Use of passive surveillance data to study temporal and spatial variation in the incidence of giardiasis and cryptosporidiosis

Objective, The goal of this study was to evaluate temporal and spatial vari ations in the reporting of cases of giardiasis and cryptosporidiosis to a p assive surveillance system, and to assess the relationship of those variati ons to source of drinking water, adjusting for socioeconomic variables. Methods. The authors analyzed temporal and spatial patterns for 4,058 cases of giardiasis and 230 cases of cryptosporidiosis reported to the Massachus etts Department of Public Health for 1993-1996. They linked each reported c ase to a database containing information on source of residential water sup ply and socioeconomic characteristics and evaluated the association between these factors and reporting rates using regression techniques. Results. Reports of giardiasis and cryptosporidiosis were highest for the m ixed unfiltered drinking water supply category, Reports of giardiasis were associated with income levels, Increases in reporting for both giardiasis a nd cryptosporidiosis were seen in summer to early fall, During a suspected outbreak of cryptosporidiosis in the city of Worcester in 1995, a significa nt increase in reported cases was also observed in the Boston metropolitan area, Following the suspected outbreak, weekly giardiasis rates increased s lightly in Worcester and the Boston metropolitan area, while reporting of c ryptosporidiosis increased dramatically. Conclusions, Consistently collected passive surveillance data have the pote ntial to provide valuable information on the temporal variation of disease incidence as well as geographic factors, However, passive surveillance data , particularly in the initial period of surveillance, may be highly sensiti ve to patterns of diagnosis and reporting and should be interpreted with ca ution.