A lawsuit that charges Zenith National Insurance Corp. with improperly discounting medical bills that were submitted for payment has been filed in federal court in Los Angeles by an Illinois chiropractor. Woodland Hills, Calif.-based Zenith said Friday that the complaint, which seeks class action status, was filed by Kathleen Roche, a Belleville, Ill.-based chiropractor who conducts business as Back Doctors Chiropractic. The insurer said it has not yet been served with a copy of the complaint. "The company believes that the complaint is without merit and intends to defend the litigation vigorously," Zenith said in a statement.