Trends and seasonal variations in the occurrence of Salmonella in pigs, pork and humans in Denmark, 1995-2000

A mandatory programme monitoring the occurrence of Salmonella in pork at sl aughterhouses and a serological monitoring of slaughter-pig herds has been implemented in Denmark since 1993 and 1995. respectively. All results are s tored in a central database. From this, aggregated weekly results of serolo gical and bacteriological samples collected in the period between January 1 995 and July 2000 were extracted. In addition. the reported weekly incidenc e of human infections with S. Typhimurium covering the same time period was obtained. The times series were analysed for trends and cyclic variations by seasonal decomposition. The association between the incidence in humans and the prevalence of Salmonella in pigs and pork, and prevailing weather c onditions, were analysed by using a general linear (glm) and a general addi tive model (gam). Explanatory variables were lagged to account for time ela psed between sampling, consumption, incubation period and case registration . The results of the seasonal decomposition showed an overall declining tre nd in all three time series. All time series exhibited a double peaked annu al cycle. The seasonal variation of the prevalence in pork and the human in cidence had a very similar course. The variables that were both biologicall y meaningful and statistically significant in both regression models were t he prevalence in pork sampled 4 to 5 weeks before case registration, the se roprevalence, measured as the average prevalence of week 15 to 35 before ca se registration, and the air temperature lagged at 2 and 3 weeks. Limitatio ns on inferences from overall surveillance data are discussed.