A massive research programme to find out whether BSE is circulating in British sheep has turned up its first suspicious result. But while scientists say the sheep did not have conventional BSE, they cannot rule out the possibility that it could have had a new form of mad cow disease that has adapted to sheep. Britain’s Department for Environment, Food and Rural Affairs has announced that the Veterinary Laboratories Agency in Weybridge, England, had found “a type of scrapie not previously seen in the UK”. Scrapie is a sheep disease similar to BSE which is not generally thought to harm people. DEFRA said the disease-causing prion detected in the sheep’s brain “had some characteristics similar to experimental BSE in sheep”, but that on other tests it resembled neither BSE nor “previously recognised types of scrapie”. The UK’s Food Standards Agency said in a statement: “Uncertainties still remain on this issue. However, based on the best scientific evidence to date, we are not advising against eating lamb and sheep meat.”
There have long been fears that sheep which ate cattle-derived meat and bone meal during Britain’s BSE epidemic in the 1980s might have acquired BSE, although they have never been confirmed. Unlike BSE in cattle, prion diseases spread directly from sheep to sheep. So any BSE in sheep could still be circulating despite subsequent bans on animal-derived feed. Furthermore, sheep experimentally fed BSE develop a disease indistinguishable from ordinary scrapie, making detection very difficult. Yet the prion from such animals still behaves like BSE, and could cause the fatal human disease vCJD. Worse, sheep carry prions in more tissues than cattle, including the muscle that people eat, so BSE-infected sheep could cause more human disease than mad cows. A previous attempt to determine whether British sheep had acquired BSE went spectacularly wrong in 2001 when sheep and cattle brains were mixed up in the lab. But since then, the VLA has tested the brains of all 1019 newly reported cases of scrapie, as well as 1125 scrapie brains dating back to 1998, with tests designed to distinguish scrapie from BSE.
The new result announced recently, from a sheep reported with scrapie symptoms, is the first to give results that resembled BSE. Danny Matthews of the VLA told New Scientist that in a prion test called a western blot, the sheep’s brain did not bind an antibody called P4. P4 also does not bind prions from sheep experimentally infected with BSE, but does bind all but one forms of scrapie tested with it. Also like BSE, the form of the prion without a sugar attached to it had a lower molecular weight than the form found in scrapie. But the ratio of prions with different numbers of sugars on them looked like scrapie, not BSE, says Matthews. Most conclusively, immunohistochemistry (IHC), in which thin slices of the sheep’s brain were stained with various antibodies, showed prions had accumulated in different parts of the brain and different kinds of cells from BSE – or any known form of scrapie.
IHC seems to be a reliable indicator of BSE, as it has given a constant pattern in the 100 sheep of different genetic varieties experimentally infected with BSE and tested so far. But so little scrapie has been tested, says Matthews, it is not known if one strain might give these results on the tests. The IHC pattern reliably indicates BSE, says Matthews, having been constant in the 100 experimentally infected sheep of different genetic varieties tested so far. But so little scrapie has been tested, he says, it is not known if one strain might give these results on the tests. One possibility, he says, is that the sheep might have been carrying a prion initially derived from BSE. Passage into new species is well known to change prions. BSE from experimentally infected sheep has so far been passed to just one more round of sheep, with no apparent change. “But we don’t know if passage through many sheep, of different genetic types, might change it so it no longer gives the same pattern in IHC or western blots,” says Matthews. “Those experiments are underway now.” Any such new incarnation of BSE in sheep may – or may not – have lost its ability to harm humans.
08 April 04
Source: NewScientist.com news service
April 27, 2004