Following the recent outbreak of the A(H7N9) strain of avian influenza in China that infected 132 people and killed 37, the world’s first-known case of human infection with H6N1 bird flu was discovered in Taiwan on May 7.
Avian influenza viruses may carry any of 17 kinds of hemagglutinin (H) antigens and nine kinds of neuraminidase (N) antigens, and their RNA consists of eight gene segments. Since time immemorial, they have infected and spread among a complex variety of hosts.
Influenza virus genes are forever reassorting and mutating, making it very easy for new strains to appear that may be highly pathogenic to humans.
Among all pathogens, avian flu could therefore pose the greatest threat to mankind.
In 1972, the H6N1 bird flu virus was isolated from sick ducks from Zhuwei (竹圍) in New Taipei City (新北市) near the estuary of the Tamsui River (淡水河), and the H1, H2, H4, H5, H8 and H12 types have also been found in birds.
Apart from some highly pathogenic varieties of H5N2, most of these strains have very low pathogenicity for poultry and are not infectious to humans.
However, could the same thing that happened in China with H7N9 happen in Taiwan now that someone has been infected with H6N1?
This question needs to be asked because the two have many features in common.
They are both of very low pathogenicity to chickens, and the genes of both these viruses evolve through reassortment in birds or other animal hosts in a given area over a certain time.
It can be inferred that before H7N9 became deadly to humans it may have been similar to H6N1 in Taiwan in that only a few people got infected with it and their very mild symptoms meant that the cases were overlooked by quarantine health workers.
Yet after a while the H7N9 virus evolved into a highly pathogenic form.
There are about 300 million domestic fowl and 7 million pigs in Taiwan. The wet and cool weather in autumn and winter make it easy for H6N1 to spread among poultry and cross over to pigs. The viral genes can be reassorted and mutate in the process.
The Center for Disease Control has thus far detected just one case of H6N1 infection in a human, and further observation will be required to tell whether this was just a one-off case or whether it means that H6N1 has become more pathogenic.
However, from an epidemiological point of view, the recent outbreak of H6N1 probably did not just infect one person.
After spreading to China’s Fujian Province on May 8 and the onset of summer’s higher temperatures, there have been no further cases of human-to-human H7N9 infection.
However, the virus is still among poultry, so there is concern that when winter arrives the disease could spread to Taiwan.
Still, H7N9 remains outside Taiwan, and if the authorities are vigilant and take the right kind of preventative measures, they may be able to keep it away from our shores.
H6N1, on the other hand, is already in Taiwan, and that may make it more serious and urgent.
Authorities should, therefore, immediately launch a deep-reaching epidemiological investigation and analysis.
This should include testing for the presence of H6N1 antibodies among people who are in contact with poultry and conducting a genetic sequence analysis of the H6N1 virus isolated from the recent case of the 20-year-old woman in comparison with H6N1 viruses isolated from chickens recently and 10 years ago.
The test results can then provide agricultural departments with a basis for handling H6N1 infections on chicken farms, and inform other countries about how to prevent the spread of low-pathogenicity H6N1.
H6N1 infection is widespread among domestic fowl in Taiwan, with a higher than 20 percent prevalence among poultry. The strain is also widespread among swine.
Test results from the year 2000 onward show that infection is present in about 10 percent of pig farms and that 1 percent of pigs have antibodies to the virus.
Six of the gene segments in the H5N2 bird flu virus that broke in 2011 originated from H6N1 through reassortment.
The spread and evolution of H6N1 should make health authorities pay careful attention to the question of whether the strain might develop into a new and virulent kind of bird flu virus, as happened with H7N9.
Lai Shiow-suey is an honorary professor in the Department of Veterinary Medicine at National Taiwan University.
Translated by Drew Cameron and Julian Clegg