New experiments suggest sheep eaten by British consumers in the early 1990s might have been infected with BSE.
The fear is that if people have eaten infected sheep, they might develop variant Creutzfeldt Jakob Disease (vCJD), the human equivalent of mad cow disease. BSE-infected sheep could pose a greater human health risk than infected cattle.
So far, the assumption has been that sheep cannot naturally contract BSE. Scrapie, the sheep equivalent of mad cow disease, is not thought to pose any risk to humans.
Researchers at the Institute for Animal Health (IAH) at Compton in Berkshire injected mice with liquefied brain tissue from 3000 sheep diagnosed with scrapie in the early 1990s. The preliminary results suggest that some of these sheep might in fact have been suffering from BSE.
“If BSE did spread to sheep it would be serious,” says a spokesman for the UK Food Standards Agency, which has issued a bulletin warning of the early results.
In cattle, the infective agent is confined to specific tissues such as the brain and spinal cord, says Peter Osterhaus, a member of the European Commission’s advisory panel on transmissible spongiform encephalopathies. These “specified risk materials” can be stripped out of the carcass. But this is more problematic with sheep because the agent would infect other tissues, such as lymph nodes.
“Once a sheep is infected, the whole carcass becomes specified risk material,” says Osterhaus. “Studies are ongoing to see if the list of SRMs should be extended for sheep.”
The FSA emphasises that, to date, BSE infection has not been detected in sheep flocks in the UK. In an ongoing study, 156 samples from sheep diagnosed with scrapie in the late 1990s have all tested negative for BSE.
Although details of the experiment are not being disclosed, it is understood that some of the mice died within a year of being injected with the sheep brain material – the same amount of time it takes for them to die from BSE.
“It would indicate, but not confirm, that they had BSE,” says an agency spokesman. “The problem is that there are many forms of scrapie, and you must be sure that what you’ve found is unequivocally BSE.”
“There could be a strain of scrapie which happens to cause death at the same time as BSE in these mice,” he adds. Lab or field contamination of the brain material with BSE could also be to blame.
There is yet another possible explanation for the results. The study was carried out ‘blind’, so the researchers do not yet know which infective agent was injected into the dead mice. Some mice were injected with cattle BSE to ‘calibrate’ the system.
“We won’t unblind the results till autumn, so it could be that they received a known sample of BSE,” says the spokesman.
The FSA says there is no reason for consumers to stop eating lamb. Changes to feeding and slaughtering practices in the UK mean the risk of meat on sale now being infected with BSE is very slim.