SALVATORE MONACO, a researcher at the University of Verona, and his colleagues, have created a stir this week by reporting, in the latest edition of the Proceedings of the National Academy of Sciences, what they think is a new prion disease in cattle.
The best known (indeed, until now the only known) prion disease of cattle is bovine spongiform encephalopathy (BSE), or “mad cow” disease. Prion diseases are odd. Although they can sometimes be passed from one individual to another as an infection, they do not appear to be caused by an organism that has genes, unlike all the other infections that are known. Instead, they are caused by misshapen brain proteins (prions) which have the strange characteristic of acting as catalysts that cause normal versions of the protein to reconfigure themselves as prions. Thus the infection spreads and, eventually, kills by affecting critical brain functions.
BSE has proved, in relatively rare cases, to be transmissible to people who have eaten the meat of infected cattle. In Britain, where BSE was first identified, some 139 people have died over the past decade of the human version, known as variant (or, sometimes, new-variant) Creutzfeldt-Jakob disease (vCJD). As the sobriquet suggests, other forms of CJD had been identified in people before vCJD came along. Some had known causes (such as the transplantation of infected tissue), but most were dubbed “sporadic”, and of unknown cause. The reason for the excitement this week is that Dr Monaco's new version of mad-cow disease shares features with one of the human sporadic forms of CJD, and might therefore be its cause.
The new disease is dubbed bovine amyloidotic spongiform encephalopathy (BASE), because it causes the formation in the brain of structures called amyloid plaques. Such plaques form in humans during Alzheimer's disease. They also form in particular patterns in the brains of people who have contracted a certain strain of sporadic CJD, known as M/V2. Those patterns are similar (though not identical) to the ones seen in BASE. And the BASE prion itself resembles the one which causes M/V2 sCJD. In particular, it has fewer sugar molecules attached to it than does the prion that causes BSE and vCJD.
So far, Dr Monaco and his team have found only two cows with BASE, so its identification as a genuinely new disease is far from secure. It could simply be BSE that has developed in an odd way.
On the other hand, it would be strange if cows only had one prion disease, as humans and sheep have several. If it does turn out that BASE is the cause of M/V2 sCJD, that would not be cause for panic. Only one or two people per million are killed each year by all forms of sCJD, a figure that has been stable for years. Those killed by M/V2 sCJD are but a fraction of that already small number.