Their finding suggests many more cows will have to be sacrificed to get rid of the disease in Britain and the cost will be several times present estimates.
The team, working in New York and Reykjavik, have found that scrapie, the sheep equivalent of bovine spongiform encephalopathy, can be spread by mites which live in hay. They observed that scrapie, which is relatively common in Iceland, can be passed from one sheep to another. They also found that scrapie can occur in flocks of sheep that have previously been free of the disease if the flock is transferred to pasture on which scrapie infected sheep have fed.
The Icelandic-American team Henryk Wisniewski, Sigurdur Sigurdurson and others working at the University of Iceland and the New York State Institute for Basic Research in Developmental Disabilities suspected that the disease must be transferred by hay mites.
They collected several species of mites from five Icelandic farms infected by scrapie and injected extracts into mice. Mice injected with mites from three of the farms developed scrapie a year later and the scientists demonstrated that the mice had the infectious agent of scrapie, a prion protein, in their brains.
Writing in the latest issue of The Lancet the scientists say: "It is possible that hay mites acting as vector and/or reservoir have played a part in the continuing occurrence of bovine spongiform encephalopathy in the UK after the ban on the use of sheep and cattle products for cattle food."
BSE is generally believed to have been passed to cows from the sheep with scrapie which were ground up and put in cattle feed. Both scrapie and BSE are caused by a similar rogue protein that induces animals to produce more of the same protein, so spreading the disease. Until now government scientists have argued that BSE had reached a dead end in cows and was not spread from one cow to another.
However, BSE has continued to be found in animals born after the ban on recycling the carcasses of sick sheep and cows in animal feed. Farmers have been blamed for using old feed after the ban, instead of destroying it. Now the government will have to consider the alternative - that infected hay is to blame.
The scientists were able to see the rogue protein in the bodies of some of the mites after they were treated with a special stain. They concluded that the rogue protein was not just attached to the bodies of the mites but was inside their bodies. They believe that the protein probably reproduces itself inside the mites so it is passed from mite to mite and the mites act as a natural reservoir for the infection.
Sir David Naish, president of the National Farmer's Union, said: "It has enormous implications for farmers if it can be proven that the BSE agent is transferred from one animal to another in hay mites. We will be looking at this new evidence very carefully."Reuse content