The recent publication of a paper describing an atypical form of BSE, referred to as ‘bovine amyloidotic spongiform encephalopathy’ (BASE) could help in explaining the mystery of the origin of the BSE epidemic. This different type of BSE was identified in two cattle, aged 11 and 15 years, in Italy, and a similar strain of BSE was also detected in a cow in Japan. Both the Italian and Japanese strains were detected by routine sampling of animals without symptoms. The pathological changes that result from experimental infection with the BASE strains are characterized by the presence of PrP-immunopositive amyloid plaques, and not by the lack of amyloid deposition in typical BSE cases. BASE also shows a different pattern of regional distribution and topology of brain PrPSc accumulation. In addition, Western blot analysis of BASE cases showed a PrPSc type with predominance of the low molecular mass glycoform and a protease-resistant fragment of lower molecular mass than BSE-PrPSc. It is interesting that the molecular signature of this previously undescribed bovine PrPSc is similar to that encountered in a distinct subtype of sporadic Creutzfeldt-Jakob disease. The BASE has been shown to convert into the classical BSE strain following serial transmission in inbred mouse lines, although BASE and BSE appear to be distinct diseases caused by different prion strains. The new strains are readily detected by existing tests for BSE.
Several strains of scrapie have been recognised, but it was generally thought that only one strain of BSE existed. BSE was first recognised and described in the UK 1986, and within a few years there were thousands of cases throughout Britain. The epidemic reached a peak in 1992 (37,280 cases in UK) after which it declined. The decline of the disease followed strict controls on the use of meat and bone meal in the cattle feed, and it generally accepted that the infected feed was the cause of the epidemic. It is thought that the disease was probably present in Britain in the 1970s and early 1980s, but was not recognised. Affected cows may have been diagnosed as ‘downer cows’ suffering from conditions such as hypomagnesaemia or parturient paresis. But the question of how the infection got into the feed in the first place, was never adequately answered. Another question that has also never been fully answered is why was the outbreak so large in UK when the practice of feeding meat and bone meal was fairly common in many countries, and when British feeds were exported throughout the world?
One factor considered to be important in the epidemiology of the disease in Britain is the fact that the UK has such a large sheep population in which scrapie was present, as well as having a large cattle population. The cattle to sheep ratio in the UK was the highest in Europe, and therefore the meat and bone meal produced was more likely to contain scrapie infected sheep. The problem with this theory is that the scrapie agent differs from the BSE agent, and even though cattle can be experimentally infected with scrapie, it causes a different range of symptoms from BSE. Cattle experimentally infected with the scrapie agent developed an encephalopathy that had a different pathology to BSE, (Clark, W. W. et al: American Journal of Veterinary Research 1995; 56(5): 606-612). In recent years different strains of scrapie have been been discovered in sheep and raising the possibility that there is agent strain variation in natural scrapie, and that BSE may have been present in the sheep population.
Unlike the BSE prions that could easily be transmitted to different strains of inbred mice, the BASE prion is not easy to transmit to different strains of mice, indicating a substantial barrier to infection. The BASE strain could be more widespread in the cattle population as it is difficult to recognise having less obvious symptoms than BSE. BASE infected cattle seem to be older and show only weight loss and reduced alertness. So could BASE have developed into BSE following the changes in rendering practices in the UK in the 1980s? Another question arising from the study concerns a possible link between BASE and sporadic CJD. The biochemical type and deposition patterns of the infecting prion show strong similarities with a distinct subgroup of patients with sporadic CJD.
The study of transmissible spongiform encephalopathies shows that the prions and prion diseases are more complex than originally thought, and that findings will continue to challenge current and orthodox views about these diseases. Follow the developments in the state of knowledge on BSE from the first case described in 1986 to the present by searching CAB Abstracts for ‘bovine spongiform encephalopathy’.
For further information on this see the following two recent papers:
Conversion of the BASE Prion Strain into the BSE Strain: The Origin of BSE? Capobianco R, Casalone C, Suardi S, Mangieri M, Miccolo C, et al. PLoS Pathogens Vol. 3, No. 3, e31 doi:10.1371/journal.ppat.0030031. [Published March 9, 2007]
Identification of a second bovine amyloidotic spongiform encephalopathy: Molecular similarities with sporadic Creutzfeldt-Jakob disease. Cristina Casalone, Gianluigi Zanusso, Pierluigi Acutis, Sergio Ferrari, Lorenzo Capucci, Fabrizio Tagliavini, Salvatore Monaco, and Maria Caramelli. Proceedings of the National Academy of Sciences (PNAS), Mar 2004; 101: 3065 – 3070. 10.1073/pnas.0305777101
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