Merck of the US is close to restructuring its lucrative animal medicines assets in a wide-ranging deal likely to be finalised in the next few weeks with its partner Sanofi-Aventis that could earn it more than $4bn.

The two pharmaceutical companies have appointed investment banking advisers and are in advanced talks over the future of their joint venture Merial, sparked by Merck's planned takeover of Schering-Plough unveiled earlier this year.