Avian influenza (AI) has appeared periodically in regions all over the world, including the United States. The virus spreads easily among wild birds, which are the natural hosts of AI viruses, but certain strains can also infect domesticated birds (including chickens, turkeys, ducks and, rarely, pet birds), humans (rarely); and a variety of other mammals. Most outbreaks of AI in the United States have been associated with milder strains of the virus (those having low pathogenicity). These infections do not normally cause clinical signs of disease in waterfowl, but may cause mild disease in poultry. Outbreaks have been resolved through the combined efforts of veterinarians, the poultry industry, and local, state, and federal governments. More serious (highly pathogenic) strains of AI virus can devastate entire flocks of poultry and result in major economic losses. The AI virus that has affected poultry flocks and other birds in Asia, Europe, and Africa since the end of 2003 has resulted in loss of human and animal lives.
Q: What is avian influenza?
A: Influenzas are broadly divided into three types: A, B, and C. Type A influenza includes most human and all avian influenza (AI) viruses. AI is a viral disease that naturally infects waterfowl and some species of shorebirds. Influenza viruses are divided into subtypes based on the two proteins, hemaglutinin (H) and neuraminidase (N), that they have on their surfaces. There are 16 recognized H types, and 9 N types, and these are known to occur in a number of different combinations.
Avian influenza is broadly divided into highly pathogenic (HPAI) and low pathogenic (LPAI) strains based on its ability to cause disease in poultry. Low pathogenic AI is a natural infection of waterfowl that may cause minimal to no signs of disease in domestic poultry and wild birds. Highly pathogenic AI is rarely found in waterfowl, and causes severe disease in domestic poultry with a high case fatality rate (death rate). Only two types of AI viruses, H5 and H7, are known to include highly pathogenic viruses. It is important to understand, however, that not all H5 and H7 influenza viruses are highly pathogenic. Due to stringent biosecurity practices, AI is uncommon in most commercial poultry flocks in the United States; it is most often identified in poultry raised outdoors or those that intermingle with or are exposed to wild birds.
The AI virus that has infected birds in Asia, Europe, and Africa since the end of 2003 is highly pathogenic H5N1 avian influenza (H5N1 HPAI). This particular form of AI is deadly to most domestic poultry and some wild birds, and can spread rapidly among an entire flock. To date, H5N1 HPAI has not been found in birds in North America, including the United States, or in South America, including the Carribean. H5N1 LPAI has been detected in waterfowl and is not a serious threat.
Q: How is avian influenza transmitted?
A: Avian influenza (AI) is most often spread by contact between infected birds and healthy birds. It may also be spread indirectly through contact with contaminated equipment and materials. The AI virus is found in secretions from the nares (nostrils), mouth, and eyes of infected birds and is also excreted in their droppings (feces). Contact with contaminated droppings is the most common means of bird-to-bird transmission, although airborne secretions are another important means of transmission, especially within poultry houses. Feces from wild ducks can introduce low pathogenic AI (LPAI) into domestic flocks raised on range or in open flight pens.
How highly pathogenic AI (HPAI) is initially introduced into poultry flocks remains unclear. However, the spread of AI between poultry facilities almost always results from the movement of infected birds or contaminated people and equipment (including clothing, boots, and vehicles). Avian influenza virus can also be found on the inner and outer surfaces of egg shells; therefore, egg transfer is a potential means of AI transmission. Airborne transmission of AI virus from farm to farm is highly unlikely.
H5N1 HPAI can be spread from birds to people as a result of extensive direct contact with infected birds, such as during home slaughter and defeathering of infected poultry. Public health concerns center around the potential for the virus to mutate or combine with other influenza viruses to a form that could easily spread from person to person. If that happens, there is a risk that the virus could rapidly spread worldwide and cause large numbers of humans to become ill or die (a pandemic).
Q: Can animals "shed" the virus before clinical signs are observed?
A: The incubation period is the time between infection and the appearance of signs of disease. "Shedding," as it applies to viruses, means that the animal's secretions and/or droppings contain viral particles that may infect other animals or people. Some animals (e.g., growing poultry) rapidly show clinical signs of disease and simultaneously shed virus. Other animals, including some species of waterfowl, may appear clinically healthy, but be shedding the virus. The incubation and shedding periods for avian influenza virus in many species are not known.
Q: How stable is highly pathogenic H5N1 avian influenza virus in the environment?
A: Avian influenza (AI) viruses are sensitive to most detergents and disinfectants, and heating and drying will inactivate them. However, AI viruses may persist in soil, feces, and pond water for varying amounts of time, depending on environmental conditions.
The ability of AI viruses to survive in the environment depends on temperature and humidity. According to a 1998 study,1 the stability of the Hong Kong H5N1 highly pathogenic AI (HPAI) virus in wet feces in the environment was more than 40 days at 4°C (39.2°F). The virus becomes less stable as the temperature increases: at 25°C (77°F) the virus was stable for 8 days and at 35°C (95°F) the virus was stable for only 2 days. The same study reported that H5N1 HPAI virus in dry feces at 25°C (77°F) was stable for only 1 day.
1 Shortridge KF, Zhou NN, Guan Y, Gao P, Ito T, Kawaoka Y, et al. Characterization of avian H5N1 influenza viruses from poultry in Hong Kong. Virology. 1998;252:331-42.
Q: Should I be concerned about highly pathogenic H5N1 avian influenza causing a problem in the United States?
A: Be aware, but not overly concerned. Vigilance is the proper response. H5N1 highly pathogenic avian influenza (HPAI) has not been detected in the United States to date, and it is highly unlikely that H5N1 HPAI will infect US commercial poultry production facilities due to strict biosecurity measures at these facilities. Multiple safeguards are in place to monitor avian influenza (AI) in commercial bird flocks, and modern production facilities are designed to minimize the likelihood of spread between flocks should infection occur. Poultry maintained outdoors is at somewhat higher risk.
Other potential sources of infection, like live bird markets (retail stores where customers can purchase live birds that are then slaughtered, dressed, and packaged on site) are closely monitored for the presence of AI by federal and state surveillance programs. Waterfowl and wading birds maintained on open ponds are at higher risk of infection if the virus reaches North America.
Everyone, including pet owners, should be aware of the potential of H5N1 HPAI to cause disease and death, as well as how it can be transmitted. If H5N1 HPAI is identified in North America, it may be wise to take some precautions, but there is no reason to abandon cats, dogs, or other pets because of concerns about contracting or spreading the virus.
Although the CDC has not recommended that the public avoid travel to currently affected countries, it is recommended that people avoid poultry farms, bird markets, and other places where live poultry are kept in countries where outbreaks have occurred. For additional information, go to the CDC's Advice for Travelers Web page.
Q: What's being done to stop avian influenza from spreading in birds?
A: In countries where H5N1 highly pathogenic avian influenza (HPAI) is prevalent infected poultry are destroyed to reduce the risk of infecting other birds and to minimize human exposure. Awareness of avian influenza (AI) virus has been heightened, and education has become a priority in affected areas around the world.
The USDA recognizes the potential threat to human health and has therefore increased its surveillance and control efforts to detect HPAI and, if/when found, contain and eradicate HPAI in domestic poultry. More information about USDA's efforts and response to AI in the United States is available at http://www.usda.gov and http://pandemicflu.gov/.
In 2006, the US commercial chicken industry started a voluntary program to test each flock on the farm before it is sent to slaughter. Any flock identified as having positive results of testing for either the H5 or H7subtype of influenza A virus will be depopulated and disposed of in a safe, environmentally responsible manner. Also in 2006, surveillance to detect H5N1 HPAI in wild birds in the United States was greatly expanded. USDA and the Department of the Interior are collecting and testing at least 125,000 samples from wild birds or their immediate environment (in all four major flyways in the United States) for the presence of AI viruses, specifically looking for H5N1 HPAI.