(Reuters) -EU antitrust regulators have started an investigation into whether Zoetis blocked the market launch of a rival pain medicine for dogs, the European Commission said on Tuesday, a move that could result in a hefty fine for the U.S. pet products maker. Antitrust regulators on both sides of the Atlantic have recently cracked down on Big Tech and Big Pharma acquiring start-ups or small rivals in order to shut them down in what is known as killer acquisitions. Zoetis' Librela is currently the first and only monoclonal antibody medicine approved in Europe to treat pain associated with osteoarthritis in dogs.