A possible fifth case of a rare and often fatal brain infection linked to Elan's multiple sclerosis drug Tysabri has been reported to federal regulators, a US Food and Drug Administration (FDA) spokeswoman said yesterday.

The case was submitted on May 16th through the FDA's adverse event reporting system, which collects reports of possible drug reactions from physicians and drugmakers.

Elan and US partner Biogen Idec have reported three confirmed cases of progressive multifocal leukoencephalopathy (PML) in patients taking Tysabri. Two of the patients died and the third is reported to have shown improvement.

A possible fourth case was reported to the FDA earlier this month but the firms said the PML diagnosis had not been confirmed.

FDA spokeswoman Lenore Gelb said the fifth report was not confirmed as PML and gave no other details about the case. Elan spokesman Brian McGlynn said the company had no comment.

Biogen spokeswoman Amy Brockelman said the company was reviewing the case. "We hope to have the findings by the end of the summer," she said.

The company would then share its safety reports with the FDA and decide its next step for the drug.

PML is a rare disease in which a virus attacks the protective sheath over cells in the central nervous system.

The companies halted sales of Tysabri, formerly known as Antegren, in February.

While Elan officials have said they are confident they can reintroduce the drug, industry analysts have said that if possible PML cases are confirmed they could hinder the drugmaker's ability to bring the product back on the market.

According to US media reports, the fifth possible case concerns a female patient, of an undisclosed age, who has been hospitalised. The FDA spokeswoman said she could not comment on the patient's condition.

Elan stock closed 3.6 per cent weaker in Dublin at €5.60. In New York, it closed 34 cents, or 4.8 per cent down, at $6.79.