Zoo officials confirmed Wednesday that the California sea lion, named Catalina, died Saturday.
Officials say Valley fever is an uncommon cause of death for their animals. Because the illness is so rare, officials say they didn't consider Valley fever as a potential cause of death. It was initially thought that Catalina died of cancer.
Valley fever is a common fungal illness prevalent in parts of Arizona and the San Joaquin Valley. Officials say it isn't contagious and it is rare in marine mammals.
Catalina came to Fresno in January 2010 after she was rescued by the Fort MacArthur Marine Mammal Center in Los Angeles.