By Kate Kelland
LONDON, Feb 5 (Reuters) - The death of a woman in China from a strain of bird flu previously unknown in humans is a reminder of the ever-present potential pandemic threat from mutating animal viruses, scientists said on Wednesday.
The new strain, called H10N8, has so far infected only two people - a fatal case in a 73-year-old and another in a woman who is critically ill in hospital. But the fact it has jumped from birds to humans is an important warning, they said.
"We should always be worried when viruses cross the species barrier from birds or animals to humans, as it is very unlikely that we will have prior immunity to protect us," said Jeremy Farrar, director of Wellcome Trust and an expert on flu.
"We should be especially worried when those viruses show characteristics that suggest they have the capacity to replicate easily or to be virulent or resistant to drugs. This virus ticks several of these boxes and therefore is a cause for concern."
Chinese authorities last week confirmed a second human case of H10N8 which was reported for the first known time in humans in December 2013.
It has emerged as another new and often fatal strain of bird flu, called H7N9, has infected at least 286 people in China, Taiwan and Hong Kong, killing around 60 of them.
Chinese scientists writing in The Lancet medical journal who conducted a genetic analysis on samples of the H10N8 virus from the woman who died said it was a new genetic reassortment of other strains of bird flu viruses, including one called H9N2 that is relatively well known in poultry in China.
Somewhat worryingly, the virus - like H7N9 - has also evolved "some genetic characteristics that may allow it to replicate efficiently in humans", said Yuelong Shu of the Chinese Center for Disease Control and Prevention in Beijing.
According to the scientists' study of her case, the 73-year-old victim, from Nanchang City in eastern China, was admitted to hospital with fever and severe pneumonia on November 30, 2013. Despite being treated with antibiotic and antivirals, she deteriorated rapidly, developed multiple organ failure and died nine days after her symptoms first started.
Investigations found the woman had been at a live poultry market a few days before becoming infected. But no H10N8 virus was found in samples collected from the market, the scientists said, so the source of the infection remains unknown.
Mingbin Liu from Nanchang City Center for Disease Control and Prevention added that the emergence of a second human case of H10N8 in a 55-year-old woman "is of great concern because it reveals that the H10N8 virus has continued to circulate and may cause more human infections in future".
John McCauley, head of the WHO Collaborating Centre for Influenza at Britain's National Institute for Medical Research said the emergence of H10N8, and of H7N9 "reminds us to be aware of human infections from animal influenza viruses.
"Previously we did not think that H7N9 infections might be so lethal. Now we also must consider H10N8 infections as well," he said in an emailed comment.
He added, however, that the risk of this virus spreading from person to person "seems low since the H10N8 virus is not expected to be transmitted well between humans".
Other flu experts not directly involved in the study on H10N8 agreed it was an important reminder of the potential threat from circulating and mutating flu viruses, but said it did not appear to be a particular concern for the moment.
Ben Neuman, a virologist at Britain's University of Reading said while the fatal H10N8 case was a "personal tragedy for the family and friends of the victim" and needed to be watched closely, "there is no cause for alarm at this time". (Reporting by Kate Kelland, editing by Clive McKeef)