UPDATE 1-Roche wins EU backing for wider use of arthritis drug

(Adds details, stock price)

* Won EU approval to treat active Systemic Juvenile Idiopathic Arthritis

* RoActemra already used in adult rheumatoid arthritis

* RoActemra knows as Actemra outside of Europe

* Roche stock up 0.8 percent, outperforms sector index

ZURICH, Aug 3 (Reuters) - Roche has won European backing for broader use of its arthritis drug RoActemra to treat children with a rare and severe form of the disease, the Swiss drugmaker said on Wednesday.

The medicine, known as Actemra outside of Europe, is already used to treat adult rheumatoid arthritis in patients who were either intolerant to or failed to respond to other medicines to treat inflammation.

The drug can now also be used in children two years of age and older suffering from active Systemic Juvenile Idiopathic Arthritis (sJIA) who have inadequately responded to previous therapy.

It is already approved in the United States, Mexico, India and Switzerland in this setting, and Vontobel analyst Andrew Weiss said he is carrying a peak sales estimate for the drug of 2 billion Swiss francs ($2.6 billion).

At 0820 GMT, Roche stock was trading 0.8 percent firmer, outperforming a 1.1 percent drop in the European healthcare index .

The ailment, which can cause fever, rash and sore joints in children, can also cause enlargement of the liver or spleen and inflammation of the lining of the heart and/or lungs, and has a 2 to 4 percent overall estimated mortality rate.

RoActemra is the first and only biological treatment to demonstrate significant efficacy in this patient population, Roche said in a statement.

Roche is the world's largest maker of cancer drugs and is seeking to bolster its position beyond oncology. ($1 = 0.771 Swiss Francs) (Reporting by Katie Reid, Editing by Mark Potter)