LONDON (AP) _ Dolly, the world's first cloned sheep, has developed arthritis at a relatively early age, renewing debate about whether cloned animals are susceptible to premature aging and health problems.
Ian Wilmut of the Edinburgh-based Roslin Institute, one of Dolly's creators, said Friday that the 5 1/2-year-old sheep has arthritis in her left hind leg, hip and knee.
``There is no way of knowing if this is down to cloning or whether it is a coincidence,'' Wilmut told British Broadcasting Corp. radio.
Dolly was born in a Scottish research compound in 1996, the world's first mammal to be successfully cloned from an adult. Roslin scientists announced her birth on Feb. 23, 1997.
Wilmut said that apart from the arthritis, Dolly remained a healthy animal who has given birth to six lambs.
She also remains a rarity. Most attempts to clone animals have ended in failure. Deformed fetuses have died in the womb with oversized organs, while others were born dead. Still others died days after being born, some twice as large as they should have been.
In 1999, scientists noticed that the cells in Dolly's body _ cloned from a 6-year-old sheep _ had started to show signs of wear more typical of an older animal.
Some geneticists said the finding provided evidence that researchers could not manufacture copies of animals without the original genetic blueprint eventually wearing out.
Wilmut said he was ``very disappointed'' by Dolly's arthritis, and called for more research into the effects of cloning.
``We know already that there's an unusual incidence of death of cloned animals around the time of birth,'' he told the BBC.
``What we need to go on studying is whether diseases like arthritis, which tend to be associated with older age, occur in a normal way or whether the incidence is changed.
``The fact that Dolly has arthritis at this comparatively young age suggests that there may be problems. We do not know and it's very important that we look.''
Animal rights groups said the development showed that developments in cloning have outstripped scientific knowledge of its consequences.
Two research teams announced this week that they have cloned piglets that were genetically modified to help prevent their organs being rejected if they were transplanted into a human.
``You can't just interfere with one aspect of an animal's system and expect the rest of the system to continue to function perfectly,'' said Dan Lyons of the animal protection group CAGE.
Shares in PPL Therapeutics, the Scottish group that worked with the Roslin Institute to produce Dolly, fell in early trading on the London Stock Exchange.