Outbreaks of highly pathogenic avian influenza A (H5N1) occurred among poultry in 8 countries in Asia (Cambodia, China, Indonesia, Japan, Lao, South Korea, Thailand and Vietnam) during late 2003 and early 2004. At that time, more than 100 million birds either died from the disease or were culled.

From December 30, 2003 to March 17, 2004, 12 confirmed human cases of avian influenza A (H5N1) were reported in Thailand and 23 in Vietnam, resulting in a total of 23 deaths.

By late February, however, the number of new human H5 cases being reported in Thailand and Vietnam slowed and then stopped. Within a month, countries in Asia were reporting that the avian influenza outbreak among poultry had been contained. No conclusive evidence of sustained human-to-human transmission was found.

Recent Developments

Beginning in late June 2004, new lethal outbreaks of H5N1 among poultry were reported by several countries in Asia: Cambodia, China , Indonesia , Malaysia (first-time reports), Thailand and Vietnam. There has not been a resurgence of avian influenza in South Korea and Japan, and the outbreaks are reported to be controlled in those countries. It is unknown to what extent H5N1 outbreaks in the other countries may be ongoing. For more information about outbreaks in poultry, visit the World Organization for Animal Health website.

The new outbreaks of H5N1 in poultry in Asia were followed by renewed sporadic reporting of human cases of H5N1 infection in Vietnam and Thailand beginning in August, 2004 and continuing into 2005. Of particular note is one isolated instance of probable limited human-to-human transmission occurring in Thailand in September, 2004. On February 2, 2005, the first human case of avian influenza A H5 infection from Cambodia was reported.

As of February 17, 2005, there have been 55 human cases of avian influenza A (H5N1) in Vietnam (37), Thailand (17) and Cambodia (1) resulting in 42 deaths. For more information about H5N1 infections in humans, visit the World Health Organization (WHO) website .

Assessment of Current Situation

The avian influenza A (H5N1) epizootic outbreak in Asia is not expected to diminish significantly in the short term. It is likely that H5N1 infection among birds has become endemic to the region and that human infections will continue to occur. So far, no sustained human-to-human transmission of the H5N1 virus has been identified and no evidence for genetic reassortment between human and avian influenza virus genes has been found; however, the epizootic outbreak in Asia poses an important public health threat.

If these H5N1 viruses gain the ability for efficient and sustained transmission between humans, there is little preexisting natural immunity to H5N1 in the human population, and an influenza pandemic could result, with high rates of illness and death. In addition, genetic sequencing of influenza A (H5N1) virus samples from human cases in Vietnam and Thailand show resistance to the antiviral medications amantadine and rimantadine, 2 of the medications commonly used for treatment of influenza. This would leave 2 remaining antiviral medications (oseltamavir and zanamavir) that should still be effective against currently circulating strains of H5N1. Efforts to produce a vaccine that would be effective against this strain of influenza A H5N1 are under way. Vaccine reference virus strains already have been made and provided to manufacturers to produce pilot lots for human clinical trials as well as to produce a larger quantity of H5N1 vaccine, but mass production and availability of such a vaccine is some time off.

Recent research findings give further cause for concern. New research suggests that H5 viruses are becoming more capable of causing disease (pathogenic) for mammals than earlier H5 viruses and are becoming more widespread in birds in the region. One study found that ducks infected with H5N1 are now shedding more virus for longer periods of time without showing any symptoms of illness. This has implications for the role of ducks in transmitting disease to other birds and possibly, to humans as well. Additionally, other findings have documented H5 infection among pigs in China and H5 infection in felines (experimental infection in housecats in the Netherlands and isolation of H5N1 viruses from infected tigers and leopards in Thailand), suggesting that cats could host or transmit the infection. These finding are particularly worrisome in light of the fact that reassortment of avian influenza genomes is most likely to occur when these viruses demonstrate a capacity to infect multiple species, as is now the case in Asia.

CDC Response to Outbreaks

Domestic Activities:

• In February 2004, CDC issued recommendations for enhanced domestic surveillance of avian influenza A (H5N1). Following the reports of human deaths in Vietnam in August, CDC issued a follow-up Health Alert Network (HAN) message on August 12 reiterating criteria for domestic surveillance, diagnostic evaluation, and infection control precautions for avian influenza A (H5N1). The HAN also detailed laboratory testing procedures for H5N1.
• CDC collaborated with Association of Public Health Laboratories on 2 training workshops for state laboratories on the use of molecular techniques to identify H5 viruses. Planning for a third training workshop is under way.
• CDC is working collaboratively with Council of State and Territorial Epidemiologists and others to assist states with pandemic planning efforts.
• CDC is working with other agencies such as the Department of Defense and the Veterans Administration on antiviral stockpile issues.

International Activities:

• CDC worked collaboratively with WHO to conduct investigations of H5N1 in Vietnam and to provide laboratory diagnostic and training assistance.
• CDC has performed laboratory testing of H5N1 viruses from Thailand and Vietnam.
• CDC has implemented a $5.5 million initiative to improve influenza surveillance in Asia.
• CDC has conducted or taken part in 9 training sessions to enhance local capacities to conduct surveillance for possible human cases of H5 and to detect influenza A H5 viruses by using laboratory techniques.
• CDC has developed and distributed a reagents kit for the detection of the currently circulating influenza A H5 viruses.
• CDC is monitoring the situation and remains in close contact with WHO and other international partners.

In addition, CDC continues to work collaboratively with WHO and the National Institutes of Health (NIH) on safety testing vaccine seed candidates and the development of additional vaccine virus seed candidates for influenza A (H5N1).

More Information

For CDC’s surveillance, diagnostic evaluation, and infection control precautions for avian influenza A (H5N1), visit http://www.cdc.gov/flu/avian/professional/han081304.htm.

For more information about infection in birds, visit the World Organization for Animal Health website or the Food and Agriculture Organization of the United Nations website.

For more information about infections in humans, visit the World Health Organization (WHO) website.

Bird Import Ban

On February 4, 2004, CDC and USDA issued an order for a ban on the import of all birds (Class: Aves) from affected areas in Southeast Asia. On March 10, 2004, CDC, in coordination with USDA, lifted the embargo of birds and bird products from Hong Kong. On September 28, 2004, the list of countries affected by the embargo of birds and bird products was expanded to include Malaysia.

Travel

Please read Updated Information for Travelers about Avian Influenza A(H5N1).

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 them, 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 influenza A (H5N1) virus was documented during an outbreak of avian influenza among poultry in Hong Kong; the virus caused severe respiratory illness in 18 people, of whom 6 died. Since that time, there have been other instances of H5N1 infection among humans (see General Information on Avian Influenza). However, H5N1 viruses thus far have not been capable of efficient human-to-human transmission; health officials continue to monitor the situation closely for evidence of H5N1 transmission between people.
  
• 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.