A pioneering retinal scan could diagnose “zombie deer” on the spot—if only scientists could fund it

Research suggests that fatal, prion-based conditions like chronic wasting disease are visible in the eye's "fluorescent signature." But more money is needed to build a tool that would work in the wild.

Hundreds of years ago, Shakespeare popularized the idea that the eyes are windows to the soul. Flash forward to 2019, and scientists will be satisfied if eyes offer a peek into the brain.

As we recently reported, “zombie deer” disease, also known as chronic wasting disease (CWD), continues to spread into new areas. The sickness, a transmissible spongiform encephalopathy (TSE)—part of the same family of rare, fatal, neurogenerative disorders as mad cow disease—gradually eats away at the animals’ brains. It’s impossible to cure, difficult to detect, and researchers worry it could infect humans.

Now, scientists hope deer’s eyes might hold the key to detecting CWD-positive animals long before they start displaying zombie-like symptoms. It all starts with a retinal scan.

As we get older, yellow-brown pigment material called lipofuscin accumulates in our eyes. The older we get, the more lipofuscin we’re carrying around. And since lipofuscin can absorb and emit light, an LED shone on an old person’s eyeball is going to refract off a lot more particles than an LED shone on a baby’s eyeball. (Don’t try this at home: The blue light can damage human eyes.)

Mice, deer, cows, sheep, and goats all accumulate lipofuscin in their eyeballs, too. So an old mouse will have a shinier eyeball than a teen mouse when held under a fluorescent light. The same concept—more lipofuscin, bigger “fluorescent signature”—applies to animals with degenerative brain diseases. An illness like CWD speeds up the accumulation of lipofuscin, meaning a two-year-old buck with CWD is likely to have an abnormally bright fluorescent eye map.

The technology, while promising, never made it into the mainstream.
Mark Rasmussen and Jake Petrich, researchers at Iowa State University, have been working for years to develop a technique called fluorescent spectroscopy, which could be used on the retinas of animals with TSE. (They’ve also tried to adopt the technology for other purposes, including a technique that may determine whether your “grass-fed” milk really came from cows on pasture.) The duo started out looking at the eyes of healthy mice, using lipofuscin levels to figure out each mouse’s age. Later, they used the technology to identify sheep infected with scrapie, a “model” TSE.

In a 2010 paper co-authored with a team of Iowa State researchers, they wrote that using fluorescent spectroscopy in a retinal scan could in fact identify animals with TSEs like mad cow. The retinas of the infected animals, they found, “contain[ed] very bright regions of highly fluorescent material, which is suggestive of neurological damage as a result of disease.”

In other words, if you want to diagnose an animal with TSE, it doesn’t matter if it’s mad cow, CWD, or another disease. The eye is the best place to look.

“Our investigations … suggest that the retina is a most promising part of the eye for revealing spectroscopic signatures indicative of neurological disease,” they wrote. “The differences in total fluorescence due to disease status greatly exceed any differences that result from other factors, such as age.”

The technology, while promising, never made it into the mainstream. A large fast food chain showed interest in developing a portable version of the test, but grant money never came through and eventually the threat of mad cow disease mostly subsided.

It’s a promising technology, but will never start screening deer if it doesn’t get built.
The testing method has some significant strengths. First and foremost, it’s way faster than a lab test. Whereas Minnesota hunters currently wait two to three weeks to get results, fluorescent spectroscopy could clear their deer on the spot. And the test—in theory—is non-invasive, meaning it could be performed on live animals if the researchers could find a way to keep the deer still and unblinking.

For now, though, there are limitations. Some in the medical profession have pushed back, arguing that the test is too general. Who’s to say a deer’s elevated lipofuscin levels are symptomatic of CWD positivity and not some other eye disease?

In response, Rasmussen and Petrich emphasize that this test is meant to be used for fast early detection, not definitive diagnoses. (If your freshly hunted buck only possibly had CWD, we’re guessing you’d seek dinner elsewhere.) Ultimately, they envision rolling it out at testing stations so hunters can instantly spot-check their harvest. Possible matches would be flagged for further analysis, but healthy animals could be butchered without fear of contamination.

Then there’s the fact that Rasmussen and Petrich still currently have to dissect specimen animals to do the scan—dislodging the intact eyeball to remove the retina. A working prototype would be portable and could scan whole eyeballs (not just retinas). But developing it will take time and money. Petrich estimates a prototype could be completed in about six months if the project were funded.

“If you say a month to do something, usually that means adding another two on, so maybe if you take three months to build a prototype, to test it out, to make sure it’s really working, maybe half a year,” he says.

For now, the project is still on ice for lack of funding. Petrich says he’s had some preliminary interest from people in the deer farming industry, but nothing official has firmed up. In the meantime, they’ve been trying to drum up support with wildlife advocates. It’s a promising technology, but will never start screening deer if it doesn’t get built.

“No one has come out of the bushes with a checkbook,” Rasmussen laughs.

H. Claire Brown is a senior staff writer for The Counter. Her work has also appeared in The Atlantic, The Guardian, and The Intercept and has won awards from the Society for Advancing Business Editing and Writing, the New York Press Club, the Newswomen's Club of New York, and others. A North Carolina native, she now lives in Brooklyn.