Drugmaker Elan and its multiple sclerosis research partner Biogen Idec have confirmed another case of the potentially deadly brain disease known as progressive multifocal leukoencephalopathy (PML).

The companies have notified regulators of the find -- the fourth such case reported globally this year.

The latest case was found in a European patient who had been taking Tysabri as a stand-alone treatment for 26 months, Biogen said in a regulatory filing.

"The patient is under physician's care," company spokeswoman Shannon Altimari said, noting it was too soon to speculate on the prognosis for the patient.

Biogen shares were down 1.2pc in morning trading, while the stock of its marketing partner, Elan Corp of Ireland, fell 3.7pc on the New York Stock Exchange.

Shares of Biogen and Elan fell sharply after the drugmakers disclosed on October 29 that a case of PML had been seen in a US patient.

In July, the companies reported two cases had emerged in Europe.

This year's four cases are the first to be announced since Tysabri was withdrawn from the market in 2005 after three earlier patients developed the brain infection.

Asked why no cases were seen in 2006 or 2007, Biogen spokeswoman Naomi Aoki said: "The drug had been taken off the market for a little over a year, and returned in July 2006."

She said Biogen and Elan believe the superior effectiveness of Tysabri in preventing relapses of multiple sclerosis symptoms justifies its use, despite the relatively low risk of PML.

Aoki said about 35,000 patients worldwide are taking Tysabri, and that the drug in clinical trials was able to produce a 68pc reduction in relapses.

Rival drugs, by contrast, are able to reduce relapses by only about a third, she said.