Laboratory-based surveillance ofCampylobacterandSalmonellainfection and the importance of denominator data

Abstract

SUMMARYLaboratory data are the cornerstone in surveillance of infectious disease. We investigated whether changes in reported incidence ofCampylobacterandSalmonellainfection might be explained by changes in stool sampling rates. Data were extracted from a national database on 585 843 patient stool samples tested by microbiology laboratories in Wales between 1998 and 2008.Salmonellaincidence fell from 43 to 19 episodes/100 000 population butCampylobacterincidence after declining from 111/100 000 in 1998 to 84/100 000 in 2003 rose to 119/100 000 in 2008. The proportion of the population sampled rose from 2·0% in 1998 to 2·8% in 2008, mostly due to increases in samples from hospital patients and older adults. The proportion of positive samples declined for bothSalmonellaandCampylobacterfrom 3·1% to 1·1% and from 8·9% to 7·5%, respectively. The decline inSalmonellaincidence is so substantial that it is not masked even by increased stool sampling, but the recent rise inCampylobacterincidence may be a surveillance artefact largely due to the increase in stool sampling in older people.