AN OUTBREAK OF CRYPTOSPORIDIOSIS IN SOUTH LONDON - WHAT VALUE THE P VALUE

An outbreak of cryptosporidiosis which affected 44 people in January a nd February 1991 was identified through local surveillance at a South London Public Health Laboratory. Preliminary enquiries revealed that m ore than half the patients were adult and that there were no common fa ctors other than geographical association. A case-control study showed a significant association between illness and consumption of tap wate r supplied by a particular water company, as well as a dose response e ffect. There were no apparent breaches or irregularities in the water distribution system and no indication of a problem through routine mon itoring indices. This incident demonstrates the problems of establishi ng the source of cryptosporidiosis outbreaks in the absence of evidenc e of environmental abnormality, as well as possibly indicating that wa ter conforming to current treatment standards may occasionally contain sufficient numbers of Cryptosporidium oocysts to cause sporadic cases or clusters.