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Bird Flu Found to Sicken Cats

Finding raises worries about transmission to humans

THURSDAY, Sept. 2, 2004 (HealthDayNews) -- For the first time, a type of bird influenza has been shown to infect and sicken house cats, which has some experts worried that there may one day be a strain of severe flu that can pass easily from human to human and create a pandemic.

The flu strain in question is the 2003-2004 outbreak in Asia of the H5N1 virus, which caused massive poultry slaughters in eight Asian countries and also led to at least 34 human infections, at least 23 of which were fatal. During the outbreak, there were also anecdotal reports of fatal infections in cats.

The news is startling to some because domestic cats had been thought to be largely resistant to actually getting ill from influenza A viruses such as this one, even though they could be infected.

But others said the findings don't point to impending calamity. "We're still a long way off from saying cats are vitally important in this chain," said Dr. Susan McLellan, a tropical medicine and infectious diseases expert at Tulane University School of Medicine and School of Public Health. This report also has nothing whatsoever to do with the average North American house cat, McLellan emphasized -- only cats that spend a lot of time on farms in Asia with infected birds. McLellan was not involved in the study, which is reported in the Sept. 3 online issue of Science.

The researchers tested whether domestic cats would become sick if the H5N1 virus was introduced into the airways or by feeding infected chickens to the cats. The six cats in the study all developed severe lung disease and transmitted the disease to two additional cats that were living nearby.

When cats were exposed to another virus, H3N2, which most commonly causes flu in humans, the felines did not get sick.

"The novelty of our study was not so much that this H5N1 virus spreads from birds to cats, because previous studies have shown that cats can be infected with other influenza virus strains," said study author Thijs Kuiken, a veterinary pathologist in the department of virology at Erasmus Medical Center in The Netherlands. "The novelty is that H5N1 virus causes disease and can be fatal in cats. In most previous experimental studies, influenza virus infection did not cause clinical signs or death in cats."

What does this mean for human health?

"First, cats should be considered as a potential source of infection for humans on farms where poultry are infected with H5N1 virus, and where cats have access to these poultry or their feces," Kuiken said. "Cats also should be considered as a potential source of infection for humans if they have been fed on carcasses of poultry infected with H5N1 virus."

Still, poultry are more likely to transmit the virus to humans than cats, as cats shed less of the virus, Kuiken added.

On the other hand, because they are mammals, infected cats may provide an opportunity for the bird virus to adapt to other mammals, including humans. "The concern about species-to-species transmission [including cats to humans] is that it increases the risk of developing a virus strain that is easily transmissible from human to human," Kuiken explained. "It is such a strain which could trigger the pandemic that many experts have been predicting."

The scenario experts have been dreading is that two different flu viruses can exchange chunks of genetic information and make a virus that spreads from human to human much quicker and easier than one virus that mutates over time. "The worrisome part is putting two different types of influenza viruses in the same organisms [for example, a cat], they then may recombine and we'll get the worst of both," McLellan said.

The chances aren't high, however.

"There was no mutation in the virus. This has probably been happening to cats for years and nobody bothered to look," McLellan said.

More information

The U.S. Centers for Disease Control and Prevention has more on the bird flu.

SOURCES: Thijs Kuiken, D.V.M., Ph.D., veterinary pathologist, department of virology, Erasmus Medical Center, Rotterdam, The Netherlands; Susan McLellan, M.D., tropical medicine and infectious diseases expert, Tulane University School of Medicine and School of Public Health, New Orleans; Sept. 3, 2004, Science online
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