During the outbreak of avian influenza A (H5N1) in Asia in 2003 - 2004, there were 34 documented cases of bird-to-human transmission, of which 23 resulted in death. Like other avian strains, the deadly H5N1 virus appears unable to pass from human to human. In the midst of this health emergency, however, a story came out of Thailand about the infection of three pet cats, as well as a white tiger from a zoo, resulting from the consumption of flu-stricken chickens. Concerns immediately arose about the possibility of cats transmitting the disease to humans.
A group of Dutch researchers at the Erasmus Medical Center in Rotterdam, the Netherlands, set out to confirm the anecdotal evidence of feline vulnerability to avian influenza. As described in a report published online today by the journal Science, Thijs Kuiken and his collaborators injected three domestic cats with a H5N1 virus that had been isolated from a fatal human case in Vietnam. After one day, the cats showed signs of illness; by the third day, virus excretions were found in their respiratory systems. One of the cats unexpectedly died only six days after infection. Autopsies of all three animals revealed pathologies similar to those seen in humans and primates infected with H5N1.
In other tests, the researchers determined that the virus was capable of spreading from cat to cat, as well as through the consumption of infected birds. The full implications of these findings are not yet known, but because cats can move between farms and often into households, the possible pathways of the disease will have to be reassessed. One cause for alarm is that cats, being mammals, are closer to humans than birds. "Whatever tricks the virus picks up in infecting cats could help it in infecting humans," says Richard J. Webby of St. Jude Children's Research Hospital. He admits that more data is necessary to establish this adaptation mechanism, but there have been indications that past strains of avian influenza used pigs as intermediate hosts before infecting humans. --Michael Schirber