Transmission of Avian Influenza A Viruses Between Animals and People

Influenza A viruses of various subtypes have a large reservoir in wild waterfowl. They are also known to infect and transmit among humans and in some other species or groups (domestic poultry, swine, horses, dogs, and bats). Influenza A virus subtypes currently circulating among people are A(H3N2) and A(H1N1) viruses. Examples of different influenza A virus subtypes currently circulating in animals include A(H1N1), A(H1N2), and A(H3N2) in pigs (different virus strains than those subtypes found in humans), A(H3N8) in horses, A(H3N2) in dogs, and A(H5N1) in wild water birds, domestic poultry, and dairy cattle. Influenza A viruses that can infect and regularly transmit among wild water birds and domestic poultry are called avian influenza viruses.

Influenza A viruses that typically circulate in wild aquatic birds can sometimes cause illness in another species (for example: pigs). Additionally, influenza A viruses from humans may be transmitted to pigs. For example, before 1998, only A(H1N1) viruses circulated widely in the U.S. pig population. However, in 1998, A(H3N2) viruses from humans were introduced into the pig population and caused widespread disease among pigs. More recently, avian influenza A viruses from birds have caused sporadic infections in mammals in the United States and in other countries and in United States dairy cattle.

Avian influenza A (bird flu) viruses may be transmitted from infected birds to other animals, and potentially to humans, in two main ways:

• Directly from infected birds or from avian influenza A virus-contaminated environments.
• Through an intermediate host, such as another animal.

In birds, avian influenza A viruses infect the respiratory and gastrointestinal tract and are shed in feces. Direct infection of wild birds, poultry, and mammals can occur from exposure to saliva, mucous, or feces from infected birds. Avian influenza A virus infections among people are rare because currently circulating avian influenza A viruses do not have the ability to easily bind to receptors in the human upper respiratory tract.

However, human infections with avian influenza A viruses can happen when enough virus gets into a person’s eyes, nose or mouth, or is inhaled. This can happen when virus is in the air (in droplets or possibly dust) and a person breathes it in, or when a person touches something that has virus on it and then touches their mouth, eyes or nose. Avian influenza A virus infections in people happen most often after close, prolonged and unprotected (no gloves or other protective wear) contact with infected birds or other animals. People with close or prolonged contact with infected birds or animals or their contaminated environments are at greater risk of infection.

Influenza A viruses have eight separate gene segments. The segmented genome allows influenza A viruses from different species to mix genes (genetic reassortment) and create a new virus if influenza A viruses from two different species infect the same person or animal at the same time.

One possible way that virus reassortment could occur is if an animal or human were infected with a human influenza A virus and an avian influenza A virus at the same time. The new replicating viruses could reassort and produce a new influenza A virus that had some genes from the human virus and some genes from the avian virus. The resulting new virus might then be able to infect humans and spread easily from person to person, but it could have surface proteins (hemagglutinin and/or neuraminidase) different than those currently found in influenza viruses that routinely circulate in humans – this could make it seem like a “new” influenza virus to people, one that had not been encountered before. Specifically, the term “antigenic shift” is applied to a new influenza A virus for which most people have little or no immune protection. If this new influenza A virus causes illness in people, is transmitted easily from person to person in a sustained manner, and if most people lack immunity to the new virus, an influenza pandemic can occur.

In the spring of 2009, a novel A(H1N1) virus emerged to infect people and quickly spread, causing an influenza pandemic. This virus had genes that were descended from viruses circulating in swine which were the result of reassortment between viruses from North American and Eurasian swine. It is also theoretically possible that the process of genetic reassortment could occur in an animal or person who is co-infected with an avian influenza A virus, or a swine influenza A virus, and a seasonal (human) influenza A virus. The genes of these viruses could reassort to create a new influenza A virus with a hemagglutinin and/or neuraminidase gene from the avian or swine influenza A virus and other genes from the seasonal influenza A virus. Viruses with such hemagglutinin and/or neuraminidase genes could potentially cause a pandemic if people lacked immunity against them, and the virus was able to spread easily from person to person in a sustained manner. Therefore, careful evaluation of influenza A viruses recovered from humans and animals who are not usually infected with avian influenza A viruses is important to identify genetic reassortment if it occurs.

Although it is unusual for people to get influenza A virus infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses and swine influenza A viruses have occurred.