Any cat that becomes ill or dies when suspected bird flu is circulating should be tested for the virus, the Navy team reported in the August issue of the journal Emerging Infectious Diseases.

The team at the Naval Medical Research Unit No. 3 or NAMRU-3, based in Cairo, have been studying bird flu viruses taken from animals and people in the region.

The H5N1 avian influenza virus spread out of eastern Asia and into Europe and the Middle East late in 2005. It has been found in 48 countries since it re-emerged in 2003, mostly in birds.

It can infect other animals as well as humans, and has so far killed at least 134 people in 9 countries. Experts are afraid it may evolve just enough to pass easily from person to person, sparking a pandemic that could kill millions.

Samuel Yingst, Magdi Saad and Stephen Felt of NAMRU-3 had been hearing stories from veterinarians in Turkey and Iraq who said cats had died where bird flu outbreaks were being reported in January.

But they could not get any samples from the cats.

"After H5N1 influenza was diagnosed in a person in Sarcapcarn, Kurdish northern Iraq, the government of Iraq requested a World Health Organization investigation, which was supported in part by Naval Medical Research Unit No. 3 veterinarians," they wrote in their report.

People told the WHO team about cats that had died in a house near the city of Erbil where 51 chickens died. The researchers got the bodies of two of the cats and a sick goose from next door.

The animals had flu virus throughout their bodies, Yingst and colleagues reported. The virus found in the cats and goose strongly resembled the virus from a person who died in Iraq, suggesting it had not become adapted to cats.

The researchers said their findings support the idea that cats can be infected with H5N1 and may play a role in transmitting it, and that the virus could possibly mutate in the bodies of cats.

Flu viruses change in two ways -- by steady mutation, which H5N1 has been seen to do, and by reassortment, which means swapping genes with other flu viruses. In 1957 and 1968 pandemic influenza broke out after the H3N2 viruses reassorted with other viruses.

Cats are mammals and biologically closer to humans than birds are, so in theory a virus that can easily infect a cat could infect a person more easily than a purely avian virus, which H5N1 remains.

"The route of infection in these cats cannot be determined definitively. How cats behave when eating birds makes both oral and respiratory infection possible," the researchers added.

A cat died of bird flu in Germany in March and Austrian experts said a cat there was infected with H5N1 a week later but did not get sick.

The Iraqi cats were infected with a distinct strain of the H5N1 virus known as Clade II, which was first found in migrating birds in Qinghai Lake in western China in 2005, the NAMRU team said. "To our knowledge, this is the first report of a Qinghai-like virus detected in domestic cats," the report reads.