Health departments investigate suspected outbreaks for a variety of reasons.
These include the need to institute control and prevention measures, the
severity of the problem and its risk to others, the opportunity for
research and training, program considerations, and public relations,
political concerns, and legal obligations.
Control and prevention. A
primary reason for a public health investigation is to control the
outbreak at hand and prevent future outbreaks. In any investigation, you
have to strike a balance between these two goals, depending on where the
outbreak is in its natural course: Are cases occurring in increasing
numbers or is the outbreak just about over?
If cases are continuing to occur, your first priority will more than likely be
controlling the outbreak, so you want will to assess its extent and the
characteristics of the population at risk so you can design measures to
prevent additional cases. On the other hand, if an outbreak appears almost
over, you may want to focus on investigating further to identify its
source and using that information to develop measures that will prevent
future outbreaks.
The balance between instituting control measures and conducting further
investigation depends on how much you know about the agent causing the
illness, the source of the agent, and its mode of transmission, since you
cannot design control measures without this information.
Severity and risk to others. Decisions
regarding whether and how extensively to investigate an outbreak are also
influenced by the severity of the problem and its risk to others. It is
particularly urgent to investigate an outbreak when the disease is severe
and could affect more people unless prompt control measures are taken. For
example, in the United States, every case of plague and botulism is
investigated immediately to identify and eradicate the source. Cases of
syphilis, tuberculosis, and measles are investigated promptly to identify
contacts and interrupt transmission.
Research opportunities. Another
important objective of outbreak investigations is simply to gain
additional knowledge. Each outbreak offers a unique opportunity to study
the natural history of the disease in question—including the agent, mode
of transmission, and incubation period. For a newly recognized disease,
there is the opportunity to study the clinical spectrum of the illness.
Investigators also attempt to characterize the populations at greatest
risk and to identify specific risk factors.
Even with familiar diseases, investigators can learn more about the impact of
control measures and the usefulness of new epidemiological and laboratory
techniques. For example, an outbreak of measles in a highly immunized
community provides a setting for investigators to study the effectiveness
of vaccine, the effect of age at vaccination, and the duration of
protection afforded by the vaccine (1).
Training opportunities. Investigating
an outbreak requires a combination of diplomacy, logical thinking, problem
solving, quantitative skills, epidemiological know-how, and judgment. These
skills improve with practice and experience. For this reason, many
investigative teams pair a seasoned epidemiologist with an
epidemiologist-in-training, who gains valuable on-the-job training and
experience while assisting in the investigation and control of the
outbreak.
Program considerations. Health
departments routinely use a variety of programs to control and prevent
illnesses such as tuberculosis, vaccine-preventable diseases, and sexually
transmitted diseases. By investigating an outbreak of a disease targeted
by one of these programs, health departments may discover populations at
risk that have been overlooked, failures in the program’s intervention
strategy, changes in the agent causing the disease, or events beyond the
scope of the program. This information can then be used to improve control
and prevention efforts.
Public, political, or legal concerns. Public,
political, or legal concerns sometimes override scientific concerns in the
decision to conduct an investigation. Increasingly, the public has taken
an interest in disease clusters and potential environmental exposures and
has called upon health departments to investigate. Such investigations
almost never identify a link between the disease and the suspected source
(2,3). Nevertheless, many health departments have learned that it is
essential to be "responsibly responsive" to public concerns,
even if the concern has little scientific basis (4,5,6). They also see
these instances as opportunities to educate the public. In some instances
(e.g., a request by three or more workers to investigate workplace health
and safety), investigations are required by law.
Next: Steps of an Outbreak Investigation
