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House Cats Can Carry the Virus for Bird Flu

By Cheryl Simon Silver

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Infectious Diseases

Dutch scientists report today that house cats can carry the strain of bird flu that killed a person in Vietnam earlier this year. They infected the animals with the virus known as H5N1, the strain of bird flu that some public health officials worry could cause a global flu epidemic in humans.

In early 2004, some strains of the avian influenza virus killed more than 100 million birds in Asia either through the disease or through programs intended to contain the outbreak by slaughtering infected animals.

Relatively few humans were infected, but the virus was deadly. In Asia this year, 37 people were infected, mostly through direct contact with diseased birds in settings such as live poultry markets, and 26 of them died.

The new research adds cats to the list of animals that can contract and potentially spread the disease. Some pigs in China were recently reported to be infected with H5N1.

There are no known cases in which cats have transmitted influenza to humans. However, some leopards and domestic cats in Thailand died of the bird flu virus earlier this year, which prompted the researchers to conduct the study.

A team led by Thijs Kuiken at Erasmus Medical Center in Rotterdam swabbed the tracheas of house cats with an H5N1 virus isolated from a person in Vietnam who died of the flu. The cats contracted the disease.

When the researchers fed other cats chicks infected with the bird flu virus, the cats, like those inoculated with the virus, developed severe pulmonary symptoms. They then passed the disease to two other cats that shared their quarters.

One cat died before the end of the week-long experiment. When the researchers examined the rest of the cats inoculated with the virus, all were found to have severe lung inflammation.

Cats are generally not susceptible to disease from influenza virus infection. Still, the finding is significant, says Klaus Stohr, who coordinates the World Health Organization’s Global Influenza Program in Geneva. If the cats excrete the virus, they may be vectors of the disease, passing it on to other farms and birds.

“Cats are not normally considered vectors, so this is something new,” Stohr says. It is not clear, he adds, whether cats could play a role in spreading the infection to humans, but says this needs to be assessed.

“From a public health perspective, we must not lose focus,” Stohr says. “The real source of the problem is poultry because that’s the major reservoir.”

The new research is relevant to areas where H5N1 is causing an outbreak in poultry because cats on a farm that has infected birds could contract the disease by eating the birds or through contact with their waste. In another scenario, cats could be fed poultry products of chickens infected with H5N1, fall ill, and potentially spread the disease.

It is also possible, though unlikely, that different strains of viruses in cats could reassort. In this process, parts from two viruses recombine into a new virus that could infect humans and be transmitted from one person to the next.

Many experts suspect that the three global flu epidemics, or pandemics, of the 20 th century occurred when new bird flu strains became established in humans. In each instance, the virus spread worldwide in less than a year after detection

In most years, about 36,000 people in the United States die from influenza. A major change in the virus could cause a pandemic.

“We don’t know what this virus needs to do to successfully infect human populations,” says Richard Webby of St. Jude Children’s Research Hospital, in Memphis, Tennessee. “But the virus is a genius at adapting to different environments. The more chance it has to infect different hosts, the more risk we have.”

Related Story: Shape of 1918 Flu Protein Provides Clues to Global Pandemic

Kuiken, T. et al. Avian H5N1 Influenza in Cats. Science Express. Published online September 2, 2004

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