An animal rights advocate has filed a lawsuit against US cable television network HBO, alleging that horses used in the filming of its series \"Luck\" often were drugged, sick and underweight. Plaintiff Barbara Casey also sued her former employer, the American Humane Association (AHA), for allegedly covering up the animals\' mistreatment. \"AHA bowed to political and financial pressure and refused to report the production defendants\' conduct to the authorities,\" the suit filed earlier this week alleged, according to local media reports. \"AHA engaged in efforts to conceal and cover up the production defendants\' criminal activities.\" The suit alleged that horses used on the HBO series routinely were \"intentionally misidentified\" so that animal welfare authorities would not be able to monitor their medical histories. HBO in a statement strongly rejected the allegations in the suit. \"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,\" the company said. The television drama starring Hollywood actor Dustin Hoffmann premiered in January 2012, but was canceled after at least three horses died during production.