Fluorescence eye test reveals neurological diseases in livestock

The eyes of sheep infected with scrapie—a neurological disorder similar to mad cow disease—return an intense, almost-white glow when they're illuminated with blue excitation light, according to a research project led by Iowa State University's Jacob Petrich.

The findings suggest technologies and techniques can be developed to quickly and non-invasively test for transmissible spongiform encephalopathies, progressive and fatal neurological diseases such as mad cow disease in cattle and Creutzfeldt–Jakob disease in humans. Petrich, in fact, is working to develop a testing device. The findings were published earlier this year in Analytical Chemistry (doi: 10.1021/ac100179u).

The research is the result of an accidental discovery while Petrich and his collaborators were developing a fluorescence spectroscopy device that's now used in slaughterhouses to test livestock carcasses for faeces and possible E. coli contamination.

"One day we were testing the apparatus by shining light on the carcass and we saw the spinal cord glow—it fluoresced," said Petrich, professor and chair of Iowa State's chemistry department. "We saw the spinal cord through the skin. The light was pretty intense. It was an amazing result."

Jacob Petrich is using fluorescence to test the retinas of animals for signs of neurological diseases. Photo by Dave Gieseke/College of Liberal Arts and Sciences.

That suggested that fluorescence technology could be used to test animals for transmissible spongiform encephalopathies such as bovine spongiform encephalopathy—often called mad cow disease. To reduce the risk of human exposure to the diseases, the brains and spinal cords of animals are removed during slaughter and processing. But there is no quick test to identify animals with the diseases. So Petrich and a team of researchers began studying the feasibility of a fluorescence test. The researchers collected 140 eyeballs from 73 sheep. Thirty five of those sheep were infected with scrapie; 38 were not. The researchers took fluorescence readings from various parts of the eyes of all the sheep.

"The bottom line is the scrapie-positive retinas fluoresced like crazy," Petrich said. "And the scrapie-negative ones did not."

A previous study published in Veterinary Pathology (doi: 10.1354/vp.08-VP-0206-W-FL) reported that the function and structure of retinas are altered in cattle infected with transmissible mink encephalopathy. Other studies have reported that lipofuscin, an intracellular fluorescent pigment, accumulates in the eyes of animals infected with the neurological diseases. Petrich and his team attribute the glow from scrapie-positive retinas to the elevated levels of lipofuscin.

Whatever the cause, Petrich said it's clear there are distinct differences in the fluorescence and spectroscopic signatures of retinas from sheep that were naturally infected with scrapie and those that were not. And so he and his research team think there's great promise for a diagnostic test based on that discovery.

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