Agency for Toxic Substances and Disease Registry
Case Studies in Environmental Medicine (CSEM)

Disease Clusters: An Overview
Answers to Pretest Questions

1. The term "cluster" is an unusual aggregation, real or perceived, of health events that are grouped together in time and space and that are reported to a public health unit.
2. The public health department investigates disease clusters. The public health department's role in the disease cluster investigation involves the following:
   • collecting accurate case information,
   • conducting active surveillance through local surveys or by using health data registries,
   • conducting environmental or occupational exposure assessments when warranted,
   • ensuring that appropriate public and health professional communication and education is occurring specifically related to the existence of a disease cluster and any associated factors, and
   • initiating timely and effective actions to mitigate factors associated with the disease cluster.
3. The steps involved in a disease cluster investigation are (1) establish a case definition; (2) confirm the suspected cases; (3) define a "population denominator" measured in person-years, search for additional numerator cases within that population, and draw conclusions about the "unusualness" of the cases; (4) review the literature for risk factors and exposure hypotheses; (5) perform an exposure assessment; and (6) generate biologically plausible hypotheses.
4. The physician's overall responsibility is to suspect a cluster of disease on the basis of clinical observation, complete an exposure history, confirm cases through accurate clinical and laboratory diagnosis, act as a sentinel in reporting cases to the local public health unit, and educate patients about occurrence of disease.
5. The first line for contact is usually the public health department.
6. Following are the most important education points:
   • The current problem and the next appropriate diagnostic step.
   • Specific factors related to the occurrence of the particular disease (e.g., latency period for cancer, significance of family history, and other confounding factors).
   • Whether it is likely or unlikely that the patient's perceived exposure might be responsible for the problem; if it is likely, discuss your role and responsibility.

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