Cats Can Catch and Spread Bird Flu, Study Says

Stefan Lovgren
for National Geographic News

September 2, 2004

A bird flu virus killed 26 people in Asia and led to the vast slaughter of poultry several months ago. Now a new study says the flu can also infect cats, and that cats can spread the flu to other cats. The finding raises the possibility that they may eventually spread the flu to other mammals, including humans.

Scientists previously believed domestic cats were resistant to diseases from influenza A virus, to which the bird flu virus—also known as H5N1—belongs.

The new research suggests that domestic cats are at risk of disease or death from the avian virus. The cats can also play a role in the transmission of the virus, scientists say.

"That cats could so easily be infected and could transmit the infection to other cats means that in areas where poultry are infected with H5N1 virus, cats could act as vectors," said Thijs Kuiken, a veterinary pathologist in the department of virology at the Erasmus Medical Center in Rotterdam, Holland.

"Cats could transmit the virus from one poultry farm to another, or could transmit the virus to people," said Kuiken, an author of the study described in tomorrow's issue of the journal Science.

Surprising Answers

The avian influenza A virus was responsible for massive poultry slaughters in eight Asian countries in 2003 and 2004. Thirty-seven cases of direct bird-to-human transmission, 26 of them fatal, were officially reported. Anecdotal reports of fatal infections in cats also emerged during the outbreak.

Kuiken and his colleagues investigated whether the virus could make cats sick when the pathogen was introduced into their airways or when the cats ate infected chickens.

The researchers introduced the H5N1 virus into the airways of three cats. Three other cats were fed an infected chick. Finally, two cats were exposed to the virus by being placed in the same cage as the first three cats.

The cats soon showed signs of disease: raised body temperature, decreased activity, and labored breathing. All developed severe lung disease. One cat died after six days of infection.

"Were we surprised? Yes!" Kuiken said. "Although we had expected to see some pathologic change in the lungs—because of the anecdotal reports of cats dying from H5N1 virus infection in Thailand—we didn't expect them to be so severe and present in all animals."

"Often an infectious agent that causes mortality in the field has a much less severe effect when the infection is performed in the laboratory," Kuiken added. "With H5N1 virus, the experimental infection also resulted in severe lesions."

Human Infection

Previously there were no reports of disease in cats from infection with an influenza A virus. Kuiken says the evidence of transmission from cat to cat is a particularly important finding.

"With all of the human cases of H5N1 virus infection, the virus was transmitted from bird to human (bird to mammal), not from human to human (mammal to mammal)," Kuiken said. "Our study shows that H5N1 virus transmission from cat to cat (mammal to mammal) is possible."

The ability of the virus to infect cats suggests that cats could enable the pathogen to adapt to mammals. If the virus could replicate more easily in cats, it also might replicate more easily in humans, increasing the risk of a human influenza pandemic.

"That this H5N1 virus caused pneumonia and death in cats suggests that the virus has increased its ability to cause disease in this species—not that the species is less resistant," Kuiken said.

Risk Remote

Some scientists downplay the threat of the new findings.

"To be a significant new threat, the infection would have to keep the cat well enough to travel and spread and yet sick enough to maintain high doses of virus so that transmission is achieved," said Ian Jones, a professor at the School of Animal and Microbial Sciences at the University of Reading in England.

"This experimental situation shows this can occur. But it is a long way from the natural situation where a sick animal may hide away or die," he added. "So the real increase in risk may actually be quite small."

Michael Lai, a professor of molecular microbiology and immunology at the University of Southern California in Los Angeles, says the report should not scare people.

"It's true that the more animals the virus can infect, the more likely the virus can spread to humans," Lai said. "However, the possibilities that a cat serves as an additional reservoir for avian flu and that it may allow human and avian flu viruses to recombine are quite remote."

If the transmission occurred easily, Lai said, we should have seen widespread flu infection in cats in the avian flu epidemic areas in the past few years. This did not happen.

The study authors also tested the effect of another type of influenza virus, H3N2, which most commonly causes flu in humans. Cats exposed in the same way to this virus did not develop disease.

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