How worried should you be about Avian Influenza (Bird Flu) and Avian Influenza A (H5N1) Virus?

H5N1 mainly affects domesticated poultry, including chickens, ducks, and turkeys. It also affects migratory birds, and we believe that the spread into Europe has been through wild ducks, geese, and swans. Ordinary avian viruses circulate within the wild migratory-bird population all the time, and they do not usually cause much disease. However, H5N1 moved into domestic birds and started killing them, and now it has moved back into the wild bird populations and is killing them as well. This development suggests that the virus has gained the ability to cause disease in wild birds, and that is worrisome.

In order for a new strain of influenza to become pandemic, three things must happen; it must infect humans whose immune systems are naïve to the virus; it must be virulent; and it must spread easily from person to person. The first two conditions have already been met. Moreover, there have been instances in which we think H5N1 went from one person to another. To date, the infection has dead-ended at the second person, usually a family member.

Infected birds shed flu virus in their saliva, nasal secretions, and feces. Susceptible birds become infected when they have contact with contaminated excretions or surfaces that are contaminated with excretions. It is believed that most cases of bird flu infection in humans have resulted from contact with infected poultry or contaminated surfaces.

However, in parts of Asia , there is a practice of drinking raw blood, and there have been a couple of cases in which we think infection may have resulted from drinking duck blood. We also think eating raw eggs might put people at risk. Because of this, people should cook all poultry and poultry products thoroughly. Presently, the United States has an embargo on poultry imported from countries where there have been outbreaks in humans.

For people to be infected, the virus has to enter the respiratory tract. They must breathe in the virus or get it on their hands and then put it in contact with the respiratory mucosa via the nose or mouth. People at the highest risk are those who are exposed to live birds, like poultry farmers, people who clean chicken coops, or those involved with cockfighting. Once someone is infected, the virus is transmitted through the respiratory route, via sneezing, kissing, and touching.

The strain of H5N1 virus found in Asia has not been found in the United States . There have been no human cases of H5N1 flu in the United States . It is possible that travelers returning from affected countries in Asia could be infected. Therefore, we want you to be aware that the threat is out there and that you should certainly make efforts to prepare. Nevertheless, you should not panic, since the risk is not immediate; it is not on our doorstep.

If you travel to an affected country, you are only at risk if you have contact with poultry. Therefore, you should avoid contact with infected birds or contaminated surfaces, and should be careful when handling and cooking poultry.

We hope this virus has to go through many cycles in humans in order to acquire the mutations that would make it a pandemic strain. We learned from reconstructing the 1918 "Spanish Flu" virus that pandemics are not the result of just one genetic change. Rather, a variety of small and large changes taken together resulted in that lethal strain. In addition, in 1918, there were societal conditions, including severe crowding and a lack of protection against contagion, which fostered the ability of a lethal virus to emerge. For all these reasons, we are hopeful that H5N1 will not acquire all the mutations that would allow it to become easily transmissible in humans.

To keep abreast of the situation during the coming months, there are a number of helpful Web sites. Besides the CDC's influenza Web site (www.cdc.gov/flu/avian), check the Web sites of the World Health Organization (www.who.int/en) and the Infectious Diseases Society of America (www.idsociety.org). For more information about the H5N1 vaccine development process, visit the NIH Web site (www.nih.gov).