BioEd Online

Avian flu ravages tissues throughout the body, confirms an autopsy of infected cats. The finding suggests that the virus might infect people's guts through what they eat, and spread via contaminated faeces.

Fears about bird flu continue to balloon, and with its arrival in Turkey, the disease has a foot in the door in Europe. The H5N1 strain of the virus has killed more than half of those people it is known to have infected.

Because of fears that the virus will spark a human pandemic, researchers want to know how it is likely to attack the body and jump between people. But they have had little opportunity to answer these questions, in part because only a handful of human victims have been autopsied.

Caught in cats

So a team led by Thijs Kuiken at the Erasmus Medical Centre in Rotterdam, the Netherlands, carried out detailed autopsies of infected cats; these mammals are thought to be a reasonable model for human infections. This allowed them to catch a glimpse of the virus at the peak of an infection, rather than waiting until after death.

The team first reported that the H5N1 flu strain could infect domestic cats¹ in 2004, a discovery that was startling because cats were previously thought to be immune to the flu. In a follow-up study, published in the American Journal of Pathology this month², they carefully probed the tissues of eight infected animals.

The virus wreaks havoc in the brain, liver, kidney, heart and numerous other tissues, they find, killing cells and triggering inflammation. By contrast, the flu viruses that strike people in winter largely limit their damage to the nose and lungs.

This discovery backs up earlier studies in mice and ferrets, and may help to explain why the bird flu kills so many of the humans it infects. "It's promiscuous," says veterinary pathologist Corrie Brown, who studies infectious diseases at the University of Georgia, Athens. "It doesn't care what type of cell it invades."

Pass it on

The cat survey highlights two more worrying facts about the virus. The team find that H5N1 can be excreted in cat faeces as well as from the lungs.

Researchers think that people mostly catch the disease by breathing in virus from contaminated bird droppings, but it has not been clear that it could spread between mammals by a faecal-oral route too.

This suggests that avian flu might spread in water contaminated with people's infected faeces or urine as well as in coughed-out droplets. Should a human pandemic begin, this could be a major problem in developing countries where poor sanitation would fuel spread of the disease. "We do need to be aware," Kuiken says.

Careful what you eat

The team also find evidence, for the first time, that the virus can directly attack nerve cells in the gut of cats fed infected chicken meat.

This suggests the virus can directly attack the human intestine too, reinforcing current advice to avoid raw, infected meat. Kuiken says the finding could also explain two reported cases of human avian flu in which patients developed diarrhoea and encephalitis rather than the classic respiratory symptoms.

Researchers already knew that the H5N1 virus, like other pathogenic avian flu viruses, spreads throughout the body of birds. But they are only beginning to identify the genetic tricks that allow H5N1 to march into so many tissues.

Experts are still struggling to predict whether the virus will begin to spread swiftly between humans at all. "I think the greatest precaution we can take now is to control it in birds," Brown says, "That's the seething cauldron of the virus." Cats are not expected to be a major reservoir for the disease.