In Switzerland there was a reduction in the number of cases of bovine spongiform encephalopathy, observed in the birth cohorts from 1995 to 1996, but no further reduction in the following birth cohorts up to 1998. From the records of 34 cases born after April 30, 1996 (bab96) and 174 cases born before April 30, 1996 and after December 1990 (bab90), observed up to April 30, 2004, the risk factors at the farm level, possible routes of exposure and the geographical distribution of the cases were analysed to try to explain the observations. No evidence was found for a rate of exposure other than feed. There was some evidence that the risk factors at farm level were different between the bab90 and bab96 cases. A large proportion of the bab96 cases was born in cantons that had reported only a few bab90 cases, but a small cluster of the bab96 cases was found in a region where there had been a cluster of bab90 cases. The spatial distribution of these cases indicated that the risk of exposure to infection had been more randomly distributed than during the period up to April 1996. Farms with mixed livestock had a higher risk of having a case born after the ban on the feeding of specified risk material in 1996. In a regression model, a trend towards an association between cases of bse and presence of small ruminants on the farm was observed for the bab96 cases, and the presence of pigs and the pig:cattle ratio were significant for the bab90 cases.
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