Avian Influenza A Virus Infections in Humans
Although avian influenza A viruses usually do not infect humans, rare cases of human infection with these viruses have been reported. Most human infections with avian influenza A viruses have occurred following direct or close contact with infected poultry. Illness in humans has ranged from mild to severe.
The spread of avian influenza A viruses from one ill person to another has been reported very rarely, and has been limited, inefficient and not sustained. However, because of the possibility that avian influenza A viruses could change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is extremely important for public health.
The reported signs and symptoms of low pathogenic avian influenza* (LPAI) A virus infections in humans have ranged from conjunctivitis to influenza-like illness (e.g., fever, cough, sore throat, muscle aches) to lower respiratory disease (pneumonia) requiring hospitalization. Highly pathogenic avian influenza (HPAI) A virus infections in people have been associated with a wide range of illness from conjunctivitis only, to influenza-like illness, to severe respiratory illness (e.g. shortness of breath, difficulty breathing, pneumonia, acute respiratory distress, viral pneumonia, respiratory failure) with multi-organ disease, sometimes accompanied by nausea, abdominal pain, diarrhea, vomiting and sometimes neurologic changes (altered mental status, seizures). LPAI H7N9 and HPAI Asian H5N1 have been responsible for most human illness worldwide to date, including the most serious illnesses and deaths.
Avian influenza A virus infection in humans cannot be diagnosed by clinical signs and symptoms alone; laboratory testing is required. Avian influenza A virus infection is usually diagnosed by collecting a swab from the nose or throat of the sick person during the first few days of illness. This specimen is sent to a lab; the laboratory looks for avian influenza A virus either by using a molecular test, by trying to grow the virus, or both. (Growing avian influenza A viruses should only be done in laboratories with high levels of protection).
For critically ill patients, collection and testing of lower respiratory tract specimens may lead to diagnosis of avian influenza virus infection. For some patients who are no longer very sick or who have fully recovered, it may be difficult to find the avian influenza A virus in the specimen, using these methods. Sometimes it may still be possible to diagnose avian influenza A virus infection by looking for evidence of the body's immune response to the virus infection by detecting specific antibodies the body has produced in response to the virus. This is not always an option because it requires two blood specimens (one taken during the first week of illness and another taken 3-4 weeks later). Also, it can take several weeks to verify the results, and testing must be performed in a special laboratory, such as at CDC.
CDC has posted guidance for clinicians and public health professionals in the United States on appropriate testing, specimen collection and processing of samples from patients who may be infected with novel influenza A viruses.
CDC currently recommends oseltamivir, peramivir, or zanamivir for treatment of human infection with avian influenza A viruses. Analyses of available avian influenza viruses circulating worldwide suggest that most viruses are susceptible to oseltamivir, peramivir, and zanamivir. However, some evidence of antiviral resistance has been reported in HPAI Asian H5N1 viruses and influenza A H7N9 viruses isolated from some human cases. Monitoring for antiviral resistance among avian influenza A viruses is crucial and ongoing. These data directly inform CDC and WHO antiviral treatment recommendations.
CDC has posted interim guidance for clinicians and public health professionals in the United States regarding follow-up and influenza antiviral chemoprophylaxis of persons exposed to birds infected with avian influenza A viruses.
The best way to prevent infection with avian influenza A viruses is to avoid sources of exposure. Most human infections with avian influenza A viruses have occurred following direct or close contact with infected poultry.
People who have had contact with infected birds may be given influenza antiviral drugs preventatively. While antiviral drugs are most often used to treat flu, they also can be used to prevent infection in someone who has been exposed to influenza viruses. When used to prevent seasonal influenza, antiviral drugs are 70% to 90% effective.
Seasonal influenza vaccination will not prevent infection with avian influenza A viruses, but can reduce the risk of co-infection with human and avian influenza A viruses. It’s also possible to make a vaccine that can protect people against avian influenza viruses. For example, the United States government maintains a stockpile of vaccine to protect against avian influenza A H5N1 vaccine. The stockpiled vaccine could be used if a similar H5N1 virus were to begin transmitting easily from person to person. Creating a candidate vaccine virus is the first step in producing a vaccine. See “Making a Candidate Vaccine Virus (CVV) for a Highly Pathogenic Avian Influenza (Bird Flu) Virus” for more information on this process.
*Avian influenza A viruses are designated as highly pathogenic avian influenza (HPAI) or low pathogenicity avian influenza (LPAI) based on molecular characteristics of the virus and the ability of the virus to cause disease and mortality in chickens in a laboratory setting.
- Page last reviewed: March 18, 2015
- Page last updated: September 3, 2015
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