Case of the eyeless squirrel provides insight into natural world

Case of the eyeless squirrel provides insight into natural world

It is a rare day when I diagnose an animal with missing parts at autopsy. In the fall of 2015, a concerned citizen found a young, female blind squirrel foraging on windfall apples near Windsor, Ont., and sent it to a wildlife rehabilitation centre. With no hope of long-term survival in the wild, the blind squirrel was humanely euthanized and submitted to the Ontario region of the Canadian Wildlife Health Cooperative for autopsy. I happened to be taking cases that day.

Suited up in my lab coat, blue disposable gloves and safety glasses, I inspected the tiny creature. To my amazement, the squirrel did not have eyes. One eye socket was completely covered with skin, while the other had only a tiny opening.

When I removed the brain, it was obvious that more was wrong. Where thick, cord-link nerves normally connect the brain to the eyes, only a thread remained. I noted the change and proceeded to examine the rest of the body, including the internal organs. I was concerned that something else would be formed incorrectly in addition to the apparent lack of eyes, but nothing was out of place or malformed. I carefully dissected the tissue from within both eye sockets and plunked these, along with the brain, in formalin preservative.

The next step was to examine the tissues under the microscope. Nothing more than a small amount of fat and muscle came out of the eye sockets—there were no eye tissues present.

The brain was also interesting. Normally, there are thick, pink nerve axons coursing through the middle brain. These act like telegraph wires to conduct information between the eyes and the brain, a crucial connection for vision.

In the squirrel, these cords were reduced to a sliver. To confirm this, our department administrative assistant brought me a fresh road-killed squirrel she found on her commute to the office. When I compared the normal brain to the blind squirrel, it was clear that the visual pathways were abnormal.

I consulted with Dr. Bruce Grahn of the Western College of Veterinary Medicine and Dr. Fiona James at the Ontario Veterinary College (OVC) along with Elizabeth Hartnett, soon-to-be veterinarian, also from the OVC. Together, we published this remarkable case in the October 2017 issue of the Journal of Wildlife Diseases.

Cases like these may seem rare and insignificant in the grand scheme of thing, but I believe that examining unusual cases like this squirrel provide insight into the natural world.

Animals are occasionally born blind. In instances where there is an apparent lack of eyes, it is usually that the eyes are tiny, rather than absent. This squirrel was incredibly rare because there were no eye structures at all.

Based on its estimated age, this squirrel was weaned and had been outside the nest for several weeks. Somehow, it avoided all kinds of hazards that come with urban existence including roaming cats and dogs, vehicles and other predators. It also had a good layer of body fat, showing that even a blind squirrel can find a nut.

While documenting brain changes was important, this case also showed that blindness isn’t an immediate death sentence for all wild animals. Most experts would tell you that blind wildlife have little chance of surviving in nature. A potent example of this occurs when predatory birds, such as owls and hawks, suffer from eye disease. Unable to see well enough to hunt, these unfortunate predators slowly waste away.

We don’t know what caused this squirrel to be born without eyes. It is possible the squirrel’s dam was exposed to some type of toxin or infection during gestation. It could also be the result of a genetic mutation. We’ll never know for sure but I’m keen to continue examining these types of unusual wildlife cases.

If you see sick or dead wildlife, please contact the Canadian Wildlife Health Cooperative.

Dr. Jamie Rothenburger is a veterinarian who practices pathology and an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger

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