Agency for Toxic Substances and Disease Registry Case Studies in Environmental Medicine (CSEM)
Disease Clusters: An Overview Definition of Disease Clusters
Unusual events such as clusters occur all the time, especially in large populations. From a statistical perspective, it is almost inevitable that some schools, church groups, friendship circles, and neighborhoods will be associated with clusters of chronic diseases. When first noticed, such clusters are often regarded as resulting from some specific, predictable process, rather than as events with independent causes that happened to have occurred by chance in one particular place (such as a coin toss).
A "cluster" is an unusual aggregation, real or perceived, of health events that are grouped together in time and space and that is reported to a public health department (CDC 1990). Several breakthroughs and triumphs in infectious disease control have resulted from the epidemiologic evaluation of clusters of cases.
Well-known examples of clusters include the epidemic of cholera in London in the 1850s (Snow 1965), the investigation of cases of pneumonia at the Bellevue-Stratford Hotel in Philadelphia in 1976 (Fraser et al. 1977), and the 1981 report that seven cases of Pneumocystis carinii pneumonia had occurred among young homosexual men in Los Angeles (CDC 1981).
Investigations of noninfectious disease clusters have also resulted in notable examples of breakthroughs linking a particular health effect to an exposure, such as angiosarcoma among vinyl chloride workers (Waxweiler et al. 1976), neurotoxicity and infertility in kepone workers (Cannon et al. 1978), dermatitis and skin cancer in persons wearing radioactively contaminated gold rings (Baptiste et al. 1984), adenocarcinoma of the vagina and maternal consumption of diethylstilbestrol (Herbst et al. 1971), and phocomelia and consumption of thalidomide (McBride 1961).
Disease clusters differ from sentinel events. Sentinel events are occurrences of unexpected diseases or disorders that are known to result from specific, recognized causes of likely relevance to the situation or setting (Joint Commission on Accreditation of Healthcare Organizations 2002). For example, the diagnosis of
lead poisoning in a child (a sentinel event) should suggest the likelihood of environmental lead contamination that might affect other children. By contrast, disease clusters are occurrences of seemingly unexpected diseases for which no immediately apparent recognized cause exists.
