SURVEILLANCE OF OUTBREAKS OF WATERBORNE INFECTIOUS-DISEASE - CATEGORIZING LEVELS OF EVIDENCE

Public health surveillance requires the monitoring of waterborne disea se, but sensitive and specific detection of relevant incidents is diff icult. The Communicable Disease Surveillance Centre receives informati on from various sources about clusters of cases of illness in England and Wales. The reporter may suspect that water consumption or recreati onal water exposure is the route of infection, or subsequent investiga tion may raise the hypothesis that water is associated with illness. I t is difficult to prove beyond reasonable doubt that such a hypothesis is correct. Water samples from the time of exposure are seldom availa ble, some organisms are difficult to detect and almost everyone has so me exposure to water. Therefore, we have developed a method of categor izing the degree of evidence used to implicate water. The categories t ake into account the epidemiology, microbiology and water quality info rmation. Thus outbreaks are classified as being associated with water either 'strongly', 'probably' or 'possibly'. This system allows a broa d database for monitoring possible effects of water and is not confine d to the few outbreaks which have been intensively investigated or hav e positive environmental microbiology. Thus, for reported incidents, t he sensitivity of classifying it as water associated should be high bu t this may be at the expense of specificity, especially with the 'poss ible' association.