Opponents of cloning say the announcement that Dolly the sheep has developed arthritis is just more evidence that scientists know more about how to make a clone than they do about the consequences of doing it.

Scientists at the Roslin Institute in Scotland, where Dolly was born in 1996, announced on Friday that the sheep has developed arthritis in the hip and knee of one leg.

Developing arthritis at five and half years of age isn't unheard of among sheep, but it is uncommon. The condition normally doesn't appear until a sheep is 10 or 11. Sheep normally live 15 to 17 years.

"What we need to go on studying is whether diseases like arthritis, which tend to be associated with old age, occur in a normal way or whether the incidence has changed," said Ian Wilmut, a scientist with the cloning team.

"The fact that Dolly has arthritis at a relatively early stage could suggest that there may be problems."

Dolly is the oldest cloned animal, but since her birth made worldwide headlines, hundreds more have been born.

Many cloned animals are living healthy lives. Dolly herself seems otherwise to be doing well, and is the mother of six lambs.

But many cloning attempts with animals have produced gruesome failures, as well. Many deformed fetuses have died in the womb, others have died within days of birth.

Wilmut says scientists will have to study Dolly and other cloned animals over their entire life spans.

Dolly's arthritis raises anew fear and doubt about the potential benefits and dangers of cloning animals for transplant organs and of human cloning.

Earlier this week, biotech companies in Scotland and the United States announced they had made progress in cloning pigs with genetic modifications that would make them better organ donors for humans.