NR AQCZ
AU MacKenzie,D.
TI Mysterious BSE-like disease found in sheep
QU New Scientist, 8.4.2004
IA http://www.newscientist.com/news/news_224234.html
VT
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."
Meat and bone
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.
Blot test
The new result announced on Wednesday, from a sheep recently 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.
Passing change
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.
Debora MacKenzie
IN Seitdem am 17. Oktober 2001 die wahrscheinlich im in Edinburgh beheimateten Neuropathogenese-Labor des britischen Institute for Animal Health geschehene überaus peinliche Verwechslung von Rinder- mit Schafshirnen bekannt wurde [AQDA,AQDB,AQDC], gab es in Großbritannien 1019 neue Scrapie-Fälle. Diese wurden genau wie weitere 1125 seit 1998 gesammelte Gehirne britische Scrapie-Fälle in Weybridge per Western blot und Immunhistochemie untersucht, um eventuell Hinweise auf eine mögliche versehentliche Übertragung von BSE auf Schafe zu finden. Aber erst am 7.4.2004 konnte bzw. musste erstmals ein Scrapie-Fall gemeldet werden, der Ähnlichkeiten mit experimentell durch Übertragung von BSE auf Schafe erzeugtem Scrapie aufweist. Genau wie bei experimentell mit BSE infizierten Schafen wurde auch bei dem jetzt gefundenen Scrapie-Fall das Prion-Protein im Western blot nicht vom Antikörper P4 markiert. Dieser Antikörper hatte bisher mit nur einer Ausnahme die Prionproteine aller Scrapie-Varianten gebunden, versagte aber bei den bisher 100 experimentell BSE-induzierten Scrapie-Fällen. Genau wie bei den experimentell BSE-induzierten Scrapie-Fällen zeigt der nun entdeckte Scrapie-Fall auch ein vermindertes Molekulargewicht des nicht glykosylierten Prionproteins. Aber die Anteil der nicht, einfach bzw. zweifach glykosylierten Prionproteine sehen bei dem neuen Scrapie-Fall anders aus, als bei den experimentell BSE-induzierten Scrapie-Fällen. Dieses Glykosylierungsmuster entsprach dem von Scrapie gewohnten. Immunhistochemisch betrachtet entspricht das Verteilungsmuster der Prionproteinakkumulationen im Gehirn des neuen Scrapie-Falles weder dem von alten Scrapie-Varianten bekannten, noch kennt man es von den experimentell mit BSE inokulierten Schafen. Man hat es also mit einer ganz neu entdeckten Scrapie-Variante zu tun, die möglicherweise durch eine versehentliche Übertragung von BSE entstand und wo sich entweder dabei oder erst während weiterer Übertragungen von Schaf zu Schaf die Stammeigenschaften des BSE-Erregers änderten. Man könnte aber auch einfach eine alte, bisher übersehene Scrapie-Variante gefunden haben, die ihrerseits die Quelle der BSE-Epidemie sein könnte.
SP englisch
PO England
ZF kritische Zusammenfassung von Roland Heynkes