Influenza A (H5N1) virus -- also called "H5N1 virus" -- is an influenza A virus subtype that occurs mainly in birds, is highly contagious among birds, and can be deadly to birds.
Outbreaks of the
avian flu H5N1 strain occurred among poultry in eight countries in Asia (Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand, and Vietnam) during late 2003 and early 2004. At that time, more than 100 million birds infected with the H5N1 virus either died from the disease or were killed in order to try to control the outbreaks. By March 2004, the H5N1 outbreak was reported to be under control.
However, since late June 2004, new outbreaks of the H5N1 virus among poultry were reported by several countries in Asia (Cambodia, China [Tibet], Indonesia, Kazakhstan, Malaysia, Mongolia, Russia [Siberia], Thailand, and Vietnam). It is believed that these outbreaks are ongoing.
Influenza H5N1 infection also has been reported among poultry in Turkey and Romania, and among wild migratory birds in Croatia. Human cases of H5N1 have also been reported in Cambodia, China, Indonesia, Thailand, and Vietnam.
H5N1 does not usually infect people, but more than 130 human cases have been reported by the World Health Organization (WHO) since January 2004. Most of these cases have occurred as a result of people having direct or close contact with infected poultry or contaminated surfaces. However, a few cases of human-to-human spread of H5N1 have occurred.
Of the few
avian influenza viruses that have crossed the species barrier to infect humans, the H5N1 virus has caused the largest number of detected cases of severe disease and death in humans. In the current outbreaks in Asia and Europe, more than half of those infected with the virus have died. Most cases have occurred in previously healthy children and young adults. However, it is possible that the only cases currently being reported are those in the most severely ill people, and that the full range of illness caused by the H5N1 virus has not yet been defined.
So far, the spread of H5N1 from person to person has been rare and has not continued beyond one person. Nonetheless, because all
influenza viruses have the ability to change, scientists are concerned that H5N1 could, one day, be able to infect humans and spread easily from one person to another.
Because these viruses do not commonly infect humans, there is little or no immune protection against them in the human population. If H5N1 were to gain the capacity to spread easily from person to person, an
influenza pandemic (worldwide outbreak of disease) could begin. Although no one can predict when a
pandemic might occur, experts from around the world are watching the H5N1 situation in Asia and Europe closely and are preparing for the possibility that the virus may begin to spread more easily and widely from person to person.
There currently is no commercially available
flu vaccine to protect humans against H5N1 that is being seen in Asia and Europe. However, vaccine development efforts are taking place. Research studies to test a vaccine to protect humans against H5N1 began in April 2005, and a series of clinical trials is under way.
The H5N1 virus that has caused human illness and death in Asia is resistant to
amantadine and
rimantadine, two
antiviral drugs commonly used for
influenza treatment. Two other antiviral medications, oseltamavir and zanamavir, may work for treatment of influenza caused by H5N1, but additional studies still need to be done to demonstrate their effectiveness.
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