Sanofi-Aventis, the French pharmaceutical group, is to buy out joint venture partner Merck’s stake in Merial, the animal health business, for $4bn to take full control of the company.

It has also acquired a call option to pay at least $9.25bn to combine Merck’s new animal health businesses, which it is poised to acquire with the purchase of Schering Plough, with Merial.