For example, clusters of chronic obstructive lung disease are sometimes suspected to be an outbreak, but upon closer examination, investigators often learn the cases are not a single type of disease. Similarly, reported clusters of cancer may turn out to be different types of cancer or cancers that do not share common causes. Increased surveillance, resulting in increased probability that a disease will be diagnosed, may lead to a suspected outbreak and an investigation.
Next the investigator must determine the number expected in the population. We can determine the number expected from different sources.
- For a notifiable disease (one that, by law, must be reported), health department surveillance records are available.
- For other diseases and conditions, look for local sources such as hospital discharge records, death (mortality) records, and cancer or birth defect registries.
- If local data are not available, estimate using data from neighboring states or national data
- An epidemiologist might survey clinicians asking whether how many cases of the disease they have seen recently.
There are mathematical and statistical methods that can be used to assess the significance of an increase in the number of cases because an increase does not always indicate that an outbreak has occurred or is occurring. Changes in reporting procedures, changes in the case definition, increased awareness, or changes in diagnostic procedures can also lead to increased detection of cases. In some areas, population fluctuates with the season (e.g. college towns, areas utilizing seasonal workers, etc.). Additional considerations include the severity of the illness, how contagious the disease is, political considerations and certainly, available resources.