Several of the horses used on HBO's canceled series Luck were drugged, underweight and sick during production, an animal rights worker who oversaw conditions on the show alleges in a lawsuit.

Barbara Casey's suit filed Monday says she was wrongfully fired from her post at the American Humane Association after complaining about the conditions horses faced on the show, which was cancelled after a series of high-profile animal deaths.

The suit claims four horses died during the show's production, not three as previously reported. Horses were "often drugged to perform," and "underweight and sick horses unsuited for work were routinely used" by producers, her lawsuit alleges.

Casey is suing HBO, which has repeatedly denied abusing horses on the show, and the humane association. The association declined to comment, citing the pending lawsuit.

"We took every precaution to ensure that our horses were treated humanely and with the utmost care, exceeding every safeguard of all protocols and guidelines required of the production," HBO wrote in a statement.

Casey's suit states she urged the humane association to report HBO and producers to authorities for possible animal-cruelty criminal charges.

The association "bowed to political and financial pressure and refused to report the production defendants' conduct to the authorities," the lawsuit states.

Casey served as director of the association's Film and Television Unit, which oversees animal welfare and often allows a notice to be attached to the end of films and television shows that says no animals were harmed during production.

The nonprofit association's film- and TV-monitoring efforts are paid for through entertainment industry grants, according to the lawsuit.

HBO canceled Luck, starring Dustin Hoffman, in March 2012 after three horse deaths were reported during production. Casey claims a fourth horse, named Hometrader, was killed in summer 2011 but its demise wasn't documented because it occurred during a hiatus in filming.

The final horse that died during the series' production had been examined by a California Horse Racing Board veterinarian shortly before it suffered a head injury while being led by a groomer to a stable.

By that point, the humane association and the People for the Ethical Treatment of Animals had both been critical of the show's safety record and praised its cancellation.

Casey's lawsuit seeks unspecified damages.

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