Disease clusters

Sometimes a greater than expected number of cases of a disease occurs in a group of people living or working in the same area. This is called a disease cluster.

Communicable and non-communicable clusters#

Communicable diseases — diseases that can spread from one person to another — often occur in clusters. Examples include colds, flu, whooping cough, chlamydia and HIV.

A non-communicable disease is one that is not infectious and cannot be passed from person to person. Examples include cancer, cardiovascular disease, asthma and diabetes. Clusters of non-communicable disease are actually very rare.

Reports of a possible non-communicable disease cluster cause concern in the community. The fear is that something unknown in the environment, such as water pollution or radiation, may be causing disease. In most cases, however, a suspected non-communicable disease cluster turns out not to be a cluster at all.

Examples of actual non-communicable disease clusters#

Genuine clusters do happen, and the common cause is usually prolonged exposure to a particular substance, such as a chemical or drug. Examples include:

• Asbestos-related mesothelioma — clusters of mesothelioma, a rare cancer of the lung lining, have occurred in groups of people exposed to asbestos, a mineral that was widely mined in the 1940s and 1950s.
• DES-related cancer — clusters of a very rare vaginal cancer occurred in the early 1970s. The cause was exposure in the womb to the hormone diethylstilboestrol (DES), which was marketed as an anti-miscarriage drug and offered to pregnant women between 1946 and 1971.
• Workplace exposures — about five per cent of cancers are due to exposure to carcinogenic substances at work, such as asbestos, formaldehyde, arsenic or wood dust. Communities that rely on one particular industry may have higher than average cases of a disease because of exposure to a hazardous substance in the workplace.

Common characteristics of non-communicable disease clusters#

Genuine non-communicable disease clusters often share certain features:

• A large number of people are affected by one specific disease.
• The large number cannot be explained by statistics.
• The disease is rare, or does not usually affect people in that group (for example, a particular cancer may be very rare in young people).
• The people share exposure to a particular substance.

Who to contact if you suspect a cluster#

If you suspect there may be a non-communicable disease cluster at your workplace or in your neighbourhood, talk to your doctor for information, advice and referral. Depending on the situation, you may also wish to contact:

• a public health authority for a suspected cancer or community cluster,
• a public health authority for a suspected birth-defect cluster, or
• the health and safety officer at your workplace, your employer, or the relevant workplace health and safety regulator for a work-related cluster.

How a suspected cluster is investigated#

When a suspected cluster is reported, an investigation takes place. It first looks at whether the reported cases are greater than expected, examining factors such as disease patterns and probability statistics. At this early stage, most reports are resolved and no further investigation is needed.

If a report is not resolved, experts investigate the cases and a health study is undertaken. Such a study relies on the input of many specialists, is expensive, and can take months or even years. In many cases, an underlying cause is never found.

Investigating a non-communicable disease cluster is challenging because:

• Diseases such as cancer are very complex, and medical science may not fully understand how various risk factors combine to cause a particular disease.
• Working out a person’s individual risk factors can be difficult.
• The time between exposure to an environmental agent and the onset of disease may be many years.
• People often move house, making it difficult to track when exposure to risk factors may have occurred.

Why most suspected clusters are not clusters#

Several factors can give the false impression of a cluster:

• Common diseases are common. Over a lifetime, about one man in three and one woman in four will be diagnosed with cancer, and cardiovascular disease is one of the leading causes of death. It is not unusual to know two or more people with the same common non-communicable disease.
• Different cancers have different risk factors. It is not a disease cluster when people in the same area have different types of cancer.
• Populations are ageing. Many non-communicable diseases are more common with age, so a high rate of heart disease among people in a retirement community is an expected outcome, not a cluster.
• Risk is not spread evenly. For example, lower socioeconomic groups tend to have higher smoking rates, which explains higher rates of smoking-related cancers. Sometimes people with the same disease simply happen to live in the same area.

Dealing with anxiety#

Whether or not a cluster is confirmed, the possibility can cause anxiety. Suggestions for managing it include:

• Discuss any fears with your doctor. Getting reliable, trusted information about a disease and its risk factors can help keep fears in perspective.
• Be sceptical of rumours. For example, an email hoax once suggested that underarm deodorants cause breast cancer, which is not true. Use reputable medical sources for accurate information.
• Understand how personal experience can skew your perception. When someone we love is diagnosed with cancer, it is easy to start noticing other cases and worry about an unknown link.
• Make lifestyle changes to reduce your risk. You can substantially reduce your risk of heart disease and some cancers by eating a high-fibre, low-fat diet, exercising regularly and not smoking.

Key points#

• Reports of possible non-communicable disease clusters cause concern, but most suspected clusters turn out not to be clusters at all.
• When a genuine cluster occurs, the common cause is prolonged exposure to a particular substance, such as a chemical or drug.
• A well-known example is mesothelioma caused by exposure to asbestos, a mineral widely mined in the 1940s and 1950s.
• Investigations are difficult and an underlying cause is often never found, partly because individual risk factors are hard to determine.
• If you suspect a cluster, talk to your doctor for information, advice and referral.

Where to get help#