The big cat was rescued by police six years ago from a circus. He was already suffering osteomyelitis -- infection and inflammation of the bone -- to his front right leg through neglect.
The head of the zoological park in San Salvador, Virna Ortiz, said the operation to clean the leg's marrow, followed by chemotherapy, could only slow the infection but not cure the animal.
The procedure takes place every six weeks, and is complicated by the fact that the jaguar cannot be medicated constantly because its previous neglect in the circus left it with renal deficiency.
The park's biologist, Raul Miranda, said Greco and his female companion Flacucha were star attractions for visitors.
Their species is threatened with extinction across Latin America, and no others are believed to exist in El Salvador.