The deadly H5N1 virus, bird flu, has resurfaced in poultry in Hong Kong for the first time in six years, reinforcing warnings that the threat of a human pandemic still exists.
During December, India, Bangladesh, Vietnam and mainland China also experienced new outbreaks. In that same period, four new human cases in Egypt, Cambodia and Indonesia were reported to the World Health Organization. A 16-year-old girl in Egypt and a 2-year-old girl in Indonesia have died.
The new cases come at a time when the number of confirmed human deaths from H5N1 bird flu have fallen for two years in a row and fewer countries are reporting outbreaks among poultry. A United Nations report in October credited improved surveillance and the rapid culling of potentially infected poultry for helping to contain and even prevent outbreaks in many countries.
Yet H5N1 has continued to "at the very least smolder, and many times flare up" since the outbreaks began in 2003, said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
H5N1 has already been a disaster for poultry farmers in Asia. Public health officials estimate that as many as a half-billion fowl have been killed by the virus or culled to contain its spread, causing economic strain and food shortages.
But the bigger fear has always been that H5N1 would give rise to a human pandemic.
In Hong Kong in 1997, the H5N1 virus was first observed to jump from chickens to humans, infecting 18 people and killing six. Hong Kong ordered its entire poultry population, estimated at 1.6 million birds, destroyed within three days.
A more recent chain of poultry outbreaks began in South Korea in 2003 and spread over the years to 61 countries in Asia, Africa and Europe.
To cause a pandemic, a virus must be able to both infect humans and spread readily from person to person. The currently circulating H5N1 strain does neither well.
The total number of verified human cases since the 2003 outbreak is 391, of whom 247 died. The Hong Kong poultry outbreak in December is significant; the government thought it had stamped out H5N1 in the territory after an outbreak in 2003. Since then, Hong Kong has vaccinated poultry against the virus and strictly regulated farm sanitation.
The government ordered the slaughter of 80,000 fowl at two large farms after the latest outbreak killed 60 chickens at one of the farms. Investigators are looking for the source of the infection and testing the effectiveness of the vaccine used since 2003 to inoculate chickens, geese and ducks.
Hong Kong's vaccine protects poultry against several flu subtypes. But some scientists believe that the H5N1 virus might have mutated to break through the vaccine. Flu viruses change constantly, which is why human vaccines for seasonal flu are modified every year, said Scott P. Layne, a professor of epidemiology and environmental health sciences at at the University of California, Los Angeles.