Influenza Pandemic Preparedness

by CDC

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What changes are needed for H5N1 or another avian influenza virus to cause a pandemic?

Three conditions must be met for a pandemic to start: 1) a new influenza virus subtype must emerge for which there is little or no human immunity; 2) it must infect humans and causes illness; and 3) it must spread easily and sustainably (continue without interruption) among humans. The H5N1 virus in Asia and Europe meets the first two conditions: it is a new virus for humans (H5N1 viruses have never circulated widely among people), and it has infected more than 190 humans, killing over half of them.

However, the third condition, the establishment of efficient and sustained human-to-human transmission of the virus, has not occurred. For this to take place, the H5N1 virus would need to improve its transmissibility among humans. This could occur either by "reassortment" or adaptive mutation.

Reassortment occurs when genetic material is exchanged between human and avian viruses during co-infection (infection with both viruses at the same time) of a human or another mammal. The result could be a fully transmissible pandemic virus-that is, a virus that can spread easily and directly between humans. A more gradual process is adaptive mutation, where the capability of a virus to bind to human cells increases during infections of humans.

What is CDC doing to prepare for a possible H5N1 influenza pandemic?

CDC is taking part in a number of pandemic prevention and preparedness activities, including the following:

• Providing leadership to the National Pandemic Influenza Preparedness and Response Task Force, created in May 2005 by the Secretary of the U.S. Department of Health and Human Services.

• Working with the Association of Public Health Laboratories on training workshops for state laboratories on the use of special laboratory (molecular) techniques to identify H5 viruses.

• Working with the Council of State and Territorial Epidemiologists and others to help states with their pandemic planning efforts.

• Working with other agencies, such as the Department of Defense and the Veterans Administration, on antiviral stockpile issues.

• Working with the World Health Organization (WHO) to investigate influenza H5N1 among people (e.g., in Vietnam) and to provide help in laboratory diagnostics and training to local authorities.

• Performing laboratory testing of H5N1 viruses.

• Starting a $5.5 million initiative to improve influenza surveillance in Asia.

• Holding or taking part in training sessions to improve local capacities to conduct surveillance for possible human cases of H5N1 and to detect influenza A H5 viruses by using laboratory techniques.

• Developing and distributing reagent kits to detect the currently circulating influenza A H5N1 viruses.

• CDC has developed and is distributing the first FDA approved test for the detection of the H5 viruses that first emerged in Asia in 2003.

CDC also is working closely with WHO and the National Institutes of Health on safety testing of vaccine candidates and development of additional vaccine virus seed candidates for influenza A (H5N1) and other subtypes of influenza A viruses.

Avian Influenza Infection in Animals

What animals can be infected with avian influenza A (H5N1) viruses? In addition to humans and birds, we know that pigs, tigers, leopards, ferrets, and domestic cats can be infected with avian influenza A (H5N1) viruses. In addition, in early March 2006, Germany reported H5N1 infection in a stone marten (a weasel-like mammal). The avian influenza A (H5N1) virus that emerged in Asia in 2003 is evolving and it's possible that other mammals may be susceptible to infection as well. CDC is working closely with domestic and international partners to continually monitor this situation and will provide additional information to the public as it becomes available.

Can domestic cats be infected with avian influenza viruses?

While domestic cats are not usually susceptible to influenza type A infection, it is known that they can become infected and die (both experimentally and naturally) with avian influenza A (H5N1) viruses and, in a laboratory/research setting can spread the virus to other cats. It is not known whether domestic cats can spread the virus to other domestic cats under natural conditions.

How do cats become infected with avian influenza A (H5N1) viruses?

All of the cases of influenza A (H5N1) infection in domestic cats reported to date have been associated with H5N1 outbreaks among domestic poultry or wild birds and are thought to have occurred by the cat eating raw infected birds.

How commonly have cats been infected with avian influenza A (H5N1) viruses?

During the avian influenza A (H5N1) outbreak that occurred from 2003 to 2004 in Asia, there were only several unofficial reports of fatal infections in domestic cats. Studies carried out in the Netherlands and published in 2004 showed that housecats could be infected with avian influenza A (H5N1) and could spread the virus to other housecats. In these experiments, the cats became sick after direct inoculation of virus isolated from a fatal human case, and following the feeding of infected raw chicken. In February 2006, Germany reported that a domestic cat had died from influenza A (H5N1) infection. That cat lived in the northern island of Ruegen, where more than 100 wild birds are believed to have died of the disease. The cat probably got sick by eating an infected bird.

About the Author

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The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.