Exclusive: FDA says working with Merck, USDA on cattle drug Zilmax WASHINGTON (Reuters) - The U.S. Food and Drug Administration said on Friday it was working with drugmaker Merck & Co and the U.S. Department of Agriculture to gather information on the cattle feed additive Zilmax and determine if it is unsafe. The FDA, in an email response to a Reuters query, said it had received "a very small number of reports of lameness or lying down" in cattle whose feed contained zilpaterol, also known as Zilmax. The agency said it would review any new information about the additive and would notify Merck and the public if it determined the product posed safety concerns. The USDA had no comment and referred questions to the FDA, which does not typically reveal its investigations. Merck said on Friday it was temporarily suspending sales of Zilmax in the United States and Canada, following concerns about the drug, which is given to cattle to increase their weight before slaughter. Last week, Tyson Foods Inc said it would stop accepting beef from Zilmax-fed cattle after it observed animals arriving at its slaughter facilities with signs that they had difficulty walking or moving. Merck on Tuesday revealed a new program to retrain and certify beef producers in administering Zilmax, which had sales of $159 million last year. On Friday, K.J. Varma, the head of research at Merck's animal health unit, told Reuters the company had not seen any safety or effectiveness issues with Zilmax beyond those observed in clinical trials before it was introduced in 2007. The FDA, which is responsible for ensuring that drugs on the market are safe and effective, requires drugmakers to report any adverse events. "We are always interested in new information about the safety and effectiveness of approved animal drugs," the regulator said on Friday. Tyson Foods, the nation's largest meat processor, said its rejection of Zilmax-fed cattle was based on animal welfare, not food safety. It said it did not know what was causing the animals' behavior, but animal health experts have suggested the use of the weight-adding drug may be a possible cause. Zilmax, approved by the FDA in 2006, is one drug in the class of beta-agonists, which is approved and deemed safe by the FDA and long used by the livestock industry to add muscle weight to cattle, pigs and turkey in the weeks before slaughter. Beta-agonists have come under scrutiny in recent months over industry concerns that animals showed signs of distress and had difficulty walking after being fed the additives. Additional reporting by Charles Abbott in Washington and Ransdell Pierson in New York; editing by Karey Van Hall, John Wallace and Matthew Lewis