Murphy, a 13-year-old Collie who lives with the Rev. Cathey and Dr. Neil Rennick in Eat Troy, Wis., had always been a happy-go-lucky dog. He loved to romp in the yard and play with the couple's three girls.

Two years ago, however, the Rennicks noticed Murphy was slowing down and in pain.

"He had trouble going up and down stairs," said Cathey Rennick. "He lost use of his hind legs and sometimes the use of all of his legs. He collapsed and fell over."

Murphy showed signs of arthritis, believed to affect 20 percent of dogs in the United States. With well cared-for dogs living longer, more cases of age-related degenerative joint disease - the most common form of arthritis - are being treated. Fortunately, Murphy's symptoms surfaced around the time treatment options began to expand. Two years ago veterinarians mostly treated pain with anti-inflammatory or analgesic drugs, or surgical removal of the ball part of the ball-and-socket hip joint or fusion of elbows or wrists to stop painful motion. Today new drugs and holistic supplements reduce inflammation and pain, and treat the underlying cause: the damaged cartilage.

Healthy cartilage gives the bones of a joint a smooth surface to glide across. But if the cartilage is deprived of blood because of joint stress or injury, it becomes inflamed damaged, eroded and roughened. This starts a cycle of damage to the cartilage and other joint tissues. Rough, abnormal bone builds up around the joint and adds to the pain and swelling.

Several factors increase a dog's risk. Injuries and genetic disorders such as hip dysplasia, elbow dysplasia and luxating patellas (loose dislocating knees) can predispose a joint to arthritis.

Other factors include wear and tear due to inappropriate or excessive exercise and joint instability from injuries to supporting connective tissues, such as anterior cruciate ligament tears - a common knee injury in dogs. Abnormal conformation, such as the straight lower back leg of breeds such as Chow Chows, can also put a dog at risk.

Weight is another factor. Overweight dogs stress their joints, increasing their risk of developing arthritis.

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