Doubts over cow disease threat to humans

Experts are still unsure how many people in Britain are infected with the deadly brain disease thought to have been contracted from cattle, according to an analysis published today.

However the numbers may be small, according to reassuring new findings.

Researchers set out to find whether there is a hidden epidemic of variant Creutzfeldt-Jakob disease (vCJD), which was thought to have been passed to humans from beef infected with BSE – so-called mad cow disease.

Dr Jonathan Clewley and colleagues at the Health Protection Agency in London, UK, looked for the agent that causes vCJD in samples of tonsils removed in tonsillectomy between 2004 and 2008.

They state: "Awareness of the prevalence of vCJD in the population is important to determine the level of risk to the population and to limit the impact of infection or plan healthcare interventions for people who may develop the disease."

In total, they tested 63,007 samples of which 12,753 were from the age group in which most vCJD cases have arisen (people born from 1961 to 85) and 19,908 were born from 1986 to 95 – the group that would have been also exposed to bovine spongiform encephalopathy through infected meat or meat products.

None of the samples was positive, suggesting little evidence of a "hidden" epidemic of the problem.

On the website of the British Medical Journal, the authors say that the prevalence of vCJD carriers in Britain may be lower than that given by previous estimates.

But they add this result is "consistent with the earlier study of appendix tissue that showed a prevalence of 292 per million".

Dr Clewley said: "It may be that we have seen the worst of vCJD already."

In an editorial, Dr Maurizio Pocchiari of the Istituto Superiore di Sanita, Rome, Italy, writes that the small number of cases and diagnostic problems make accurate measurement difficult.

He believes that these negative results "indicate that public health authorities in other countries should not carry out such studies because an enormous number of samples would be needed to yield useful information."

Clewley, J. P. et al. Prevalence of disease related prion protein in anonymous tonsil specimens in Britain: cross sectional opportunistic survey. The British Medical Journal, 2009;338:b1442.

Pocchiari, M. Prevalence of variant CJD in the UK. The British Medical Journal, 2009;338:b435.