The 6-month old cub, Kamau, and his brother Winspear became sick a week ago and had been treated by the zoo's veterinary staff.
While Winspear started to recover, Kamau became sicker and died Tuesday afternoon.
The two cubs were born on July 8 at the Smithsonian Conservation Biology Institute in Front Royal, Va., and had become favorites of people who visited the zoo.
Both cubs had been vaccinated against both canine and feline distemper and other respiratory illnesses. Officials say other cats at the zoo are healthy.
Zoo officials say the cubs' puppy companion, a black Labrador retriever named Amani, shows no sign of illness.
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