BOVINE SPONGIFORM ENCEPHALOPATHY (BSE) - THE CURRENT SITUATION AND RESEARCH

Bovine spongiform encephalopathy (BSE), discovered in Great Britain in 1986, was to pose one of the most serious threats to the well-being of the British cattle industry this century. The disease is now established as a member of the group of diseases known as the sub-acute spongiform encephalopathies caused by unconventional, transmissible agents and which includes scrapie of sheep. It is from scrapie of sheep that it appears BSE has resulted though it is possible BSE may have existed in a sub-clinical form in cattle. The vehicle of transmission is meat and bone meal prepared from infected ruminant carcases and included in the protein concentrate rations of cattle, especially dairy cattle in which the disease predominates. Most animals become exposed as calves and the incubation period is typically 4-5 years with most cases occurring at this age. The increase in exposure of cattle to infection that resulted in disease in 1985/1986 was 1981/1982. The factors that contributed to this increase were an increasing sheep population, possibly an increase in the prevalence of scrapie and changes in the industrial processing of animal waste to prepare meat and bone meal. The clinical signs of BSE include abnormal behaviour, posture, gait and an increased sensitivity to visual and aural stimuli. There is loss of condition and milk yield. In only one of 28,197 cases of BSE confirmed by 10 May 1991 can feed be almost certainly ruled out as a source of infection. This therefore may be a singleton case of maternal transmission though we cannot be certain. Every other case so far has been exposed to meat and bone meal in the diet and thus resulted in an extended common source epidemic. A large research programme is underway to investigate the epidemiology, clinicopathology, transmission and molecular biological/genetic aspects of the disease. Much is collaborative between Institutes and Member States of the European Community, in which the disease, as in Britain, is notifiable. In Britain the lynch pin of control for animal health has been the ban established in July 1988 on feeling of ruminant protein to ruminant animals. Though there is no evidence that either BSE or scrapie is a hazard to humans, as a precautionary measure, suspect animals are compulsorily slaughtered and destroyed (except all brains are taken for diagnosis) with compensation being paid at 100% of value. Milk from such animals is also destroyed. The offals from all cattle over 6 months old likely to contain the agent of BSE are not permitted to be fed to any animal, poultry or bird. As a result of these measures the epidemic in Britain is expected to decline from 1992/1993 onwards and be extinct by the end of the century provided no cattle to cattle transmission occurs and for which at present there is no evidence.