Brain-destroying prion disease can spread among deer through faeces shed by animals which have been infected by the deadly pathogen but are not sick, a study published on Wednesday suggests.
The paper implies that the mysterious disorders known as chronic wasting disease (CWD) and scrapie -- cousins of mad-cow disease -- could be transmitted in a hitherto-overlooked way, posing a headache for farmers.
The scientists are led by University of California neurologist Stanley Prusiner, who won the 1997 Nobel Medicine Prize for research into prions, the rogue proteins blamed for turning brains spongey.
Prion diseases are well-researched disorders in deer (CWD), cattle (bovine spongiform encephalopathy, or BSE), sheep and goats (scrapie), cats (feline spongiform encephalopathy, FSE) and in humans (Creutzfeldt-Jakob disease, CJD).
Many aspects about them are unclear, though. In deer, CWD is known to be handed on from mother to offspring, but other paths of infection are deemed possible.
Publishing in the British journal Nature, Prusiner's team probe a theory that deer may pass on the prion in their faeces, thus creating a risk that other deer will pick up the pathogen in the soil when they graze.
Over the course of months, the investigators collected the faeces of five mule deer (Odocoileus hemionus) and then orally infected them with CWD prions.
They continued taking samples until the animals died or developed signs of the disease and were euthanised.
Faecal samples -- irradiated to kill germs and viruses but leave the prions untouched -- were injected into the brains of genetically modified mice.
Samples that had been collected before the deer were infected and three to four months after infection had no effect.
But 14 out of 15 samples that were collected after the four-month mark caused the mice to fall sick with CWD-like symptoms.
The results, say the authors, point to a "horizontal" way by which CWD could spread among deer in the wild -- and, possibly, by which scrapie could spread among sheep and goats.
The "faecal-oral route" was given credence by a 1994 Icelandic study in which sheep with scrapie were held in a field, removed and then replaced with scrapie-free sheep, which then went on to develop the disease.
But an unresolved question is whether prions can survive long enough in the soil to infect other animals. Past studies have sent back conflicting data on this; some say harsh weather and microbial enzymes in the soil degrade the pathogen.
The new paper did not touch on BSE. That disease, which erupted in Britain in the 1980s, unleashed a scare in 1996 when BSE-infected beef was linked to a rare form of fatal neurodegenerative disorder in humans, variant CJD.
The causes of BSE have been fiercely debated. One idea is that herds became infected by eating the ground-up carcasses of scrapie-infected sheep.
BSE has been fought through a culling of animals and a change of feeding practices. Several studies into cattle suggest there is no "faecal-oral route" for BSE.