The sharp decrease in the number of cases born after December 1990 indicates that the measures then implemented were highly effective in hampering the spread of the disease. Henceforward, only importation from suppliers producing according to domestic standards will be allowed. The animals to be imported must have been born at least 18 months after the implementation of such feed ban. These measures became effective on 1 January 1997. - on farms, which have experienced or will experience a BSE case, all animals born before 1 December 1990, i.e. prior to the implementation of the ruminant feed ban, must be culled. For farms with a BAB case, all cattle must be culled.
Analogous measure is carried out in the herds of origine of the case.
Except few cases this measure has now been fully implemented. Acknowledgements The authors thank Josef Schmidt for critical comments. An exploratory case-control study on risk factors for BSE on Swiss dairy farms. Annual meeting of the Society for Veterinary Epidemiology an Preventive Medicine, University College, Chester, England, 9-11 April.