Jay Fishman is stepping down as CEO of The Travelers Companies on Dec. 1, due to a neurodegenerative condition that appears to be a form of ALS.

Alan Schnitzer is set to replace him as chief executive officer. Currently, Schnitzer is CEO of Travelers’ Business and International Insurance segment – the largest at the company. He’s also in charge of Travelers’ Field Management, Corporate Communications and Public Policy divisions, and has been with Travelers since April 2007.

The property/casualty insurance giant said that Fishman will become Executive Chairman once the CEO transition takes place.

In January, Fishman, 62, disclosed that a “neuromuscular condition” first announced in November would force him to reduce any activities not directly connected to Travelers. In an Aug. 4 letter to employees released by the company, Fishman said he was leaving his CEO slot much sooner than he had expected, and also explained the nature of his illness.

“Given the progression of the symptoms I have been experiencing since informing all of you of my illness, it appears likely that I am dealing with a variant of Amyotrophic Lateral Sclerosis, or ALS,” Fishman wrote. “While this is a progressive disease, anyone who has seen me can tell you that I am more than able to play an active role in Travelers’ future.”

ALS, as defined by the AL Foundation, is a progressive neurodegenerative disease that affects nerve cells in the brain and spinal cord. As it advances, ALS can affect speech, the ability to eat, move and breathe.

Fishman wrote that “at every company there comes a time for new leadership. Because of the progression of my neuromuscular condition, this time has come a little earlier than I had hoped.”

He explained “given the strength of the company and with an exceptional leader in [Schnitzer] more than prepared to become our CEO, there is good reason to make a change now.”

The change is a substantial one for Travelers. Fishman has been CEO since the April 1, 2004 merger of The St. Paul Companies, Inc. with Travelers Property Casualty Corp. Fishman took on the added role of chairman in Sept 2005. Additionally, from October 2001 until April 2004, Fishman was chairman, CEO and president of The St. Paul Companies. He also helped run Travelers’ insurance business when it was a part of Citigroup.

Schnitzer, 49, said in prepared remarks that he’d continue to work closely with Fishman, “who is an invaluable resource and colleague.”

At the same time, Lead Director John Dasburg said that Schnitzer is “a proven leader, with demonstrated readiness to lead the company.”

“He has made exceptional contributions to Travelers’ success, and the Board has complete confidence in his ability to lead the company forward,” Dasburg said in prepared remarks.”

Dasburg added that he and the board are pleased that Fishman will remain as executive chairman.

“It is a pleasure and privilege to continue to work with him as a member of management and our board,” Dasburg said.

Fishman, in his letter to employees, said he and his family “could not be more grateful for the outpouring of support and good wishes we’ve received from people both inside and outside the company over the past several months. This means more than you could ever imagine.”