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Bird Flu in Pennsylvania Poses Uncertain Risk to People
Gene Analysis Is Underway for New Strain

By Nancy Touchette

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Infectious Diseases

Chicken infected with the avian flu virus.
A new strain of bird flu virus has cropped up in a flock of chickens in Lancaster County, Pennsylvania. The virus was detected during routine surveillance and no chickens or humans have shown any symptoms of disease. State health authorities believe it poses no risk to human health.

A related strain, however, triggered a pandemic in 1957, causing nearly 70,000 deaths in the United States and 30,000 deaths in the United Kingdom. The virus was a bird strain that apparently acquired the ability to infect people.

Researchers at the United States Department of Agriculture National Veterinary Services Laboratory (NVSL) in Ames, Iowa, identified the Pennsylvania flu strain earlier this week. It has not been analyzed yet and it may take some time to determine whether it poses a risk to humans.

“Viral isolates have just been made,” says NVSL virologist Dennis Senne. “We have identified the virus as an H2N2 strain, but that’s all we know. We have yet to sequence the virus or do further testing.”

Nevertheless, Calvin B. Johnson, Pennsylvania’s Secretary of Health, stated in a February 13 press release that “Based on our information from Ames, Iowa, we believe no health risk to humans exists at this time.” There is no full explanation for this assertion.

Avian influenza has been a problem for several months.

A recent outbreak of bird flu, or avian influenza, has infected 31 people and caused 22 deaths in Thailand and Vietnam so far, and millions of chickens throughout Asia have been infected or destroyed. That epidemic is caused by infection with the highly pathogenic H5N1 strain, a different type of avian influenza virus.

Another outbreak was caused by the H7N2 form of the virus and has infected two flocks of chickens in Delaware and has appeared in live chickens in New Jersey marketplaces. That form of the virus has never been transmitted to humans.

An H7 influenza virus has also been detected on a farm in British Columbia, Canada. The exact type of strain and its transmissibility to humans is unknown.

Electron micrograph of avian influenza virus.
The discovery of the H2N2 virus in Pennsylvania has prompted officials there to reassure the public and international trading partners. Following the Delaware outbreak, some countries, including Korea, Japan, and China, banned poultry imports from the United States.

“This is not the strain currently in Asia or Delaware,” Pennsylvania Agriculture Secretary Dennis C. Wolff said in the press release. “We believe this indicates no threat to human health and a low threat to the poultry industry.”

But many researchers are not so sure. They fear that the current H2N2 Pennsylvania strain may be similar enough to the 1957 pandemic strain to cross over to humans or that it might easily mutate to a form that can.

“I do not know how [Pennsylvania authorities] could conclude that this strain poses no threat to humans,” says Ron A.M. Fouchier of the National Influenza Center in Rotterdam, the Netherlands. “I would be surprised if they had any real evidence that this virus is not dangerous to humans.”

The NVSL researchers determined the type of virus by testing which types of viral antigens—proteins made by the virus—react with specific immune proteins. All flu viruses carry a protein known as hemagglutinin on the surface. Fifteen forms of this protein have been found in birds and are designated as H1, H2, H3, and so on. Nine types of a second protein called neuraminidase have been identified. These are designated as N1, N2, N3, and so on. The particular combination of H and N proteins defines the virus subtype.

The NVSL has determined that the Pennsylvania virus is an H2N2 subtype. The virus is now being evaluated for its pathogenicity to assess its potential for causing disease in poultry. NVSL researchers will sequence the genome of the virus and compare it to similar viruses to get an idea of how it has evolved. They will also look for any sequences that might signal a potential for infecting humans.

Related GNN Article
Shape of 1918 Flu Protein Provides Clues to Global Pandemic
All known human influenza viruses have evolved from avian viruses. The virus can mutate rapidly and switch genes with human forms and that’s what has researchers worried. However, although the H2N2 virus caused the 1957 pandemic and has emerged in chickens from time to time, it has not been found in humans since 1968.

Claims by Pennsylvania officials that the virus poses no risk to humans may come more from a historical perspective than from any current data, according to Senne.

“It’s fairly safe to say that it’s not going to cause a problem until there is evidence that people are infected,” says Senne. “But nothing that we have tested for so far, tells us anything—one way or another—about its risk to humans.”

In the meantime, the U.S. Centers for Disease Control has contacted state and local health departments and asked them to be doubly vigilant in monitoring patients for suspect cases of avian flu.

Health officials note that the virus was detected by routine surveillance, before it had a chance to cause disease in either humans or chickens. Agriculture officials first detected antibodies—immune proteins that attack the virus—in eggs. They then found the virus in chickens. The flock is under quarantine and chickens within a five-mile radius are also being tested.

Schafer, J.R. et al. Origin of the pandemic 1957 H2 influenza A virus and the persistence of its possible progenitors in the avian reservoir. Virology 194, 781-788 (1993).
Webby, R.J. and Webster, R.G. Are we ready for pandemic influenza? Science 302, 1519-1522 (November 28, 2003).
Webster, R.G. The importance of animal influenza for human disease. Vaccine 20, S16-S20 (2002).

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