However, field investigations place a burden on a health department, so the decision also hinges on the availability of staff and resources, and competing priorities. In addition, some health departments have a practice of aggressively investigating outbreaks and hence have experience in doing so, while other health departments may lack such experience.
Regardless, field investigations are usually justified for one or more of the following reasons:
• Control or prevention of the health problem
• Opportunity to learn (research opportunity)
• Public, political, or legal concerns
• Public health program considerations
• Training
Each of these reasons is discussed in more detail below.
Control and prevention
The most important public health reasons for investigating an outbreak are to help guide disease prevention and control strategies. These disease control efforts depend on several factors, including knowledge of the agent, the natural course of the outbreak, the usual transmission mechanism of the disease, and available control measures. For example, if a health department learns of an outbreak of hepatitis A (known agent) in which one of the victims is a restaurant cook, the department can offer immune globulin to the restaurant patrons to prevent a second wave of cases (control measure), but only if they are within 14 days of exposure (timing). On the other hand, if an outbreak appears to be almost over, the health agency may not need to implement control measures, but may be interested in identifying factors that contributed to the outbreak in order to develop strategies to prevent similar outbreaks in the future. For that outbreak of hepatitis A, investigators may find that the poor personal hygiene that led to the outbreak was the result of lack of soap or water in the workplace washroom, which could be addressed in public health messages to other worksites.
The balance between control measures and further investigation depends on how much is known about the cause, the source, and the mode of transmission of the agent.10 Table 6.1 illustrates how public health emphasis on investigation versus control is influenced by these factors. In particular, if the source and/or mode of transmission are known, then control measures that target the source or interrupt transmission can be implemented. If the source and/or mode of transmission are not known, then you can’t know what control measures to implement, so investigation takes priority.