The Origin of BSE
It’s not often that I use MicrobiologyBytes to say “I Told You So”, but today, I’m going to make an exception. When BSE emerged in British cattle in the mid-1980s, the theory was that cows had contracted the disease by eating feed containing the remains of sheep infected with scrapie. This is the part of the story I have never believed as it is purely circumstantial does not fit any of the facts. Having made this species jump, the theory was BSE spread as infected cattle carcasses were processed into animal fodder and fed back to cows.
BSE is one of the so-called Transmissible Spongiform Encephalopathies, or TSEs. These diseases involve an infectious agent which (almost certainly) does not have a nucleic acid genome. It seems that a protein alone is responsible. The infectious agents are known as prions – “small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids”. The discovery that proteins alone can transmit an infectious disease came as a considerable surprise to the scientific community. Prion diseases are called spongiform encephalopathies because of the post mortem appearance of the diseased brain, which contains large vacuoles in the cortex and cerebellum. Probably most mammalian species develop these diseases. Other examples include scrapie in sheep, transmissible mink encephalopathy and chronic wasting disease in mule deer and elk. Humans are also susceptible to several prion diseases including Creutzfeld-Jacob Disease (CJD, although most cases of CJD are sporadic and not transmissible), Gerstmann-Straussler-Scheinker syndrome, fatal familial insomnia, Alpers syndrome, and most notably, kuru.
Animal experiments involving various species clearly show the existence of a species barrier, i.e. it is easier to transmit a TSE from animals of the same species than from a different species. Yet attempts to transmit BSE by deliberately giving scrapie to cows have failed, and many countries outside the UK included sheep remains in cattle feed without creating BSE. This led to elaborate explanations to try to fit the facts. Traditionally, meat and bonemeal (MBM) for animal food was prepared by a rendering process involving steam treatment and hydrocarbon extraction, which generated a protein-rich fraction called ‘greaves’ from which MBM was produced and a fat-rich fraction called ‘tallow’ which was put to a variety of industrial uses. In the late 1970s, the price of tallow fell and the use of expensive hydrocarbons in the rendering process was discontinued, producing an MBM product in which the infectious material may not have been inactivated. As a result, a ban on the use of ruminant protein in cattle feed was introduced in the UK July 1988.
The trouble is, BSE is not scrapie. The BSE prion is biochemically distinct from the scrapie prion, no matter what species it is in. So all the theories about the origin of BSE (and, most importantly, whether it could happen again), are pretty tenuous.
A paper recently published in PLoS Pathogens describes a disease similar to BSE but distinct from it, bovine amyloidotic spongiform encephalopathy, or BASE (Conversion of the BASE Prion Strain into the BSE Strain: The Origin of BSE? 2007 PLoS Pathogens 3, e31). In 2003 two Italian cows were found to have a prion disease distinct from BSE. The prion had a lower molecular weight and one, rather than two, sugar molecules atached to it. The brains of cows with BASE were also damaged in different places from those with BSE. Similar prions have also turned up in France, Germany and Japan.
The PLoS paper describes experiments in which mice were infected with either the BSE or the BASE prion. In mice genetically altered to make the normal cow prion, BSE and BASE produced different symptoms, confirming that BASE is a distinct disease from BSE. If BASE can mutate into BSE, this could be how mad cow disease first emerged. BASE is reminiscent of sporadic CJD in humans, and the change from BASE to BSE is presumably similar to the origin of kuru in New Guinea.
The existence of more than one agent associated with prion diseases in cattle and the ability of the BASE prion to convert into BSE has important implications with respect to the origin of BSE, and for spongiform encephalopathies in other species, including humans. One thing is for sure, if we don’t want the BSE disaster to happen again, we had better learn these lessons.