Even Eagles Get Cataracts

By Kathy A. Svitil|Thursday, December 01, 1994
Papa the bald eagle had already lost a wing, and he was losing his right eagle eye to a cataract--until the surgeons stepped in.

At the Ellen Trout Zoo in Lufkin, Texas, there is a male American bald eagle called Papa. Papa couldn’t survive in the wild; over a decade ago, a human with a gun left him with a wing missing. But he seemed to be a happy enough eagle until a few years ago, when he began having trouble with his right eye. In the beginning we chose not to do anything about it, says zoo veterinarian John Wood. After all, he’s not like a bird in the wild that needs perfect vision to survive. Earlier this year, though, he began keeping that eye closed and tilting his head to one side. The eye appeared to be giving him trouble.

Wood and zoo director Gordon Henley called in ophthalmologist Thomas Duncan. Duncan diagnosed a cataract: a clouding of the lens. And he proposed to fix it by putting in a new lens.

The lens of the eye, be it human or eagle, sits just behind the cornea and the iris. Its job is to focus light onto the photoreceptive cells of the retina, and it is an architectural masterpiece: an onionlike structure of concentric layers, each made up of fibers so exactly aligned that the lens is almost perfectly transparent. But trauma (from humans with guns, for example), chemical imbalances (from diseases like diabetes), and even ultraviolet light can all disrupt this alignment. When that happens, a protein that was once distributed uniformly through the lens gets concentrated in light-scattering cataracts.

Bald eagles have more to lose from cataracts than we do; their vision allows them to spot a scampering rabbit from an altitude of hundreds of yards. The main difference between our vision and theirs, though, lies not in the lens but on the retina. Our acute vision is limited to a tiny area of densely packed photoreceptors--one in each eye--called the fovea. But eagles and their kin have two foveae in each eye, says Christopher Murphy, an avian vision specialist at the University of Wisconsin at Madison. Focus on something directly in front of you. Now hold your left thumb 45 degrees to the left. Do the same with the right thumb. If you had another fovea in each eye, you’d be able to see both your thumbs and the central image at the same time and with the same resolution. That’s what an eagle can do.

But not if the bird’s lens isn’t transmitting any light. Over one and a half million Americans each year get their cataract-riddled lenses replaced with artificial ones. Until Papa’s operation last February, though, the procedure had never been done on a bald eagle.

Papa was first anesthetized, and his talons were wrapped in towels. (We weren’t sure if he was going to wake up and drive them through our arms, Duncan says.) Then Duncan measured the curvature of the eagle’s cornea and the length of the eye. That told him what strength of artificial lens best corresponded to the eagle’s own lens.

Getting the damaged lens out proved difficult. The muscles in a raptor’s iris don’t respond to the muscle-relaxing eyedrops that are used to dilate the human pupil. So we injected a tiny amount of curare, the muscle-paralyzing drug used in poison darts, directly into the eye through the cornea, says Duncan, and prayed that the eagle didn’t die.

He didn’t. Duncan then made a small incision in the cornea and cut open the sac holding the lens with a pair of microscopic scissors. Next he broke up the lens with an equally minute ultrasound-emitting probe and vacuumed out the smashed-up bits with a suction tip. In a human this part of the procedure takes about 15 minutes. But the eagle’s lens, Duncan says, was sort of past due. It was tough and stringy and took nearly two hours to get out. Finally Duncan slipped the plastic, quarter-inch-wide artificial lens into the lens sac. Two polypropylene loops curving out of either side of the lens pressed up against the inner wall of the sac, anchoring the lens in place.

After the eye was stitched up, Papa was up and about in two hours. Within a week he was back on exhibit. His vision still isn’t perfect, of course. His acuity is probably not as good as it once was, says Wood. He still has some floaters in his eye--bits of tissue that we weren’t able to get out. But he’s improved tremendously. He seemed a little depressed before, just sitting around in a daze. Now he pays attention to every cricket that jumps in his pen.
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