Being a long time pug and french bulldog parent I have witnessed first hand the spread of pigment throughout my dogs' eyes. Mackenzie, my senior pug who passed away a couple years ago, had severe corneal pigmentation and now Paige my 8 yr old frenchie is facing this same issue. As a result, I have a personal interest in better understanding what causes this to happen, especially as dogs get older.
To start things off, corneal pigmentation, also known as pigmentary keratopathy, is a very non-specific response by the cornea to a variety of irritating stimuli. It has been compared to getting mud on your windshield - when there are small specks there is still visibility, but when the entire windshield is covered you are in many ways blind. This is what happens to our dogs as the pigmentation expands and thickens.
It is very common in brachycephalic breeds such as french bulldogs, pugs, boston terriers etc which explains why I have seen this in more than one of my dogs. With Paige I know the cause - she has a history of eye ulcers and the pigment that has formed is her eyes' way of protecting itself from further damage. In other dogs dry eye is a common cause and it can also be brought on by eye disease, infections and problems with a dog's eyelids.
With Mackenzie, while he did have an eye trauma in one of his eyes, the other eye was quite healthy but he still experienced rapid pigmentation which always puzzled and saddened me. He was treated with a variety of medications to slow down the process, but in the end his eyes were completely covered and he eventually had little to no visibility. As our dogs get older, their eyes degrade in similar ways to humans leaving them more susceptible to the underlying causes of corneal pigmentation which is why this condition can appear to be worse in older dogs. With treatment, while it cannot be cured (yet), the process can be slowed so that it takes many years for the pigmentation to take over, which for Mackenzie and dogs like him, ends up occurring when they are well into their golden years.
In researching corneal pigmentation, I came across some interesting and recent research specifically on pugs being conducted by Amber L. Labelle, DVM, MS, DACVO; Christine B. Dresser, DVM; Ralph E. Hamor, DVM, MS, DACVO; Matthew C. Allender, DVM, PhD, DACZM and Julia L. Disney, BS which looked at the Characteristics of, prevalence of and risk factors for corneal pigmentation (pigmentary keratopathy) in pugs. In this study they looked at 295 pugs from different geographical locations in an attempt to better understand the prevalence of pigmentary keratopathy in the pug population. The results were quite shocking in that roughly 82% of the pugs in the study showed signs of corneal pigmentation, and while the majority of those evaluated are considered to be mild cases, they found that 25% of the affected pugs are moderately or severely affected. The median age of pugs in the study was 4 years with 25% of the dogs older than 7 years, however I can't help but wonder if more seniors were involved, if the number of moderate to severe cases detected would have been higher.
I had a chance to connect with Dr. Amber Labelle who helped answer some additional questions and shared how her love for pugs (she is owned by 2 pugs named Dexter and Sheldon) and her experiences as a a veterinary ophthalmologist in examining many pug patients with corneal pigmentation, led her towards research on pigmentary keratopathy. The interesting and challenging aspect and why this particular research is so important is that while there are identifiable causes to the presence of corneal pigmentation, like in the case of Paige, when it comes to pugs the cause is often unknown. Therefore the hope of this research is to better understand the why when it comes to pugs, with a long term goal of improving treatment and possibly prevention.
Dr. Labelle explains, "when we started the study, one of the questions that we wanted to answer is whether or not the more common diseases (such as dry eye and eyelid problems) are associated with pigmentary keratopathy in pugs. What we found in the study is that pugs are equally likely to have pigment on their corneas whether or not they have dry eye or eyelid problems. This leaves us in a bit of a quandary—we know that pigmentary keratopathy affects the majority of the pug population, but we still aren't sure what causes it." While more research is needed, this study did point towards genetic factors as the root cause. "The findings of iris hypoplasia (which means the iris, the colored part inside the eye, did not develop normally) and iris-iris persistent pupillary membranes (a birth defect that leaves little strands of tissue on the surface of the iris) being very common in pugs was very unexpected. Those were findings that I started to notice after I examined the first 20-30 pugs. We started tracking those findings in the remainder of the pugs that participated in the study, and were surprised to find that 70-80% of pugs also had iris hypoplasia and persistent pupillary membranes. In fact, there was only ONE pug in the WHOLE study that had no corneal pigment, no iris hypoplasia and no persistent pupillary membranes! We are unsure of the relationship between corneal pigmentation and these other abnormalities, but it is possible they are related and more investigation is needed," states Dr. Labelle.
So while pugs can have underlying conditions causing the pigmentation, we may be looking at genetic causes as the core reason behind its overwhelming prevalence among pugs. With this research we are one step closer to finding the answers we need and for that I am very grateful for the commitment and efforts of Dr. Labelle and her team.
Ann-Marie Fleming is the Founder & CEO of Dog Quality, a provider of products focused on improving the quality of life for older dogs.
