ROTTERDAM, STOCKHOLM, LONDON–– “Cats could fuel bird flu pandemic,” headlined the April 5 edition of The Times of London, sparking similar headlines worldwide––but the risk is small, responded the European Centre for Disease Prevention and Control in Stockholm, Sweden, after reviewing the evidence.

“A distinction needs to be made,” reminded the European Centre, “between species which can occasionally be infected by a particular influenza, but who rarely transmit it,” such as cats, “and those species in which it seems that the viruses are better adapted and transmitted,” such as birds.

Cats were first known to be vulnerable to H5N1, the European Centre response continued, in December 2003, “when a few leopards and tigers died in a zoo in Thailand after being fed infected poultry.” Later came “a much larger H5N1 outbreak in zoo tigers, also in Thailand, who had been fed chicken carcasses. Over 140 tigers died or were euthanised. There was convincing evidence of tiger to tiger transmission.

“Experimentally,” the European Centre acknowledged, “it has been shown that domestic cats can be infected with H5N1 through eating infected material, and can transmit influenza to other cats. These experimentally infected cats, though limited in number, all became seriously ill when infected, and did not seem to shed the virus until they had symptoms. To date the only domestic cats who have been conclusively shown to be infected have been those found ill or dead in the intense epizootic of H5N1 in wild birds on Rugen Island,” off the German coast, “in February 2006.

“There have been anecdotal reports of increased mortality in cats during H5N1 outbreaks in other countries,” the European Centre warned, “ but these have not been confirmed by laboratory tests. Unconfirmed reports of infections and deaths from H5N1 in cats elsewhere should always be interpreted cautiously. A recent preliminary report of infected cats in Austria was eventually not confirmed.”

The Times article, by science correspondent Mark Henderson, summarized a paper published in the April 6 edition of Nature by virologist Albert Osterhaus and colleagues at the Erasmus Institute in Rotterdam.

“As soon as the virus becomes endemic in wild birds or poultry, it would be wise to realise that cats are susceptible,” Osterhaus told Henderson. “As soon as you have birds that become sick, cats are very effective at catching and eating them. In endemic areas you should keep cats indoors and dogs on a lead.”

“Affected regions of Germany and France have already ordered that cats be kept indoors,” Henderson reported, “but the [British] Department of the Environment, Food and Rural Affairs has not yet said whether it would make similar provisions.”