Sir: Until now the debate on BSE and CJD has been characterised by a lack of clear statements of the uncertain basis on which most strategy decisions have been made.
Much strategy has been based on the assumption that BSE was derived from scrapie, an encephalopathy of sheep caused by "prion" infective agents. It was assumed that its properties have been maintained in cattle. The evidence that scrapie was the source of BSE is essentially that it is the only common form of prion disease in the food chain. However, any species with a prion gene (ie all known mammals) can mutate to produce an inherited encephalopathy and this would potentially be infective (this is true of the mutations causing human CJD). Cattle could themselves have been the source of the infection giving a new disease with no species barrier into cows and a debatable situation for humans.
Despite these doubts over the relevance of scrapie, much was based on the fact that the agent would be scrapie-like. It was argued that, like scrapie, it would be unlikely to cross the species barrier.
Curiously, it was not argued that the pattern of infection would be similar to that of scrapie, ie that there would be vertical transmission (mother to calf) or horizontal transmission (between unrelated cattle), the normal mode of transmission of scrapie. Presumably this was ignored in order to argue against an expensive slaughter policy in infected herds. In terms of a proposed slaughter policy, it is imperative to note that pasture that has carried infected sheep remains infective for some years after the sheep are removed. Slaughter alone does not eliminate scrapie.
The arguments about the efficacy of removing offal and hence eliminating the risk of transmission are based on the fact that muscle and milk contain very low amounts of infective agent, as measured by animal infection experiments. Relatively low numbers of animals are used in these experiments, but the "experiment" we have been conducting on the human population involves 60 million people, and it is not at all clear what level of disease low amounts of infectious agent would produce in this target population.
While infected cattle are killed and burnt, this is only when they show recognisable symptoms. It is like the difference between Aids and HIV infection - the asymptomatic phase provides the public health problem.
Professor Hugh Woodland
University of WarwickReuse content