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Nick Nolte appears in a scene from the HBO original series Luck. Picture: AP

An animal-rights advocate who oversaw working conditions on the cancelled HBO series Luck has sued the US network and the American Humane Association, claiming horses on the show were grossly mistreated.

Barbara Casey's lawsuit filed on Monday in Los Angeles claims underweight, sick and drugged horses were used to film the series focused on the horse racing industry.

The series starring Dustin Hoffman was cancelled in March after four horses died while in production.

Casey was working for the American Humane Association overseeing the well-being of the horses and says she was wrongfully fired after complaining about inhumane conditions on the show.

The association declined comment, citing the pending lawsuit.

HBO told The Hollywood Reporter that it took every precaution to ensure the horses's safety on the show.