DOGS FED on a shellfish diet have fewer problems with arthritis, according to a study showing that the New Zealand green-lipped mussel can alleviate canine aches and pains.

Scientists have found that powdered extracts of the mussels, when added to a dog's normal food, allow those suffering from osteoarthritis to live more normal lives after six weeks of treatment.

The results, to be presented at the festival this week, show that painful swelling and inflammation improved in four out of five dogs fed on the diet.

The green-lipped mussel was long thought to contain substances that can relieve arthritic symptoms. Maoris who eat them regularly have a low incidence of the disease, but this is the first time the effect has been shown on dogs.

Arthritis affects most dogs over five years of age and can severely hinder their movement. Karyl Hurley, a specialist in animal medicine at the pet food company Waltham, said conventional treatments with anti-inflammatory drugs could cause side-effects such as stomach irritation, anorexia, vomiting and ulcers. "Managing this condition through diet is relatively new and much more holistic than conventional medicines," she said. "Vets and dog owners have embraced this alternative and have been thrilled with the response."

Nobody has identified the active ingredient in New Zealand mussels but scientists suspect it is a fatty acid that counteracts the degradation of cartilage and bone in arthritic joints.

In a study in the United States involving 30 dogs with arthritis, scientists fed half of them with the powdered extract and the other half with a placebo. After six weeks, independent vets scored the relatively ability of the dogs to perform tasks, without knowing which had been fed on the mussel extract.

Dr Hurley said that over half of those taking the treatment showed improvement on a "score" of arthritic symptoms. Nearly 70 per cent showed improvement in a score of joint pain and nearly a quarter were better in terms of swellings.