BSE agent: there may be even more trouble ahead

BMJ 1996; 313 doi: (Published 27 July 1996) Cite this as: BMJ 1996;313:189

Newspaper reports suggest that the European Union is poised to ban the human consumption of brain, spinal cord, and spleen from goats, sheep, and deer. This follows publicity given to recent laboratory experiments showing that BSE is transmissible to sheep. Kenton Morgan, professor of epidemiology at Liverpool University's faculty of veterinary science, said that the possibility of transmission was not in doubt, although its clinical significance is. “We have not yet seen any change in the clinical presentation of scrapie in sheep,” he said.

Accepted facts about the agent responsible for bovine spongiform encephalopathy (BSE) are rare; two of those on which there is good agreement took a knock from scientists at a meeting organised by the North West Public Health Alliance in Liverpool last week. Suspicion is increasing that the agent may not have originated from scrapie and that cows may transmit it to their calves.

Until now, BSE has been explained by the contamination of animal feed by the carcases of sheep affected by scrapie. Its appearance in the mid-1980s was attributed to changes in the rendering process, which had previously destroyed the scrapie agent. But Kevin Taylor, deputy chief veterinary officer at the Ministry of Agriculture, Fisheries and Food said the problem with this theory is that none of the 20 strains in scrapie resembles BSE.

Few researchers doubt that feeding rendered down bovine carcases to cows was an extremely efficient way of spreading disease, but the disease in question may have originated in cows rather than sheep. Strain typing on the cases found in humans of the new variant of Creutzfeldt-Jakob disease (CJD) would be available later this year and should shed more light on the situation, said Martin Zeidler, research registrar at the National CJD Surveillance Unit.

In view of this risk, the government has sought to exclude from cattle feed any material that might convey BSE. If contaminated animal feed is solely responsible for cases of BSE then this strategy should lead to its eradication. Yet two thirds of cattle with recent notifications of BSE have been born after the ruminant feed ban, said Mr Taylor. Poor compliance by farmers is one explanation, but two other possibilities exist.

The first is vertical transmission, whereby cows pass the disease on to their calves. The second is environmental contamination, a theory supported by Iceland's experience. Some Icelandic farms replaced their sheep population with new stock, yet within a few years the new stock was affected with scrapie. Unless poor compliance with the feed ban was solely to blame then BSE could remain endemic within British herds, said Professor Morgan.—TONY DELAMOTHE, BMJ

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