* Patient took Novartis' Gilenya MS pill
* Developed progressive viral disease
* First incidence in 71,000 patients
* Gilenya facing competition from Biogen's Tecfidera
ZURICH, July 30 (Reuters) - A patient taking Novartis' multiple sclerosis pill Gilenya developed a rare and potentially fatal viral disease, the Swiss drugmaker said on Tuesday, an unexpected setback as it faces growing competition from new oral treatments.
Gilenya is one of Novartis' big new drug hopes, growing 66 percent in the second quarter to $468 million. But the drug faces competition from new medicines such as Biogen Idec's Tecfidera.
Novartis said it had been informed of a case of progressive multifocal leukoencephalopathy (PML) in a patient who had been taking Gilenya for MS for seven months.
It said it was working with the reporting physician to understand all possible contributing factors, including those beyond treatment, given several atypical features of the case.
"The course of the underlying neurological disease was rapid with some atypical findings for MS on the MRI scans of the brain and spinal cord, as well as some unusual clinical features," Novartis said in a statement.
Novartis said all previously reported cases of PML among the approximately 71,000 patients treated with Gilenya thus far had been attributed to prior treatment with Biogen Idec's Tysabri, which bears a known risk of PML.
Deutsche bank analyst Tim Race said the case may provoke some concerns about Gilenya's future growth potential. But he noted the incidence of reported PML cases for Gilenya has so far been extremely low.
"By the time there was a similar level of patient experience with Tysabri there had been 298 cases reported. Thus, even if the risk proves to be real it is likely to be of a very different order of magnitude," Race said in a note.
Shares in Novartis were trading down 0.8 percent at 66.10 Swiss francs by 0904 GMT, compared to a 0.4 percent weaker European healthcare sector.