September 12, 2010
Cattle Eyes May Reveal Mad Cow Disease
A cow's eyes may show signs of neurological disorders such as mad cow disease, according to scientists.
Noticing the signs early enough may help prevent infected meat from reaching the food supply.
Researchers at the Iowa State University examined the retinas of sheep infected with scrapie -- a disease similar to mad cow disease. The researchers stated in the journal Analytical Chemistry that the sick sheep's eyes had a unique "glow."
People who eat contaminated meat may contract a brain-wasting disease similar to bovine spongiform encephalopathy (BSE) - mad cow disease - and scrapie.
Jacob Petrich of the department of chemistry at ISU led the study team. He said that although the research was carried out in sheep, there were hopes that similar procedures could one day be used to spot symptoms of neurological diseases in humans.
Together with colleagues, Dr Petrich examined brain tissue of 73 dead sheep and used standard pathological methods to detect the infectious prion protein.
Once the researchers confirmed that a number of animals were scrapie-positive, they analyzed 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 distinct "glow."
"The scrapie-positive retinas fluoresce a lot - they gave a lot of light back, and this light was very structured," said Dr. Petrich.
He explained that instead of the usual way of dissecting the brain in order to analyze the tissue and detect prion proteins, the new technique was an indirect way of looking for a neurological disease.
Transmissible spongiform encephalopathy 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 experimentation, said Dr Petrich.
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 colored 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 color," he told BBC News.
Field experiments would be the next step scientists would take, he said, to see if it was possible to spot the disease by simply shining light into the eye of living animals. But the problem then would be to get the animal to stop moving long enough to get a good look into the eye.
If performing the test on living animals turned out to be too tricky, Dr Petrich said, "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.
But how certain could one be that by simply shining a light on the retina, it would be possible to confirm an animal is in fact sick or not?
According to Dr Petrich, 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," said Dr Petrich.
He said, in theory, it would be possible to apply similar techniques to humans, but that it would be 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 colors 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 colorations to its central nervous system tissue would be some kind of TSE," noted Dr Petrich.
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