Darcy has snored and breathed loudly since puppyhood, but this sounded different. We knew that her heart disease had started to progress, but was this actually labored breathing or just louder breathing? And did it warrant a trip to the emergency clinic at 11 p.m.? The fact that Darcy, age 6, couldn’t seem to get comfortable convinced me it was time to go. I drove her to the emergency clinic, and by the time we arrived, 15 minutes later, it was obvious she had worsened. The staff hustled her into an oxygen cage, where she stayed for the next three days.

Heavy bleeding and broken bones are obvious, but what are the other signs your pet needs to see the veterinarian or even make a trip to the emergency clinic? Cats and dogs are good at hiding pain and other signs of illness, so knowing what to look for can mean the difference between life and death. Veterinarian Vicki L. Campbell, who is board-certified in emergency critical care and teaches at Colorado State University’s College of Veterinary Medicine in Fort Collins, says changes can be subtle.

Among the signs to watch for, she says, are increases in respiratory rate (the normal rate in a cat is 20 to 30 breaths per minute and for a dog 16 to 24 breaths per minute); changes in gum color from a healthy pink to white, yellow, gray or blue; unusual or sudden changes in attitude or behavior, such as aggression or withdrawal; increased or decreased appetite or thirst; unexplained weight loss; changes in urination; and a wobbly gait. But before you can recognize what’s abnormal for your pet, you have to know what’s normal.

So make a habit of observing your cat or dog, from the way it eats to the way it sleeps, says veterinarian James R. Richards, director of the Cornell Feline Health Center in Ithaca, N.Y. “It’s a lot of fun to watch kitties, but it can also be very helpful, because it establishes what’s normal for that cat,” he says. “Some are going to sleep more than others, some are going to be more active than others, some are going to play in different ways than others, so there’s probably an infinite variation, but whatever is normal for your cat is an important thing to try to discover. Any kind of variation from that baseline should raise some concern.” The same advice goes for dogs, he says.

Keeping a journal of your pet’s behavior is a good way to keep tabs on changes. It’s also important to know what potential problems are common in your pet’s breed. For instance, Campbell says, older Labrador retrievers and other large dog breeds are prone to a condition called laryngeal paralysis, which consists of increased noise during inhalation, and respiratory distress. It frequently gets worse during exercise and on hot days.

Giant-breed dogs and deep-chested dogs, such as standard poodles, greyhounds, great Danes and Labrador retrievers, are prone to gastric dilatation volvulus, also known as bloat. Dogs with GDV have bloated abdomens and retch without producing anything. Often, only immediate surgery can save their lives. Many toy breeds are prone to tracheal collapse, which causes coughing and difficulty breathing. Flat-faced dogs such as pugs and bulldogs often have breathing difficulties as well, especially in hot weather.

Heart disease is common in some cat and dog breeds. Boxers are prone to heart arrhythmias and can suddenly collapse or faint. Male Maine coon cats often fall victim to hypertrophic cardiomyopathy, which can show up as early as 2 to 4 years of age and frequently causes heart failure. Cavalier King Charles spaniels, like Darcy, are prone to a condition called mitral valve disease, which also leads to heart failure.

Subtle signs may go unnoticed. Some pet health problems often go untreated until it’s too late because people don’t know what to look for. “The subtle signs of cancer frequently go unnoticed until the disease is very advanced,” Campbell says. “Signs like muscle wasting, especially along the back and on the head, and weight loss despite a normal appetite may indicate cancer.” Other diseases that may go unnoticed at first are kidney disease, Cushing’s disease and diabetes. That’s because the initial signs of increased thirst and urination may not seem unusual until the pet starts having potty accidents in the home.

“It all comes down to the apparent and the inapparent,” Richards says. “With apparent things, people say ‘That’s obviously not right, and I have to get my pet to the vet.’ It’s the inapparent part that makes it more difficult. Pets are good at hiding their illness. They’re hard-wired to do that because it served them well in the wild … but it doesn’t necessarily serve them well when they are living with us and counting on us to be good stewards of their health.”

After being hospitalized twice in one week, Darcy is now stable and back to her sweet, happy self. Her condition can never be cured, but for now it’s being managed with medication and restricted activity, and we’re savoring every day we have with her.

Kim Campbell Thornton is an award-winning author who has written many articles and more than a dozen books about dogs and cats. She belongs to the Dog Writers Association of America and is past president of the Cat Writers Association. She shares her home in California with three Cavalier King Charles Spaniels and one African ringneck parakeet. Creature Comforts appears the third Monday of every month. © 2006 MSNBC Interactive