British duo probes origin of mad cow disease
Did human remains in food spawn the infection in cattle?
Human remains in cattle feed could have caused the first case of mad cow disease, two UK researchers propose. The hypothesis seeks to answer lingering questions about the fatal infection, which has affected 180,000 cows in Britain alone since the mid-1980s, and has gone on to cause more than 100 deaths in humans.
Alan Colchester of the University of Kent and his daughter Nancy Colchester, of the University of Edinburgh, point out that during the 1960s and 1970s Britain imported hundreds of thousands of tonnes of whole and crushed bones and animal carcasses. These were used for fertilizer and to feed livestock.
Nearly 50% of these imports came from Bangladesh, where peasants gathering animal materials may have also picked up human remains, the researchers say.
Other experts in the field view the idea with scepticism, saying that proof remains circumstantial. "The argument isn't very compelling because there's no smoking gun evidence," says Surachai Supattapone, an expert in infectious diseases at Dartmouth Medical School in Hanover, New Hampshire.
Transmissible spongiform encephalopathies (TSEs) are a group of fatal neurodegenerative diseases that affect mammals. Yet until 1986 no such illness had been spotted in cattle.
Once recognized, bovine spongiform encephalopathy (BSE) became widely known as 'mad cow disease'. The incidence of the disease then rocketed, peaking in late 1992.
Tests indicated misshapen prion proteins in the brains of the cows as the source of the problem. And when authorities banned the practice of recycling animal remains into cattle feed the number of sick livestock began to drop.
But experts continue to puzzle over how BSE arose in the first place.
One of the most widely believed theories is that prions responsible for sheep scrapie got incorporated into cattle feed. Scientists argue that ingested scrapie prions radically altered the normal, analogous proteins in one cow, which then developed the first case of BSE.
But the Colchesters point out that cows have been exposed to scrapie for 70 years, so it is hard to explain why BSE emerged only recently.
They propose that a more likely source is recent exposure to human remains carrying sporadic Creutzfeldt-Jakob Disease (CJD), a TSE thought to arise spontaneously in people.
Religious customs in Bangladesh and surrounding areas mean that many corpses are disposed of in rivers. People may have collected remnants from such bodies when foraging for animal carcasses, the Colchesters argue in The Lancet1. Any prions in these corpses might then have caused mad cow disease.
Experts agree that the theory needs to be checked. More information needs to be collected, they say, about the number of deaths from CJD in the Indian subcontinent, what happened to the bodies, and whether prions could have been transmitted in the way proposed. When human sporadic CJD prions were injected into mice in previous studies, the mice did not become ill2.
- Colchester A. & Colchester N. et al. Lancet, 366. 856 - 861 (2005).
- Scott M . R., Peretz D., Nguyen H. O., Dearmond S. J. & Prusiner S. B. J. Virol., 79. 5259 - 5271 (2005).