LOS ANGELES, Feb 18 (Reuters) - Media mogul Sumner Redstone was under "undue influence" from people around him when he replaced his designated healthcare agent in October 2015, his ex-girlfriend argued in a court filing on Thursday.

The ex-girlfriend, Manuela Herzer, has sued over her removal as Redstone's healthcare agent in favor of Viacom Chief Executive Officer Philippe Dauman.

"Mr. Redstone is vulnerable to, and has become the victim of, undue influence, fraud, manipulation and chicanery," Herzer's attorneys wrote.

(Reporting by Lisa Richwine; Editing by Sandra Maler)