The female lion was being treated for a gastrointestinal disorder, but her condition worsened. She died July 19.
"The lion was under veterinary treatment and received full medical care," Mike Kerr , the zoo's director, told Patch.
"The lion underwent a physical exam, blood work, ultrasound, radiographs and supportive care."
The other lions are in good condition, Kerr also said.
The lions at the zoo are housed at a $3.6 million habitat opened in June 2017. The zoo acquired the cats as part of a species survival program.
Click here to follow Daily Voice Essex and receive free news updates.