Cattle may have BSE without symptoms for years

Government advisers on "mad cow" disease are to investigate fresh evidence that cattle can be badly infected with bovine spongiform encephalopathy (BSE) for years without showing any symptoms.

Government advisers on "mad cow" disease are to investigate fresh evidence that cattle can be badly infected with bovine spongiform encephalopathy (BSE) for years without showing any symptoms.

New research has shown that certain strains of prions, the infectious agents thought to cause BSE, can infect laboratory mice yet the animals never develop the disease.

The findings indicate that BSE can exist in a "sub-clinical" form, where high levels of infectious agent are present in an animal yet fail to result in any symptoms. The same may also be true of variant Creutzfeldt Jakob Disease, vCJD, the human equivalent of BSE.

Sub-clinical disease is different from the incubation period for BSE, when relatively low levels of the prion agent begin to build up and spread within an infected animal, eventually to cause symptoms.

Professor Peter Smith, the acting chairman of the Spongiform Encephalopathy Advisory Committee, said the latest findings, published in the Proceedings of the National Academy of Sciences, are of "considerable interest" and will be discussed in detail when it meets at the end of next month.

The research, by scientists at the Medical Research Council's prion unit, based at St Mary's Hospital in London, demonstrated that mice remain healthy when infected with large amounts of prions from infected hamsters.

Previous research had shown that mice never develop the disease when injected with hamster prions, leading scientists to believe that there was a large "species barrier" protecting mice from this source of infection.

Furthermore, when infective material from the symptomless mice was subsequently injected into other mice, these animals did develop symptoms, suggesting that a new strain of prion had been created.

John Collinge, the leader of the research team, said that the results show that just because a species appears resistant to BSE, it does not mean that they are free of infection. "This research raises the possibility, which has been mentioned before, that apparently healthy cattle could harbour, but never show signs of, BSE," he said.

Professor Collinge said current measures to protect the public from BSE did not need to be changed, but he wanted to see a national system of testing brains of cattle and other species exposed to BSE for signs of prion infection.