The percentage of cats and dogs 6 years of age or older jumped between 1987 and 2001, according to U.S. Pet Ownership & Demographics sourcebooks. The sourcebooks draw on data from national studies performed by the AVMA, the next of which is due to be conducted in 2007.
The medical conditions that afflict aging cats and dogs have been a focus of much research in recent years—and are the subject of many current studies.
Reasons for the research include the increasing longevity of pets, the willingness of owners to address chronic illness in their pets, and the implications for modeling human disease.
Some studies have examined issues that commonly affect older pets such as diabetes, heart disease, and pain management. Other studies have delved into conditions more specific to geriatric animals—cognitive dysfunction, glaucoma, and osteoarthritis.
Dr. Gary Landsberg, president of the American College of Veterinary Behaviorists, closely follows research on cognitive dysfunction.
He said behavioral changes in senior pets can indicate a number of welfare issues or disease conditions, from pain to brain aging, before any medical signs appear. So guidelines on senior care now advise pet owners and veterinarians alike to watch for behavioral changes.
Dr. Landsberg, who is a partner at Doncaster Animal Clinic in Thornhill, Ontario, said old-age medical conditions, such as cognitive dysfunction, might never have had a chance to appear when animals were living shorter lives.
"We did not identify a specific brain-aging component," he said.
He said studies conducted about 15 years ago were the first to find that aging dogs could develop brain changes similar to those of humans with Alzheimer's disease, primarily ß-amyloid deposits. Tests were developed in the 1990s to detect deficits in learning and memory of dogs in the laboratory setting, allowing researchers a more accurate way to evaluate treatments for slowing or stopping cognitive dysfunction.
Drugs such as selegiline hydrochloride, as well as the specially formulated diet Canine b/d, are now available for the management of cognitive dysfunction in dogs. Studies also are examining other compounds, such as�drugs that enhance the cholinergic system and phosphatidylserine. Antioxidants show promise for protecting against memory loss.�Researchers have begun to test�learning and memory in young dogs rather than focusing only on senility. Research on learning, memory, and cognitive dysfunction in cats also is under way.
Dr. Landsberg said the next stage will be to look into the genetics of why some animals develop cognitive dysfunction.
Dr. Ellison Bentley, a clinical associate professor of comparative ophthalmology at the University of Wisconsin School of Veterinary Medicine, said glaucoma has long been a frustrating problem for veterinarians. She said advances in drugs and surgical treatments have stimulated new research about this disease that can affect the optic nerve.
"Studies of prophylactic medical therapy are the greatest advance we have made in this field in the last few years," Dr. Bentley said.
Researchers have found medical treatments that can delay the onset of glaucoma in the other eye, she said. Better imaging techniques help with early diagnosis, while a better understanding of physiology has led to therapeutic drugs—with the most dramatic being latanaprost, which takes advantage of alternative pathways for fluid outflow to reduce intraocular pressure.
Dr. Robert Peiffer, a senior investigator at Merck & Co. and director of Bucks County Animal Ophthalmology in Pennsylvania, said glaucoma in domestic animals has major modeling implications for humans—although the diseases greatly differ between domestic animals and humans.
"Also, it is potentially blinding and without a doubt the most significant therapeutic challenge that the veterinary ophthalmologist faces," he said.
He said the basic mechanisms of glaucoma remain speculative, and treatments lack comparative efficacy data. The placement of silicone drainage tubes, or gonioshunts, into an affected eye can relieve intraocular pressure—but problems include tubal obstruction and fibrous encapsulation. Freezing and laser ablation of secretory cells are unpredictable, and many surgeons prefer to combine the techniques.
In human medicine, Dr. Peiffer said, glaucoma therapies involving prostanoids are the most important recent advance. These compounds work well in dogs but are not as efficacious in cats. He added that pharmacologic neuroprotection of the retinal ganglion cells is another emerging science, along with novel delivery methods to enable the drugs to reach the back of the eye.
Dr. Denis Marcellin-Little, associate professor of orthopedics at North Carolina State University's College of Veterinary Medicine, said the relationship between osteoarthritis and mobility has been the subject of a number of recent studies.
Dr. Marcellin-Little said lack of mobility, particularly for large dogs, is a common reason for euthanasia. The first studies of arthritis in dogs date to the '50s, but veterinarians didn't actually treat dogs for arthritis at that time. Research in succeeding decades described hip dysplasia, elbow dysplasia, and several other joint diseases.
Now, veterinarians have many methods for managing arthritis, including drug and nondrug treatments.
"This is an area where we are not too far behind human medicine in understanding that you attack the problems from as many angles as you can," Dr. Marcellin-Little said.
Researchers are examining a variety of new drugs, as well as nutritional supplements such as omega-3 fatty acids, for the treatment and management of arthritis. Other researchers have been investigating new techniques for screening, surgery, and physical therapy. The emphasis is primarily on quality of life, with the introduction of functional scoring—which is scoring how the pet functions in the home environment.
Dr. Marcellin-Little said studies also are being conducted to identify markers that will help researchers develop a better understanding of the relationship between osteoarthritis and mobility.
The new generation of studies has delved into the molecular basis of hip dysplasia in dogs, too, while arthritis in cats is becoming an important research topic.