World Health Organization director- general Margaret Chan Fung Fu-chun warns bird flu H7N9 is particularly worrying as it could be a flu pandemic strain.
This is because H7N9 is unique as it does not make chickens sick but is deadly in humans. Sick birds could usually provide early warning for imminent outbreaks, Chan told The Standard.
This comes as Macau reported its first human case of H7N9 yesterday.
"The biggest challenge for the world is the next influenza pandemic," Chan said.
The last flu pandemic was the human swine flu H1N1 which originated in Mexico in 2009, but was considered milder than most pandemics. Chan, who was the first to notify the WHO of a new bird flu H5N1 when she was Hong Kong's director of health in 1997, said that of the various bird flu viruses, she has paid attention the most to H7N9.
"H7N9 is unique as compared to H5N1. Why? Because when it affects the poultry population, it does not kill the bird. So we lost one early warning signal. We only see human morbidity and mortality," she said.
"In other words this is not causing disease in birds, it is only causing disease when it jumps to humans. So we need to watch it. We have been blindsided (by H7N9) as we don't have early warning signs in animals as we do in H5N1."
Chan said the world could not get complacent with flu as the viruses are "highly unpredictable and always full of surprises, starting from H5N1, H1N1 to H7N9, H5N6."