Obie, my 6-year-old Paint gelding, had loads of potential, so last summer my husband and I sent him to my friend Jason to be tuned up for sale. We were excited when

Obie caught the eye of an excellent eventing trainer seeking a new prospect, and we agreed to deliver him to her farm on July 9th for an evaluation period.

Jason bathed Obie the evening before he was scheduled to leave and then, to keep him clean, placed him in a stall for the night instead of turning him back out in his dirt paddock. Noticing that the flies were bad that evening, Jason went back to the tack room, grabbed the nearest fly mask and put it on our horse.

When I arrived early the following morning, Obie came out of the stall looking great. Even with the barn lights turned off I could see that he was gleaming. He was still wearing the fly mask, and we decided to keep it in place for the trip. My trailer has stock sides, and the mask would provide protection from any debris that might come up off the road. Jason took a quick peek under the mask to make sure Obie’s eyes weren’t gooey, and since everything looked good, we loaded up and left.

We pulled into the trainer’s farm two hours later, and Obie, always a good traveler, came out looking relaxed and regal. It wasn’t until we led him into the barn and took the fly mask off that we discovered a problem: Obie’s left eye looked strangely blue. On closer inspection, we saw that the blue appearance was the result of huge dilation of the pupil.

Although more oval than round, a horse’s pupils function just as our own do: They expand to take in more light when necessary, then constrict to limit the glare in brighter conditions. And, also as in people, both of a horse’s pupils ought to be the same size at the same time. Any noticeable difference between the appearance of the pupils can be a sign of serious trouble.

That was exactly the problem confronting us: Obie’s right eye looked normal, and the left pupil was wide open and completely unresponsive to light.

No clues
My first thought was that Obie must have hit his eye or whacked his head in the trailer. But we found no other signs of trouble—no tearing, swelling, discharge or any marks that would indicate trauma. Obie didn’t appear to be in pain; he let us touch his face all around the eye without protest. In fact, he was acting perfectly normal—interested in his surroundings and even more focused on the possibility of getting some of the hay stacked in the corner. Neither Jason, the trainer nor I—all lifelong horsepeople—had ever seen anything like this.

I was truly alarmed, so we called the trainer’s veterinarian, who arrived within the hour. After her initial examination, she agreed there was no obvious sign of trauma or disease and that Obie didn’t seem to be in pain. She did offer one possibility—glaucoma, an increase of fluid pressure in the eye that is a common cause of pupil dilation. In horses, glaucoma is usually secondary to a disease such as uveitis or a tumor within the eye. If the pressure within the eye remains elevated long enough, the optic nerve can be damaged, leading to blindness. The good news was that Obie didn’t show any other signs of glaucoma, such as pain or squinting.

To get a closer look at the internal structures of the eye, the veterinarian gave Obie a dose of tranquilizer and a local anesthetic. Once the gelding was relaxed and numbed up, she examined the retina more closely and found it to be perfectly normal. She also stained the eye with a fluorescein dye, which would settle into any scratches in his cornea to appear as bright yellow streaks or spots. But the veterinarian saw no evidence of even subtle trauma.

Next, she used a small penlight to check what is known as the “consensual response.” Normally, if you shine a light into one eye, the pupils in both eyes will react in exactly the same way. If they don’t, then the horse may not have vision in the eye being tested. Fortunately, shining the light in Obie’s left eye caused his right pupil to constrict, so he still had at least some sight on the left side. But we still had no clue what was wrong—or if he was in danger of going blind.

As she finished her examination, the veterinarian was unable to make a diagnosis. Based on the clues she had, her best assumption was that the dilation of the pupil was due to some kind of trauma, despite the lack of cuts, bumps or scrapes on Obie’s face. She prescribed a course of phenylbutazone, in case there was any inflammatory process at work, and suggested that, if there was no improvement within a few days, we take Obie to the ophthalmology department at the University of California-Davis for a more thorough workup. As she packed up to leave, she told us to keep a fly mask on Obie to protect the eye—too much sunlight entering through the dilated pupil could damage the retina and create a very real danger of blindness.

A flash of insight
After conferring with the prospective buyer we decided to leave Obie at her place rather than trailer him again with this undiagnosed ailment. We were also hoping that the problem would resolve on its own, as quickly as it had appeared. Unfortunately, after three days there was no change in Obie’s pupil, so my husband and I brought him home.

Back at our place, Obie still seemed completely normal except for that ominous eye. I was worried about the possibility of sun damage, so I made a “pirate patch” by cutting up an old fly mask and sewing two extra layers on top of his existing mask. He accepted this impediment to his vision with his usual good nature. Meanwhile, I went into research mode and read up on every equine eye problem I could find on the Internet. Obie’s single sign of trouble didn’t seem to match up with anything.

I also called the ophthalmology department at UC-Davis and was put in touch with Ann R. Strom, DVM. I explained Obie’s case and asked what they could do that the trainer’s veterinarian hadn’t already done. Strom explained that the UC-Davis clinic had specialized equipment that would enable them to detect problems not visible with the tools available to a veterinarian in the field. If the physical exam turned up nothing, they would start looking for possible neurological causes, such as a brain tumor or a tumor between the eye and the brain, which could best be viewed with a CT scan. Unfortunately, the cost of all this could easily run into the thousands, which we simply couldn’t afford. I had no idea what to do.

My friend Jason had also been racking his brain, trying to think of anything he had seen or done that might offer clues. He was absolutely certain Obie’s eye had been fine after his bath on the evening before the problem was discovered. What he didn’t know was whether the eye was dilated before the trailer ride the next morning
—although he had glanced under the fly mask, it was too dark for him to have noticed any changes in the pupil.

Then Jason had a light bulb moment: He remembered that the last horse to occupy the stall Obie had been in was a mare who was being treated for an eye problem. He dug through the ranch’s medical records and discovered that six weeks previously, this mare had sustained an injury to her left eye. She had been treated with atropine ointment, a powerful and long-lasting drug used to dilate the pupil. Could that horse have rubbed her medicated eye on something in the stall, something that Obie then rubbed on, too, causing him to get some residual atropine into his eye? It didn’t seem entirely implausible.

Jason and I ran this scenario by several veterinarians, all of whom seemed to think the idea was too far-fetched. My own veterinarian, Jen Powers, DVM, of Above and Beyond Animal Care in Redding, California, was also dubious, but she didn’t discount the idea entirely: “I think it’s highly unlikely, but stranger things have happened.”

Powers, like Strom, mentioned the possibilities of glaucoma or a tumor, although she agreed that it would be unusual for a horse to have either condition without showing any additional signs of trouble. She was fairly sure that the dilation was not due to trauma. In her 18 years of experience as an equine veterinarian, she said, she had never seen a traumatic eye injury with a dilated pupil as the only sign. Any blow severe enough to cause the pupil to dilate that much should have caused some other obvious injury to the head or eye itself.

If exposure to atropine had caused the dilation, Powers said, we would start to see gradual improvement within seven to 10 days of the initial exposure, although it could take a couple of weeks to fully resolve.

Jason then contacted us with another important clue: After consulting with his staff, he discovered that the fly mask he had put on Obie that night was very likely the same one the injured mare had worn during the course of her treatment. We found no sign of any ointment on the mask, but it seemed more plausible that he might have come in contact with residue from the fly mask than from some random spot in the stall.

With this news, my husband and I decided to wait a few more days to see if Obie’s eye showed any improvement. If it didn’t, we were going to have to bite the bullet and haul him to Davis.

Back to normal
To our joy, we found that the dilation of Obie’s left pupil did begin to subside, all on its own, exactly one week after that fly mask was first put on him. The progress was slow, but the eye continued to improve day by day.

On July 19, the 10th day after the trouble began, I emailed Powers a photo of Obie’s eye and asked if she felt it was OK to continue to wait or whether we should go ahead and take him to Davis. She responded, “The amount of dilation Obie shows at this point is not severe, which decreases my fears of glaucoma, tumor or blindness. I think you are fine to wait another week.”

We continued to see improvement with each passing day, and by July 21, his pupil was able to constrict enough that we felt we could safely remove the extra layers of material on his fly mask, and five days later, his eye seemed completely normal again. The entire ordeal had lasted 18 days.

Powers now agrees that atropine was the most likely cause of this strange problem. “Without the stimulation of pain [there was no injury] or light [due to the eye patch], there was no stimulus for the pupillary muscles, so it is possible for the effects of atropine to have lasted this long,” she told me. “The atropine theory is also supported by the resolution course and the story of the fly mask—a good example of why it is important to wash and disinfect shared items!”

I am happy to report that Obie now shows no sign of having ever had a problem with his eye, and he has a wonderful new home.

This article first appeared in EQUUS issue 424.