Crocodile Tears and Dry Eye in Other Species
BY JASON J. NICHOLS, OD, MPH, PHD, KELLY K. NICHOLS, OD, MPH, PHD, & CARMEN M.H. COLITZ, DVM, PHD, DACVO
Most practitioners would agree that dry eye is a fairly prevalent condition in humans, especially for aging women. Researchers have revealed new insights into the structure and components of the tear film and the molecular mechanisms of dry eye disease. Specifically, new protein and lipids species have been identified, which will give rise to a fuller study and understanding of the mechanisms of the disease (its inflammatory nature).
What we often fail to remember is the other 5,500 (and counting!) confirmed species of mammals of over 50,000 confirmed vertebrates. Does the clinical care or study of these other species provide any insight into dry eye disease in the human species?
Tear Systems among Mammals
A recent search on Wikipedia revealed that "a species is a reproductively isolated population that shares a common gene pool and common niche." The tear system of vertebrates seems very species dependent. Our laboratory recently conducted some biochemical analyses of chick tears, and we found striking differences in the profiles of the chick tears compared to human tears. Had we reminded ourselves that birds (which are mammals, but not primates) lack some of the tear film glands that humans possess, we wouldn't have been so surprised. Within the taxonomic classification system, it seems likely that other mammals would share in the dry eye disease processes associated with the complex tear film and ocular surface system.
Cats rarely get dry eye syndrome, and if they do it's secondary to chronic feline herpes virus-1 infection. Dogs seem particularly prone to dry eye problems; a recent study showed that 35 percent of canines presenting to general veterinary practices had tear deficiency. Particularly afflicted are Bull Dogs, American Cocker spaniels, Cavalier King Charles spaniels, Shih Tzus, West Highland White terriers and Lhasa apsos, among others.
Most cases of canine dry eye result from immune-mediated lacrimal adenitis. But it's also interesting that the conformational anatomy of many of these breeds predisposes the ocular surface to exposure to the environment, which suggests an additional etiological role for evaporation, tear film thinning and increased tear film osmolality.
Testing Pets for Dry Eye
Pets can't discuss dry eye symptoms as humans can; thus veterinarians often assess and diagnose dry eye using the Schirmer test along with clinical histories from pet owners. Because most canine dry eye has an autoimmune or inflammatory component, our colleagues in veterinary ophthalmology have very successfully used cyclosporine A to treat dry eye in our canine friends for more than a decade.
Other species closer to humans in the taxonomic structure (other primates, for example) seem less afflicted by dry eye conditions compared to our canine friends. While not all dogs maintain the same environments as their owner, the high prevalence of dry eye in canines seems to suggest that there could be an environmental component associated with the biological mechanisms leading to the condition in these pets.
About Those Crocodile Tears
Crocodiles do have lacrimal glands and can indeed produce tears, but unlike the old anecdote, they apparently don't cry for their victims.
For references, please visit www.clspectrum.com/references.asp and click on document #132.
Dr. Jason Nichols is assistant professor of optometry and vision science at The Ohio State University College of Optometry. Dr. Kelly Nichols is an associate professor at The Ohio State University College of Optometry in the area of dry eye research. Dr. Colitz is an assistant professor of Comparative Ophthalmology at The Ohio State University, Department of Veterinary Clinical Sciences, College of Veterinary Medicine.