Jack Hanna's Family Announces He Has Alzheimer's Disease

Jack Hanna -- the famed and beloved animal expert -- is battling with Alzheimer's disease ... and his family says it's progressed "much faster in the last few months."

Jack's three daughters -- Kathaleen, Suzanne and Julie -- made the sad announcement Wednesday and said he's "no longer able to participate in the public life as he used to" ... largely in part to the fact the disease progressed faster than anyone could have anticipated.

The girls added, "Even though Dad is no longer able to travel and work in the same way, we know that this infectious enthusiasm has touched many hearts and will continue to be his legacy."

Jack, of course, dedicated his life to the Columbus Zoo and Aquarium since he first arrived in 1978 to serve as the director ... until retirement from that role in 1992. He'd go on to become a spokesperson for the zoo and even embarked on a tour -- from late-night shows and beyond -- with some of the most exotic animals.