Staff at the Arignar Anna Zoological Park near Chennai noticed signs of loss of appetite, nasal discharge and occasional coughing among a pride of lions last week.

The zoo sent samples of the animals for testing. Nine of the lions tested positive for coronavirus and one of them, nine-year-old Neela, died on Thursday, a zoo official said on Saturday.

The lioness appeared asymptomatic and had showed some nasal discharge the day before she died. Samples have been sent for further testing at two research institutes.

All the lions that tested positive were being kept under close observation and undergoing treatment supervised by an expert team from the Tamil Nadu Veterinary and Animal Sciences University.

They included two elderly animals - one 23 and the other 19 - who were being monitored carefully.

The zoo had been meticulously following all COVID-19 guidelines and it was not clear how the animals were infected, the zoo official said.

All animal keepers and helpers are vaccinated against COVID-19. Staff distributing the animals' feed are regularly tested and the feed is treated in an ultraviolet room.

Animal keepers, veterinary doctors and field staff always wore personal protective equipment when visiting the enclosures and safari areas.

The zoo has been closed to visitors since April 20 as the number of COVID-19 infections increased in Tamil Nadu amid the second wave of the pandemic.

Australian Associated Press