Sir, - Your continued extensive coverage of BSE crisis is welcome but it continues to perpetuate a dangerous misconception that the post-mortem tests presently in use can show an animal to be "BSE-free". This is simply not the case. These tests were validated under very strict conditions where the positive tissue came from animals with a clear clinical diagnosis of BSE which was confirmed at post-mortem. The negatives were healthy animals where post-mortem examination of the brain showed no evidence of BSE. In this rigorous study, these tests showed 100% accuracy and 100% precision; i.e. no "false" positives or "false" negatives.
At this point we have no data on the accuracy and precision of these tests when applied to younger animals which might be infected with BSE but who, by virtue of their young age, show no clinical symptoms. It is evident from the use of these tests in the active BSE surveillance programme that they can detect some infected animals which do not appear at ante-mortem veterinary inspection to have any clinical symptoms. In that regard they will considerably boost our ability to exclude cases of BSE from the human food chain. However, when the test is negative in these younger animals, there is simply no scientific basis for believing that BSE is absolutely absent and that the animals are "BSE-free".
The infectivity of BSE in clinical cases is confined to tissues such as the brain, spinal cord etc. Collectively, these tissues, known as specified-risk material (SRM), are removed by law from the human food chain irrespective of the age of the animal. In other words, present risk management strategies assume any animal could be infected and therefore all SRM should be removed at all times.
There is a danger that if this misconceived belief that tests will guarantee an animal "BSE-free" is hyped up, risk-reduction measures further down the chain may become lax. We have seen carcases exported to Northern Ireland being returned because some SRM (spinal cord) was still present. The repeated assertion that post-mortem tests can ascertain an animal to be "BSE-free" will not help sustain rigorous application of the key to protecting consumer health, total and complete SRM exclusion. - Yours, etc.
Michael J. Gibney, Department of Clinical Medicine, Trinity Centre forHealth Sciences, St James's Hospital, Dublin 8.