Definition of unusual cluster or changing pattern of illness

A medical practitioner and a clinical director of a diagnostic laboratory are required to notify to the medical officer of health any unusual clusters or changing patterns of any illness, and individual cases thereof, that may be of public health concern.

An unusual cluster or changing pattern of illness means an aggregation of health events, grouped together in time or space, that is believed or perceived to be greater than could be expected by chance.

Features that should alert healthcare providers to the possibility of an unusual cluster or changing pattern of illness include:

  • A rapidly increasing disease incidence in a normally healthy population
  • An unusual increase in the number of people seeking care, particularly with fever, respiratory or gastrointestinal complaints
  • An endemic disease rapidly emerging at an uncharacteristic time or in an unusual pattern (e.g. an increase in what appears to be chickenpox-like illness among adult patients, but which might be smallpox)
  • Clusters of patients arriving from a single locale
  • Large numbers of potentially fatal cases (e.g. a large number of acute flaccid paralysis with prominent bulbar palsies, suggestive of a release of botulinum toxin)
  • Any one patient presenting with a disease that has bioterrorism potential (e.g. anthrax, smallpox, botulism, plague or tularemia)