RESEARCHERS have proven manure from deer infected with a form of mad cow disease is infectious.

The startling finding is set to re-ignite debate on how a raft of deadly brain wasting diseases, caused by minute misfolded proteins called prions, are spread among deer, sheep and cattle.

The University of Californian research team, led by Nobel prize winner Stanley Prusiner, has proven laboratory mice can be infected with prions taken from the manure of deer with chronic wasting disease.

The team's research raises the prospect of prion diseases being spread to healthy animals that graze prion contaminated manure and soil, especially in confined systems with infected animals.

In an article published in today's edition of the noted scientific journal, Nature, the US team states: "The foregoing mechanism (ingesting manure from infected animals) is consistent with observed conditions under which captive mule deer have shown remarkably high rates of prion infection and explains how CWD could effectively transmit among mule deer in the wild.

Not only did the team show susceptible mice could be infected with prions, but that infected deer may shed nearly as many prions in their faeces as accumulated in their brains at the time of death.

"These data support the faecal-oral route as a likely natural mechanism for the transmission of CWD prions among deer and other susceptible cervid species, and possibly for scrapie prions among sheep and goats,'' the researchers reported.

The US research raises serious questions about the effectiveness of present practices banning the feeding of ruminant meal to cattle and sheep feed to prevent the spread of a raft of prion diseases.

Until now animal health experts have assumed prion diseases only spread as a result of infected animals being rendered down and fed to other ruminants, a practice that is now banned following the mad cow epidemic of the 1980s.

In the UK alone more than 4.4 million cattle were slaughtered during the mad cow outbreak, which led to fears hundreds of Britons had been infected with a new variant of the human brain wasting Cruetzfeldt-Jakob Disease after consuming the brains or spinal chords of  infected cattle.

The new variant of CJD  linked to cattle has been blamed for killing 164 Britons, with more expected to die in the future, given the disease has an incubation period of 40 years.