The report, which suggests the dog became infected by eating ducks killed by the virus, also underscores a need to figure out whether the virus can be transmitted through consumption of infected animals, a World Health Organization scientist said Wednesday.

“This is the third species or fourth species that has been infected by eating carcasses. So I think we really have to think about the risk of oral ingestion,” said Michael Perdue, an avian flu expert with the WHO's global influenza program.

“I mean, these guys are getting infected somehow and we don't know how.”

Since H5N1 flared up in Asia in late 2003, tigers, leopards, domestic cats and now dogs have become infected with the virus by eating infected chicken or duck carcasses. Other mammals — a stone marten, and a small number of pigs — have also been shown to be susceptible to infection, though in those cases the mode of transmission isn't yet documented.

There have also been some human cases where it's thought ingestion of virus was the mode of infection — most notably a trio of brothers in Vietnam who fell ill after eating uncooked soup made from duck's blood.

Influenza infection occurs in the respiratory tract, when the mucous membranes of the nose and throat come in contact with viruses propelled through the air by sneezes and coughs. A person can also become infected by touching items onto which viruses have been sneezed and then touch their nose or mouth.

It's not thought that infection can occur in the human gastrointestinal tract. And the WHO's official position is that there is no evidence people can become infected by eating properly cooked poultry or eggs.

When tigers and leopards cats in Thai zoos died after being fed infected chickens in late 2003 and again in 2004, scientists speculated infection occurred when the animals ripped apart the carcasses. They believe the cats breathed in viruses that had been lodged in the birds' feathers.

Perdue said that may indeed be the way these infections have occurred. But he questions whether something else might be at play — and thinks the scientific community ought to nail down the answer.

“Is there tissue that's infectible before it (the infected meat) gets to the stomach? That's the other option. Esophogeal tissue or mucosal tissue (in the gastric tract) that allows the virus to infect?” he wondered in an interview from Geneva.

The case of the dog, reported in the November issue of the journal Emerging Infectious Diseases, actually occurred in October 2004. Scientists and people who follow H5N1 developments on Internet blogs and websites were aware of it, but this article by scientists from Thailand's Kasetsart and Chulalongkorn universities is the first scientific publication showing canine infection with the virus.

The dog developed high fever, panting and lethargy about five days after eating infected duck carcasses and died the following day, the scientists reported. H5N1 viruses were recovered from the dog's lung, liver, kidney and urine.

The authors say the proof that dogs too can be infected with this virus “warrants concern and highlights the need for monitoring domestic animals during outbreaks in the future.”

Other scientists said that while it is important to determine whether dogs are becoming infected and could potentially transmit the virus to other animals or back to poultry, the evidence to date suggests they aren't falling victim to nearly the same degree as cats.

There have been multiple reports of domestic and stray cats dying from H5N1 infection in several Asian countries, especially badly hit Indonesia which has the highest H5N1 death toll of any country.

“We'll have to look into it,” said Dr. Peter Roeder, an animal health officer with the UN's Food and Agriculture Organization.

“(But) we've not seen any evidence for dogs . . . becoming sick. So it doesn't feature very highly in our understanding at the moment. But perhaps it's something else we ought to look at.”

Another expert, Dr. Ab Osterhaus, said it would be important to try to infect dogs in a laboratory, then see if they are able to transmit the virus to nearby uninfected dogs. Such work, which Dr. Osterhaus's lab undertook with cats, would indicate how easy or difficult it is to infect dogs and whether they are able to spread the virus.

“That type of experiment will tell you a little bit about the relative risk for dogs to become infected,” said Dr. Osterhaus, director of the Institute of Virology at Erasmus University in Rotterdam. He said his lab intends to conduct this experiment.

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