The current outbreak of highly pathogenic avian influenza in the U.S. has claimed nearly 50 million head of poultry so far. The virus spread rapidly from the West Coast to the Midwest in both domesticated poultry and wild birds, and this has led to a lot of speculation as to how the virus has infected so many farms. 

Dr. David E. Swayne, director, U.S. National Poultry Health Research Center, said that viral isolates from all of the North American avian influenza findings from the current outbreak have been analyzed to see how related they are to one another. He told the audience at the International Conference on Avian Influenza and Poultry Trade that this analysis can lead to estimates of whether or not the virus came directly from wild birds or from farm-to-farm transmission. He said that clusters of farms with identical avian influenza isolates are a likely indication of farm-to-farm virus transmission, and that this information can be used to help guide the epidemiological investigations on the farm level.

Origin of the outbreak

The current worldwide outbreak of avian influenza is the largest in 50 years. Since 2012, 41 countries have reported outbreaks, with 38 of these tied to a virus that originated in a goose from the Guangdong province of China. Six primary clades of avian influenza virus have now evolved from the Guangdong goose virus.

Influenza viruses, including avian influenza, evolve and change relatively easily. The H5N8 highly pathogenic virus that came to North America from Asia late in 2014 has reassorted into H5N1 and H5N2 viruses. These viruses have a combination of genetic material from the highly pathogenic Asian virus and low-pathogenic North American viruses. The reassortment that has occurred in North America gave us viruses that are highly pathogenic for poultry but are not pathogenic for waterfowl. So, infected waterfowl shed lots of virus, but don’t die.


Swayne said that usually when a low-pathogenic strain of avian influenza is adapted from wild birds to domestic poultry, it doesn’t go back into wild waterfowl. In the current outbreak of avian influenza in Korea, however, he said the influenza virus went from waterfowl to domestic poultry and back to waterfowl. Then when the migratory waterfowl came back the next season, they brought three new influenza viruses with them. It is possible that this type of scenario could repeat in North America when migratory birds return to their winter homes in the fall of 2015.

Farm-to-farm transmission of avian flu

Swayne said that the easiest way to transmit the avian influenza virus is by bringing infected birds into the same environment as uninfected birds. He said that flies and rodents are also potential vectors for the virus, as are water, air and feed. But, he said that the number one means of spreading avian influenza is movement of people and equipment from farm to farm.

Swayne said that introduction of the avian influenza virus from wild birds to domestic poultry is usually only how an outbreak gets started; he stressed that the primary means by which the avian influenza virus is spread is by human and equipment movement from farm to farm, not from other introductions from wild birds.

The International Conference on Avian Influenza and Poultry Trade, held June 22-24 in Baltimore, was sponsored by the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS).