Three rare civet cats in Vietnam have died of bird flu, the first time the species has contracted the disease.
Civets are eaten as a delicacy in China and Vietnam
The civets died in June at the Cuc Phuong National Park.
An advisor at the park said no other animals or people had become ill, and experts had yet to determine the cause of the infection.
According to the WHO, 57 people have now died - and millions of birds have been culled - in the latest outbreak, which began in Asia last year.
The virus has also spread to Kazakhstan and regions of Russia.
Some experts fear migratory birds could spread the virus to the European Union as they move to warmer areas for winter.
This is not the first time civets have been associated with an infectious disease. Health experts in China concluded that the weasel-like animals were a primary source of the flu-like virus Sars [Severe acute respiratory syndrome], which killed hundreds of people in 2003.
Civets are often captured in the wild and served as a delicacy in restaurants in Vietnam and China.
A mother civet at the park and two of her offspring died of the H5N1 virus, laboratory tests in Hong Kong have confirmed.
"How they were infected remains unknown, as they were raised together with 20 other civets, their cages close to each other, but the remaining civets are strong," Do Van Lap, a manager at the park, told Reuters news agency.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
"It's another good example of how dangerous this thing is," said Scott Roberton, a technical advisor for the civet conservation programme at the park.
"No animals are ill, no people are ill. We're still trying to figure out where the source was," he told the Associated Press.
Officials said the civets had not been fed with poultry, and that park staff would now be tested to see if any of them could have transferred the virus to the animals.
Cuc Phuong park also houses peacocks, pheasants, deer and turtles, but Mr Do said all the remaining animals remained uninfected.
"We reckon the three civets are isolated cases," he said.
Peter Horby, an epidemiologist for the World Health Organization based in Hanoi, said that the fact civets had been infected would not necessarily increase the chance that humans would contract the disease, as people have far less contact with civets than poultry.
"The interesting thing is that it's a new species," he told the Associated Press. "It continues to surprise."
In the past, bird flu has occasionally been found in other mammals, such as cats and tigers, but is far more common in ducks and birds.
The latest outbreak of the virus has so far killed 61 people in Asia, with the majority of deaths in Vietnam.
So far humans have only contracted bird flu after coming into contact with infected animals.
But the real fear is that the virus might develop into a form which can be transmitted from person to person, raising the possibility of a global pandemic.