ROGUE proteins responsible for mad cow disease have been discovered in the saliva of cows infected as part of an experiment. The finding might pave the way for a simple test for BSE before the symptoms are apparent.
The result from a team led by Yuichi Murayama at the National Institute of Animal Health in Tsukuba, Japan, also suggests, not for the first time, that saliva may be one way some prion diseases can spread. This group of diseases includes scrapie, chronic wasting disease and variant Creutzfeldt-Jakob Disease (vCJD), the human form of mad cow disease.
However, all available evidence suggests this method of transmission is highly unlikely. So far, the team stress there is no epidemiological evidence that saliva, milk, blood or spinal fluid from BSE-infected animals is infectious.
"Data from sheep with scrapie and deer with chronic wasting disease suggest the infectivity levels are likely to be very low," says Neil Mabbott of the Roslin Institute in Edinburgh, UK, who investigates infectious disease.
At present, diagnosing BSE is only possible by examining brain tissue after death, when the prions are visible as plaques. To find out if the disease could be detected in live animals, the Japanese team deliberately infected three cows. Then every four months, they screened samples of the cows' saliva using PMC, or protein misfolding cyclic amplification, which ramps up tiny amounts of prion to measurable levels.
In one cow, they detected prions two months before typical symptoms of mad cow disease would be expected to emerge. In the other two, prions were detectable just as the first symptoms began to appear.
"Once the infectious agent reaches the central nervous system, it may spread [away] from the brain to the salivary glands," the researchers wrote in their report (Emerging Infectious Diseases, DOI: 10.3201/eid1812.120528).
When BSE spiralled out of control in the UK 20 years ago, the source was incontrovertibly traced to cattle feed contaminated with brain tissue from infected cows. The Japanese research raises the possibility that it could also spread in body fluids through licking - a theoretical possibility that can't be ruled out by their current data.
The crux of the matter, says Richard Bessen at Colorado State University in Fort Collins, who is evaluating an alternative test for prion detection, is whether there's enough infective material in saliva to spread the disease. "Sure, it may occur in unusual cases, but it is probably not a major pathway for BSE transmission," he says.
"Epidemiological data indicate that in cattle, spread of BSE from animal to animal is very limited or absent, and so any shedding of BSE prions from cattle is very unlikely to spread the disease environmentally," says Kevin Gough, a pathologist at the University of Nottingham, UK.
Although BSE is now practically extinct globally, it still crops up unexpectedly. All researchers contacted by New Scientist said that a test to detect the disease in live animals before they developed symptoms would be invaluable to keep the disease in check.
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