An Australian outback zoo was Wednesday investigating the sudden and mystifying deaths of four white rhinos who showed "neurologic abnormalities" like stumbling.
The Taronga Western Plains Zoo, a safari-style animal park about 400 kilometres (248 miles) west of Sydney, said toxins, bacterial infection, snake venom and organ failure had so far been ruled out in the deaths.
"Currently tests are underway to investigate possible viral causes, although several types including Hendra virus and West Nile virus have also been ruled out," the zoo said in a statement.
As a precaution, it said its remaining three white rhinos "have been removed from display and placed in a quarantine area at the zoo. They are being closely monitored by keeping staff and the veterinary team".
The animals all died "over the past couple of weeks".
The zoo said a team of vets was working "around the clock" with specialists from Africa and North America, virologists and pathologists to try to determine what had killed the rare African creatures.
It said that no other species at the zoo had been affected and the rest of its animals were in good health.
"Obviously the rhino keepers and veterinary staff know and care for every individual in the herd, so this has been a huge shock and we're all very sad," said Matt Fuller, general manager of the zoo.
"Our focus is on continuing this investigation to pinpoint the cause of the sickness and to care for the remaining animals in the herd."
The zoo's rhinos are southern whites, the less endangered of the two white rhinoceros species.
There are estimated to be some 20,000 southern whites surviving in the wild according to environmental group WWF, which says that northern white rhinos are virtually extinct and can now only be found in captivity.
Explore further: Rare new species of plant: Stachys caroliniana