Image caption Mad cow disease is caused by abnormally shaped prion proteins

The eyes of cattle may reveal signs of neurological disorders such as mad cow disease, say scientists.

Noticing the symptoms early may help prevent infected meat from getting into the food supply.

Researchers, led from Iowa State University, US, examined the retinas of sheep infected with scrapie - a disease similar to BSE, or mad cow disease.

They write in the journal Analytical Chemistry that sick sheep's eyes had a distinctive "glow".

My wife can't stand still to have her eyes checked by an eye doctor, [so] I think it's going to be much harder to do this for an animal Jacob Petrich, Department of chemistry, Iowa State University

People may contract a brain-wasting disease similar to bovine spongiform encephalopathy (BSE) and to scrapie if they eat contaminated meat.

Just like BSE, the Creuzfeldt-Jakob disease (vCJD) in humans is a type of transmissible spongiform encephalopathy (TSE) - a neurodegenerative disorder caused by abnormally shaped prion proteins.

These mutated prions destroy brain tissue, causing fatal brain and nerve degeneration and eventually death.

Jacob Petrich of the department of chemistry at ISU led the study team.

He told BBC News that although the research was carried out on sheep, there were hopes that in the future similar procedures could be used to spot symptoms of neurological diseases such as Alzheimer's in humans.

Together with his colleagues, Dr Petrich examined the brain tissue of 73 dead sheep and used standard pathological methods to detect the infectious prion protein.

Once the scientists confirmed that a number of animals were in fact scrapie-positive, they analysed 140 eyeballs by shining a beam of light on the retina, a light-sensitive layer of tissue at the back of the inner eye.

They found that the retinas of infected sheep emitted a characteristic "glow".

"The scrapie-positive retinas fluoresce a lot - they gave a lot of light back, and this light was very structured," said Dr Petrich.

Non-invasive

He explained that contrary to the usual way of dissecting the brain in order to analyse the tissue and detect prions, the new technique was an indirect way of looking for a neurological disease.

A TSE usually manifests itself in the central nervous system tissue and since the optic nerve is directly "plugged" into it, the eye becomes the only direct non-invasive point of entry for an experimentalist, said Dr Petrich.

"Otherwise you have to tap into the brain or the spinal cord and all those things are difficult and painful," he explained.

Image caption Prions (pink balls, upper right) are a different shape to a normal cell protein (tiny green balls, lower centre)

The scientist said that a prion-based disease caused an insult to the central nervous system tissue and this insult then caused damage that manifested itself in production of coloured pigments.

"So we're not detecting the prion itself. We're looking at the neurological damage that we believe the prions are causing, [which results in] pigments in the retina and when they absorb light, they then also emit light of a certain colour."

The next step, the scientist said, would be to conduct field experiments, to see if it was possible to spot the disease by simply shining some light into the eye of living animals.

But the problem then would be to get the animal to stop moving - at least for a short period of time.

"My wife, for example, can't stand still to have her eyes checked by an eye doctor for a glaucoma test, and if a human can't sit still for a test for five seconds, I think it's going to be much harder to do this for an animal," said Dr Petrich.

If performing the test on living cattle turned out to be too tricky, he added, "you would have to do it immediately post-mortem".

This way, it would be possible to spot the infection and stop the meat from getting into the food supply.

'A long shot'

But how certain could one be that by simply shining some light on the retina, it would be possible to say that an animal was sick?

In Dr Petrich's opinion, there was a "good" correlation between sheep confirmed with standard tests to be scrapie-positive and the spectral signatures.

"We looked at a lot of eyes, and we think that spectral signature is a very good marker of a neurological disease," he said.

"If you were doing a post-mortem scan, then at the very least such measurements would give an indication that an animal should be tested, to have a veterinarian dissect the brain and look for the disease the traditional way."

He said that, in theory, it would be possible to apply similar procedures to humans, but that it would be a lot more difficult.

"By the time humans are susceptible of getting some neurological disease, they're in their late forties or fifties. And the older you get, the more colours accumulate in your neurological tissue.

"An animal is not going to be any more than two years old by the time it's slaughtered, and the only thing that could cause colourations to its central nervous system tissue would be some kind of TSE.

"So doing humans is a long shot."