Merck (NYSE:MRK) and Sanofi-Aventis (NYSE:SNY) announced this morning the mutual termination of their agreement to form an animal health joint venture, which the two companies had been working on for over a year.The companies had planned to combine Merial, Sanofi-Aventis' animal health business, with IntervetSchering-Plough, Merck’s animal health unit, into a single entity.The companies cited the increasing complexity of implementing the proposed transaction, both in terms of anticipated divestitures and the long length of time necessary for worldwide regulatory approval.The termination is without penalty to either party, each of which is responsible for their own expenses. Both Merial and IntervetSchering-Plough will continue to operate independently.Sanofi-Aventis is trading down 0.44% to $33.85 per share, while Merck & Co is trading up 0.56% to $32.57 per share.