Bird to Human Flu

Flu viruses are normally highly species-specific, that is, they are confined to an individual species. Natural reservoirs for flu viruses are humans, certain species of birds, pigs, horses, and seals.

Since 1959, instances of human infection with bird flu virus have been documented on only 10 occasions. Of the hundreds of strains of bird flu viruses, only four are known to have caused human infections: H5N1, H7N3, H7N7, and H9N2. In general, human infection with these viruses has resulted in mild symptoms and little severe illness, with one notable exception – the highly pathogenic H5N1 virus.

The first human H5N1 infections were recorded in 1997 in Hong Kong – there were 18 cases, with six deaths. In 2003 more infections in people exposed to sick birds were identified. Since then there have been 270-odd confirmed cases world-wide, more than 160 of which were fatal. Up until January 2006 all recorded cases of H5N1 infection in humans were located in Southeast Asia; more recently confirmed cases have turned up in Turkey, Iraq, Egypt and Nigeria.

Fortunately, the virus doesn't easily jump from bird to human, nor does it spread rapidly and sustainably between humans. Those who have succumbed so far have mostly been healthy people who have been in close contact with dead or dying poultry.

Indeed, the transmission of flu viruses between birds and humans, sometimes involving pigs as a second vector, is an age-old process and thought to be the evolutionary source of all human flu strains.

But if H5N1 evolves to a form that is capable of fast human to human transmission, like seasonal flu, then a flu pandemic of frightening proportions could take hold.

Human flu pandemics have occurred in the past when a new subtype of bird flu arises that is able to infect and be transmitted by humans. Bird flu is not the same thing as pandemic flu, but it is usually a precursor.

Work on previous pandemic strains has shown that the 1918 Spanish flu was a bird virus that developed the ability to infect humans. Whereas the strains of the 1957 and 1958 pandemics arose when human and bird viruses infected the same person, which allowed the two strains to combine to form a novel one.

Flu pandemics have long been part of human history. Scientists are closer than ever to understanding how the flu virus works, but probably not close enough to prevent the next pandemic – whenever that may be.

Pandemic Flu »»

^ Top