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Background
on Influenza Viruses
There are 3 types of influenza viruses: A, B and C.
Influenza type A viruses are divided into subtypes based on two proteins on the surface of the virus. These proteins are called hemagglutinin (HA) and neuraminidase (NA). There are 15 different HA subtypes and 9 different NA subtypes. The viruses are named according to their surface proteins. For example, an “H7N2 virus” designates an influenza A subtype that has a hemagglutinin 7 protein and a neuraminidase 2 protein. Wild birds are the natural host to all subtypes of influenza A viruses.
Influenza B and certain subtypes of influenza A (H1N1, H1N2 and H3N2) normally circulate among humans and cause yearly epidemics of disease. Type C influenza viruses are milder and do not cause epidemics. Type A viruses historically have been the ones responsible for influenza pandemics.
In addition to birds, type A influenza viruses can infect several animal species, including pigs, horses, seals and whales. Types B and C influenza viruses do not affect domestic animals.
Avian Influenza Viruses
Influenza viruses that infect birds are called “avian influenza viruses.” These viruses are genetically distinguishable from influenza viruses that normally infect people. Avian influenza viruses can be classified into “low pathogenic” and “highly pathogenic” forms based on the severity of the illness they cause in poultry.
Birds that are infected with avian influenza viruses shed virus in saliva, nasal secretions and feces. Disease spreads when susceptible birds have contact with contaminated excretions.
Avian influenza A viruses do not usually infect humans; however, several instances of human infections and outbreaks of avian influenza have been reported since 1997. It is believed that most cases of avian influenza infection in humans have resulted from contact with infected poultry or contaminated surfaces.
Public health authorities monitor outbreaks of avian influenza closely because of concerns about the potential for more widespread infection in the human population. So far, human infection with avian influenza viruses detected since 1997 have not resulted in sustained human-to-human transmission. However, because influenza viruses can change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is important.
The Influenza A H7 Subtype
- Influenza A H7 is a subtype of the Type A influenza virus.
- Infection in Birds: The virus circulates among birds worldwide. Some strains of H7 can be deadly to birds, particularly domesticated birds such as chickens and other poultry. H7 outbreaks among poultry occur from time to time. It is thought that poultry may become infected with H7 through contact with waterfowl, direct or indirect contact with infected poultry (for example, contact with contaminated water).
When these outbreaks occur, control strategies that include “culling” (the
killing of infected poultry), quarantine (poultry cannot
move off the farm) within a certain geographic proximity,
and surveillance are used to contain the outbreak.
- Infection in Humans: H7 subtypes do not typically infect humans. In 2003, however, the Netherlands reported outbreaks of influenza A (H7N7) in poultry on several farms. Later, infections were reported among pigs and humans. In total, 83 people were confirmed to have H7N7 influenza virus infection associated with this poultry outbreak. These cases occurred mostly among poultry workers. H7N7-associated illness included: 79 cases of conjunctivitis (eye infections); 6 influenza-like illnesses with cough, fever and muscle aches; and 1 death. (A veterinarian who visited one of the affected farms died of acute respiratory distress syndrome and complications related to H7N7 infection.)
The majority of these cases occurred as a result of direct
contact with infected poultry; however, Dutch authorities
reported 3 possible instances of transmission from poultry
workers to family members.
Since that time, there have not been other instances of H7N7 infection among humans.
Recent Developments
- H7N2 in Delaware: The state of Delaware
has reported an outbreak of avian influenza A (H7N2) among
poultry. Affected poultry are being culled and quarantine
measures have been instituted. This H7N2 virus is significantly
different from the H7N7 virus that caused an avian influenza
outbreak in the Netherlands in 2003 that resulted in human
infections.
- H5N1 in Asia: Outbreaks of influenza A (H5N1) are currently occurring among bird populations in countries throughout Asia and in humans in Thailand and Vietnam. H5N1 avian influenza viruses have previously caused human illness in Hong Kong in 1997 (18 cases) and among 2 people from Hong Kong who traveled to China in 2003. Similar H5N1 viruses have not been identified in the Americas.
More Information
Or visit www.cdc.gov/flu or call the CDC public response hotline at (888) 246-2675 (English), (888) 246-2857 (Español), or (866) 874-2646 (TTY) for additional information.
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