Key messages
- Numbers of cases of a disease in a community are sometimes higher than expected, raising concerns about a possible disease ‘cluster’.
- What initially appears to be a cluster of disease may actually reflect the expected number of cases in the community.
- If an apparent cluster relates to a particular cancer or birth-related effect, an initial investigation may be done to study disease trends. The majority of these do not lead to further investigation.
Numbers of cases of a particular disease in a community sometimes are higher than expected. This can raise concerns about a possible disease ‘cluster’.
A disease cluster is defined as a greater number of cases of a disease than expected within a group of people in a geographical area in a specific time period.
When a family member, neighbour or work colleague is diagnosed with a noninfectious disease, such as cancer, Alzheimer disease or a birth defect, it can make you more aware of others who have been diagnosed with the same condition. This may raise concerns about a possible disease cluster.
Sometimes, we need to investigate to determine whether cases are indeed a disease cluster or unrelated incidents.
Disease trends and disease clusters
Diseases do not occur evenly in the community. What initially appears to be a cluster of disease may actually reflect the expected number of cases in the community.
For example, cancer is a common disease, affecting at least one in three Victorians over their lifetime.
Cancer describes many different diseases with a range of known or suspected causes. If a number of people in a workplace develop cancer over a period of time, this may not actually be a disease cluster. Rather, it could represent the number expected, given that cancer is a common disease.
Suspected cluster investigations
If an apparent cluster relates to a particular cancer or birth-related effect, an initial investigation may be done to see whether the number of reported cases is greater than that expected in the general community.
Most initial investigations do not lead to further investigation.
Investigations and when they are necessary
If certain conditions are met, the department makes an assessment to determine whether investigation is feasible. This type of health study or investigation:
- requires medical, epidemiological and environmental specialists
- can take months to years
- is expensive.
The conditions that point to an investigation being warranted are:
- identifying an unusually high number of disease cases that would not be expected by chance, age, gender, or social or lifestyle factors
- multiple cases of an unusual (that is, rare) disease or condition
- identifying a likely cause (that is, an environmental factor) with plausible and sufficient exposure
- adequate time between exposure and development of the disease or condition.
More commonly, these conditions are not met, and further investigation is not undertaken.
Suspected cluster reporting
If a cluster of a noninfectious disease is suspected in your workplace or local community, discuss your concerns with the relevant specialist area:
- If you suspect a cancer cluster, contact the Department of Health.
- If you suspect a cluster of birth-related effects, contact the department’s Clinical Councils’ Unit.
- Speak to your employer, your health and safety representative or WorkSafe Victoria if you have concerns in the workplace.
- Speak to your local doctor or health service, or contact the department’s Environmental Health Unit if you have concerns regarding a possible community cluster.
Reviewed 09 April 2024