Cats can catch avian influenza HN51 by eating infected birds, and can pass it on to each other, according to a report published in Science this week. But experts are divided over the threat posed to humans by the discovery.

Thijs Kuiken's team at the Erasmus Medical Center in Rotterdam investigated anecdotal reports of cats dying from the flu virus–unusual because cats are not normally susceptible to influenza type A infections. "One of the messages [on ProMED-mail] reported that in an area where poultry were infected with H5N1 virus there was a household of 15 cats of which 14 died–and one of those had been eating a dead chicken," Kuiken told The Scientist.

Kuiken's team simulated the infection conditions in the laboratory by feeding infected chicks to cats. All of the cats developed inflammation in the lungs, which in some cases were quite severe. "In fact one of the cats died after 6 days," Kuiken said.

As well as confirming bird-to-cat transmission of the H5N1 virus, Kuiken's group went on to demonstrate the potential for cat-to-cat transmission. "One implication of the results is for cat health," Kuiken said. "Cats are at risk from H5N1 infection: they can become ill and die from it." He said the second implication is that the epidemiology of H5N1 needs to be reassessed based on the findings in these areas because cats have never been implicated as the cause of a human influenza infection.

But more serious were the economic and humanitarian implications, he said. The main concern is that the virus may become adapted to humans, may replicate easily in humans, and may be transmitted between humans. "The fact that the virus can infect cats offers the opportunity to become adapted to replication in mammals and by that, increase the risk of a human influenza pandemic," Kuiken said.

An additional report late last month from the World Health Organization (WHO) records infection in pigs, and Klaus Stöhr, from the WHO's Department of Communicable Disease Surveillance & Response said this certainly complicates the issue. "What it shows is that this is a very unusual H5N1 virus," he said. Stöhr, who was not involved in the study, said that not only was the virus infecting more hosts, but that it was also gaining pathogenicity.

He said that the virus with a higher pathogenicity had also "made it" into domestic ducks. "Normally ducks as aquatic birds would only be infected with the milder form of the H5N1 virus–so a low pathogenic strain–but data from China would clearly indicate that it has got another foothold in domestic ducks. So this virus is full of surprises," Stöhr said.

"Exactly how significant it is I'm not really sure that we can tell. If it's true that these cats can get sick, it would seem to be another way of spreading the virus around, that would be my major concern," Richard Webby, whose group was instrumental in developing vaccines for the virus, told The Scientist.

Webby, in the Department of Infectious Diseases at St. Jude Children's Research Hospital, Memphis, Tenn., said that the main concern with the report was not so much the fact that cats could catch the infection, but the fact that this virus can infect different hosts. "It's just that the more hosts the virus can get to, and evolve and tinker around with its genes, the more possibilities it has to do what it needs to do to get into humans," said Webby, who was not involved in the study.

Stöhr said that the additional information from the cat study should not be ignored. "But compared with the potential for causing epidemic in humans as well as in pigs, in comparison this is certainly nothing that should cause alarm," he said.