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An outbreak of avian influenza, more commonly known as bird flu, is affecting bird populations in countries throughout Asia. The outbreak is caused by the H5N1 subtype of influenza A. Human cases also have been reported.

• In birds: Outbreaks of avian influenza A (H5N1) have been confirmed among poultry in Cambodia, China, Hong Kong (in a single peregrine falcon), Indonesia, Japan, Laos, South Korea, Thailand, and Vietnam.
  
  Visit the World Organization for Animal Health website at http://www.oie.int for more information.
  
  
• In people: The outbreak of bird flu has resulted in human cases of H5N1 infection in Vietnam and Thailand. Deaths have been reported. At this time it is believed that these cases resulted from contact with infected birds or surfaces contaminated with excretions from infected birds. An investigation is ongoing to determine the source of human infections.
  
  Visit the World Health Organization (WHO) website at http://www.who.int/en/ for more information.
  

Information on Influenza A (H5N1)

• Background: Influenza A (H5N1) is a subtype of the Type A influenza virus. Wild birds are the natural hosts of the virus, hence the name avian influenza or bird flu. The virus was first isolated from birds (terns) in South Africa in 1961. The virus circulates among birds worldwide. It is very contagious among birds and can be deadly to birds, particularly domesticated birds like chickens.
• Infection: The virus does not typically infect humans. In 1997, however, the first instance of direct bird-to-human transmission of H5N1 was documented during an outbreak of avian influenza among poultry in Hong Kong; the virus caused severe respiratory illness in 18 people, of who 6 died. Since that time, there have been other instances of H5N1 infection among humans. (See "Basic Information about Avian Influenza.") But so far, H5N1 viruses have not been capable of efficient human-to-human transmission. This is something that is being watched carefully and is being investigated during this outbreak.
• Spread: Infected birds shed virus in saliva, nasal secretions and feces. Avian influenza viruses spread among susceptible birds when they have contact with contaminated excretions. It is believed that most cases of H5N1 infection in humans have resulted from contact with infected poultry or contaminated surfaces.

Current H5N1 Strain

The H5N1 strain implicated in the current outbreak has been genetically sequenced. Following is a summary of what has been learned:

• All genes are of bird origin. This means that the virus has not acquired genes from human influenza viruses, a development that would make person-to-person spread more likely.
• There are likely different variations of H5N1 virus circulating at this time. Genetic sequencing of virus samples from South Korea and Vietnam show that the viruses in these two countries are slightly different.
• Genetic sequencing of A(H5N1) virus samples from human cases in Vietnam and Thailand show antiviral resistance to amantadine and rimantadine, two of the antiviral drugs commonly used for influenza. The remaining two antivirals (oseltamavir and zanamavir) should still be effective against this strain of H5N1.

Containment

Key to containing the outbreak is the culling (killing) of sick and exposed birds. This was done to contain the 1997 H5N1 outbreak in Hong Kong. Many experts believe this was crucial to averting many more human cases. For the current outbreak in Asia, governments are culling poultry to try to contain the virus. Patients are being treated and isolated, and investigations are underway to determine the source of infection.

What Will Happen

All influenza viruses can change. It is possible that an avian influenza virus could change so that it could infect humans and could spread easily from person to person. Because these viruses commonly do not infect humans, there is little or no immune protection against them in the human population. If an avian influenza virus were able to infect people and gain the ability to spread easily from person to person, an "influenza pandemic" could begin.

CDC Response to Outbreak

CDC is working closely with WHO and other partners in an effort to address the outbreak. For its part, CDC activities include the following:

• Activation of CDC’s Emergency Operations Center to provide round-the-clock coordination and response.
• Deployment of 6 staff members (epidemiologists, laboratorians and a data manager) as part of the WHO investigation team in Vietnam working with Vietnam's human and animal health authorities to assess the extent of the outbreak among humans and animals, identify risk factors for human infection, and determine the characteristics of influenza A (H5N1) viruses isolated from human and poultry cases.
• As 1 of 4 WHO Collaborating Centers in the WHO Global Influenza Network, CDC laboratories are conducting extensive testing of specimens from Asia to determine the characteristics of the recent H5N1 viruses compared with other avian influenza viruses. This information is needed to help develop an H5 vaccine for the current situation.
• CDC’s WHO Collaborating Center is developing reagent kits that will be distributed to laboratories throughout Asia, the U.S., and elsewhere so that they can detect recent H5N1 viruses.

CDC Recommendations

Travelers: CDC advises that travelers to countries in Asia with documented H5N1 outbreaks should avoid poultry farms, contact with animals in live food markets, and any surfaces that appear to be contaminated with feces from poultry or other animals. (For more information, visit CDC’s Travelers’ Health page at http://www.cdc.gov/travel/.)

Health Departments and Health Care Professionals: CDC has provided U.S. health departments with interim guidance on enhanced surveillance and laboratory testing to help identify possible cases of H5N1 influenza that might be imported into the United States. (For more information about these recommendations, please see the CDC Health Update for Feb 3)