(NEW YORK) -- Millions of Americans are living with the pain of osteoarthritis, and so are millions of pets.
According to veterinarians, it's one of the most under-diagnosed conditions in their field, and unlike their human counterparts, animals must suffer in silence.
Since pets can't express that they're in pain, vets say there are certain warning signs owners should watch out for, ranging from physical signals to behavioral changes. And while the joint damage caused by osteoarthritis can't be reversed, the pain can be treated. But early diagnosis is critical.
"Arthritis is easier to diagnose in dogs," said Duncan Lascelles, professor of surgery and pain management at North Carolina State University College of Veterinary Medicine. "We ask them to perform certain activities, like going for walks or jumping in the car to go for a ride. We go and do activities together, so if we're observant, we can see alterations in the animals' ability to perform them."
Dogs may not want to walk as far, or may appear to tire easily, he said. They may also hesitate before jumping or walking.
"Dogs may also be slow to rise on their back legs, or may limp, or they may bunny-hop instead of using their normal stride," said Marty Becker, a veterinarian in Coeur d'Alene, Idaho, and a columnist for Vetstreet.com.
Cats don't move around as much and are lower to the ground, but there are certain cues that they may be in pain.
"They may be less likely to jump on window ledges or onto furniture, or people may start moving furniture to help the cat and not realize the cat is actually in pain," said Lascelles.
Cats may also not use the litter box if it's too high and may stop grooming themselves, Becker said.
Arthritis isn't common only in dogs and cats. Rabbits and horses often suffer from the condition, and Lascelles explained that food-producing animals are known to suffer from it as well, but don't survive long enough to experience the disease's negative effects.
Pets in pain may also act differently than they normally do.
"People become irritable and short-tempered when they're in pain, and the same thing happens to pets," said Lascelles.
The pain may cause pets to snap, growl or exhibit other aggressive behavior, even toward their owners.
Copyright 2012 ABC News Radio