UPDATE 1-Antiviral prescriptions spike after flu scare-data

* Prescriptions up ninefold

* Bigger spikes in Los Angeles, New York (Recasts; adds information on flu season)

NEW YORK, May 1 - Americans rushed to fill prescriptions for antiviral medicines after news spread last week about a possible flu pandemic, new data shows.

U.S. antiviral prescription numbers rose ninefold on Monday compared to average daily volume in the rest of April, according to pharmaceutical market research firm SDI, reaching the highest point so far this year for such drugs.

Prescriptions edged down on Tuesday from Monday, but were still nearly nine times daily volume for the month, SDI said.

The upward trend started on Friday, April 24, a day after the U.S. government's first public announcement of cases of a new strain of swine flu in California and Texas, SDI said. Prescription sales rose far higher in major metropolitan areas.

In Los Angeles, daily prescription volume jumped 16-fold on Monday to the highest level it has reached on any single day in the past two flu seasons.

In New York, volume climbed more than 14 times the daily April average on Monday, and rose to 15 times on Tuesday, according to SDI, which did not release data for later in the week.

"Although it is not uncommon for the number of antiviral prescriptions to increase at different times throughout the season, we don't expect to see such a dramatic increase in one day at the end of the season," said Laurel Edelman, SDI's vice president of clinical accounts.

Antiviral prescriptions for October through April 24 had been lower than the same period in the previous season, reflecting a milder flu season.

The antivirals in the data include Roche Holding AG's Tamiflu and GlaxoSmithKline PLC's Relenza as well as several older drugs.

Shares of Roche and Glaxo jumped earlier this week as investors bet the threat of a flu pandemic would provide a windfall for the drugmakers.