Differences between avian influenza and pandemic human influenza

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Types of influenza viruses found in birds

All influenza viruses ever found in birdds have been influenza “Type A” viruses. Many different dub-types and strains occur, with variable ability to make birds ill. These sub-types are given names labeled with “H” and “N”, to reflect the genetic makeup of the virus and its ability to invade the birds’ cells. Some avian influenza viruses circulate naturally in wild birds, generally with little effect. When these get into poultry (eg chickens, geese, quail), some are highly pathogenic and infect and kill large numbers of birds.

Wild birds

Wild birds would not ordinarily present a threat to human health. The main threat to public health lies in a human “pandemic” of influenza that may have its origin in a bird. The virus will then be adapted to humans and unlikely to affect birds. Such an adaptation is highly unlikely to occur in Australia and would be brought here in humans rather than in birds.

Human and Avian influenza viruses

• In humans, influenza viruses can be of type “A” or “B”.
• It is thought that all human influenza “A” viruses have originated from birds.
• Seasonal influenza epidemics occur every year and result from minor changes to the make-up of existing viruses. The changes are minor, such that the virus can continue to spread from person to person, but the viruses are sufficiently different so that the human population is not immune. That is, past experience has been with influenza viruses that cause a different immune response. Vaccines are produced each year to protect people from seasonal influenza. The vaccine is produced with knowledge of what the seasonal influenza virus is like.
• Health authorities generally believe that another pandemic is inevitable. The widespread nature and behaviour of H5N1 has set it as a likely candidate virus to change sufficiently to become a pandemic human virus. A pandemic arising from another influenza virus is also possible.
• Purely on mathematical grounds, such a change from an avian to a human virus is likely to occur within those Asian countries with very large human populations and very large poultry populations, and where people have regular close contact with poultry. By comparison, Australia would be an extremely unlikely place to be the origin of a pandemic.
• The possibility of change occurring in an intermediate species (like a pig) exists, especially if the pig were to be simultaneously infected with two influenza viruses.
• This change of virus need only occur once. It will then be spread onwards by human to human transfer. It is likely that the virus will then no longer be capable of infecting birds.

Take home message is a human pandemic virus, if it were to arise, would come to Australia with a human and it would not involve poultry.

Original author: Ron Glanville, QDPI&F