Sean Healey, chief executive of the $831bn Affiliated Managers Group, has been diagonised with motor neurone disease and will be succeeded by the firm's chief operating officer as he stands down to pursue treatment.

Nathaniel Dalton, one of the company's founders, will take over as CEO with immediate effect, the company said in a market statement on Tuesday. Dalton is AMG's operations chief, but has held a variety of senior roles since joining in 1996 as general counsel, a year after Healey.