Multiple change-point analysis applied to the monitoring of salmonella prevalence in Danish pigs and pork

In the nation-wide Salmonella Control Program in Denmark, the occurrence of Salmonella enterica in pork, pigs at slaughter and herds is monitored, The objective of the present study was to retrospectively evaluate changes in sere-prevalence of meat juice samples and in the occurrence of Salmonella enterica in pork in 1995 and 1996. Three sets of data were used in this work: (1) serological test results of meat juice samples from pigs at slaughter (approximately 14 000 samples per week); (2) bacteriological test results of pork (approximately 550 samples per week); and (3) data on the salmonella level of all Danish herds with an expected kill of over 100 pigs per year. The change-point analysis was applied to detect the change-points that divided the study period into intervals in which the prevalence was constant and to estimate the average prevalences in those intervals, Progress in the Danish Salmonella Control Program was visualised when using the results of the change-point analysis (1995-96) as baseline prevalences and compared with the current year (1997). The change-point analysis provided an indication of a seasonal pattern in salmonella occurrence with lower sere-prevalence in summer than in winter. The sere-prevalence (percent positive meat juice samples) might be a better predictor of prevalence in pork than the proportion of herds with moderate or high sere-prevalence. (C) 1998 Elsevier Science B.V.