"A strain" poses a threat

Humans can become infected by the three types of influenza viruses (A, B, and C). Influenza B and C viruses are much less dangerous and have never caused pandemics. The A strain, however, poses a problem.

Avian influenza is an influenza A virus. Most strains of bird flu only exist in bird populations, but three of them have been transmitted to humans:

• H1N1
• H1N2
• H3N2

Understanding subtypes

Influenza A viruses are divided into several subtypes, based on the kinds of hemagglutinin (H) and neuramidiase (N) proteins on the virus’ surface. There are 16 possible hemagglutinin types and each can have up to nine neuramidiae subtypes. Doing the math, 144 possible major combinations are possible.

Listed below are all of the influenza A viruses known to infect humans:

H1N1 -- a variant of this virus caused a Spanish flu pandemic in 1918 and 1919. Now it is in common circulation among people, causing a standard mild flu.

H1N2 -- this virus caused so-called “Asian flu” back in the 1950s. It caused between one and four million deaths.

H3N2 -- this variant caused a Hong Kong flu that killed up to 750,000 people in 1969. Now it is a dominant subtype of influenza A virus in circulation.

H5N1 -- this highly pathogenic subtype of influenza virus circulates mostly among birds. It is responsible for most severe and fatal cases of bird flu.

H7N7 -- this is another bird flu virus, less common than H5N1 and much less dangerous. However, H7N7 is responsible for at least one case of fatal bird flu in 2003 in the Netherlands.

H9N2 -- a low pathogenic virus, it normally doesn’t infect people; however in 1999 and 2003, three children in Hong Kong and China contracted the virus. All of them recovered.

H7N2 -- a low pathogenic virus, there is only one known case of H7N2 infecting a human.

H7N3 -- this is a highly pathogenic virus, albeit less dangerous than H5N1. In 2004, it affected 18 poultry farms in British Columbia. Two known cases of H7N3 infection were hospitalized; both patients recovered.

H10N7 -- the first report of H10N7 attacking humans showed up in 2004, when two children of an Egyptian poultry merchant became infected with it. No known casualties.

How the flu virus changes

Viruses mutate in the following two ways:

Antigenic drift: A continuous shift caused by the point mutation in the two genes coding hemagglutinin and neuramidiase. The result leaves unpredictable small changes on the viruses’ surface, letting it fool our antibodies (at least for some time). Antigenic drift is the main reason why we can be infected by flu several times -- our antibodies are always prepared to fight against the older version of the virus, but can’t assimilate the new one.

Antigenic shift: A quick, major change that creates a new type of flu virus, capable of circulating among both animals and people. The shift may occur in two situations:

1. During the transition between poultry and humans
2. When the genes of two influenza A viruses mix their genes -- for example, when a person already infected by some “human” flu virus becomes infected with bird flu

One of the biggest concerns about H5N1 is that it seems to have the tendency to mutate, allowing it to infect different types of animals and humans. This ability to change means that it could transform into a strain that could be easily passed from person to person.

If that happens, a global outbreak could occur, infecting and killing millions of people. Because humans have not developed an immunity against this strain, we would have little natural defense against it.

The virus’ geographical spread

More than 100 million birds throughout the world have contracted avian influenza, with the heaviest concentrations of infection in Asia. H5N1, which is the most dangerous subtype of influenza A, has occurred in Asia, Africa, and Europe.

There have also been several cases of bird flu reported in North America. The subtypes H7N3 and H7N2 have infected people, while H5N2 infected poultry in Texas, fortunately not causing any infections in humans.

A closer look at bird flu's victims
Wall Street Journal (date June 21, 2006)
Researchers will never know precisely how all the 130 people who have died of H5N1 avian influenza acquired the disease. But the details of their investigations offer glimpses into the lives of the victims -- who have been largely young, in close contact with poultry, and mostly from Southeast Asia. While the fatality count has slowly ticked higher, the victims' stories -- or what little we know of them -- have sometimes been overshadowed by broader concerns about food safety, bird migratory patterns and feared mutations that could make the virus easily transmissible between people, potentially sparking a global pandemic. Visit the Wall Street Journal to take a look at the victims, based on updates on avian flu from the WHO.

What can be done?

Birds catch avian influenza when they come in contact with infected saliva, feces, or nasal secretions. The infection can be spread through bird-to-bird contact or from contact with contaminated items, such as cages, food, or soil.

To prevent further spread of the virus among birds, millions of infected or possibly infected birds have been destroyed. The World Health Organization (WHO) considers the destruction of sick or exposed birds the first line of defense against the spread of avian flu.

Read the article Cat bird flu risks 'overlooked' to learn more about the transmission of bird flu and the risks of our pet and other cats contracting bird flu.

See also

• Mutation and antigenic shift
• Bird flu and SARS
• Infection in humans
• Diagnosis and treatment