Possibilities to manage the BSE epidemic: cohort culling versus herd culling--experiences in Switzerland.

Bovine spongiform encephalopathy (BSE), a disease of cattle first reported in 1986 in the UK [1], has been found in the cattle populations of most European countries. To date, Japan, Israel and Canada are the only nonEuropean countries where BSE has been reported in indigenous cattle [2]. Given all available evidence, the BSE agent is not transmitted horizontally between cattle; rather it is primarily transmitted through feed. Meat-and-bone meal (MBM) is the main source of infectivity and cattle [3] are usually infected during calf-hood. Although it is likely that the other calves in a herd of a similar age to a BSE case also consumed feed from a contaminated batch of MBM, the incidence of BSE within most herds is very low. The most probable explanation is that exposure to the BSE agent was, on average, very low. Recycling of the BSE agent is likely to have resulted in an increased number of ‘packets’ of infectivity rather than an increase in the average titre of infectivity within a contaminated batch of MBM. As a result, the average within-herd incidence would be unlikely to increase [4] (see also the chapter of Bradley). Several strategies, including those that focus on feed as a source of infection and various culling programmes, have been adopted with the aim of eradicating BSE. Culling strategies vary from culling the affected animal only, to culling all animals within the birth cohort of the BSE case, to culling the entire herd.