Debate over pets' role in spread of avian flu
No cases passed yet by dogs or cats, but experts want study
Bernadette Tansey, Chronicle Staff Writer
Monday, June 12, 2006
If the bird flu virus arrives in the United States on the wings of migrating wildfowl, pet owners may need to take special precautions to protect their household members -- both human and animal.
No case has been reported anywhere in the world of a human being infected by a cat or dog, but researchers are calling for heightened study of the question before a potentially deadly avian flu virus extends its global reach.
Scientists are looking at animals in general as possible contributors to the spread of the dangerous H5N1 virus that originated in birds in Asia. Pigs raised close to infected poultry, or wild animals living near sick migratory birds, might help the virus break out of the bird population and invade mammal populations in farms or forests.
But health authorities are also looking at pets because they often live a dual life -- roaming outdoors, chasing birds and then returning home to nuzzle and play with people. Cats have caught the H5N1 virus by eating infected birds, though the reported cases are rare. So far, most experts think the risk is very low that pets could pass the disease to humans. But they can't say for sure.
"Cats can serve as an intermediary to move the virus from essentially a wild bird environment into people's houses,'' said Walter Boyce at the UC Davis School of Veterinary Medicine.
Health authorities say the most likely scenario would place any risk of infection solely with the pets. People who live near an outbreak of the virus in birds may need to keep their cat or dog indoors for a time, just to protect the animal from getting sick. But other scientists say pets can't be dismissed as possible carriers of the virus into the human population. They also speculate that cats or other animals could incubate an H5N1 strain that would be better equipped to infect mammals, including humans.
So far, the H5N1 virus is still primarily a bird disease that can devastate poultry flocks, but is hard for people to catch. Of the 225 people infected since 2003, almost all have been in close contact with stricken birds. The virus does not appear to have gained genetic traits to ease its direct transmission from person to person. That's the feared mutation that could touch off a deadly human pandemic, making the lethal bird virus as easy to catch as the ordinary winter flu.
But the disease's death rate is so high, even in its current form as a bird-borne virus, that scientists are examining every other possible route by which it could infect people -- even if that route seems unlikely. Of the 225 cases in humans, 128 have died, according to the World Health Organization.
Cat species have come under particular scrutiny because they are known to be vulnerable to H5N1 infection. Tigers and leopards in Thailand zoos died in 2004 after they ate infected raw chickens. By the same route, cats have caught the disease and died in scattered cases in Germany, Iraq and possibly in other countries.
Much less is known about the bird flu's possible impact on dogs. The American Veterinary Medical Association says a single unpublished paper from a health agency in Bangkok noted that dogs can be infected with the virus, but they don't seem to get sick.
The close, affectionate contact between many pets and their owners makes them a candidate for the study of disease transmission. "I know people who get woken up because the cat's lying on top of their nose,'' Boyce said.
Right now, the risk that an animal in the United States will contract the virus by hunting birds is near zero, because the virus hasn't been detected here.
In countries where the virus has arrived, no human infections have been linked to the cats that sickened or died of the disease. At this point, U.S. and world health authorities say it's unlikely that cats will infect people.
"We don't believe that cats pose a risk to humans at this time,'' said Ben Sun, a public health veterinarian with the California Department of Health Services. "But we have so little information that we can't make a confident recommendation that they do not pose any risk.''
A team of scientists from the Netherlands raised a stir in April when they accused government health agencies of doing too little to monitor the presence of H5N1 in cats, dogs and other carnivores. Officials shouldn't downplay the risk to humans when so many questions remain unanswered, said virologists Thijs Kuiken and Albert Osterhaus and their colleagues in a commentary in the science journal Nature.
The research team reported that cats can pass the virus to each other and can also shed virus particles before they show symptoms. Fortunately, cats release far fewer virus particles than chickens, which may make them much less efficient transmitters of the disease. Whether asymptomatic cats or dogs could infect people is still an open question.
That said, the biggest worry about H5N1 is that the virus will mutate into a form that passes easily from one person to another. That's the critical step that scientists fear could unleash a pandemic killing millions worldwide, and that crucial modification, say the virologists in the Netherlands, could take place in a cat or other host animal.
Some scientists postulate that pigs are the mammals most likely to incubate a human-adapted pandemic virus. In Asian countries where bird flus in the past have evolved into human influenza strains, pigs are often kept in close proximity to poultry and people. A pig might simultaneously become infected with a deadly bird virus and an ordinary human-adapted seasonal influenza, the theory goes. Viruses in the same host have the ability to swap genes, so the bird flu could acquire genes that make it easily transmissible from person to person.
If those key mutations occur, in no matter what host, it would be people -- not animals -- who would become the most effective vehicles spreading the disease worldwide, as they cram into commuter trains or hop on jets to other countries.
"Then we may be the ones spreading it to the cats,'' said Bruno Chomel, a UC Davis expert on zoonoses, the study of the role of animals in disease transmission.
No vaccine against H5N1 is available at doctors' offices, although the government has ordered a stockpile of experimental vaccines that it would carefully dole out in the event of a U.S. outbreak in people. In addition, antiviral drugs like Tamiflu might alleviate symptoms of avian flu.
Although experts aren't able to give definitive answers about the possible role of pets in bird flu transmission, Boyce at the UC Davis veterinary school wants cat and dog owners to learn about the issue now. Then they can take appropriate precautions, he said, but avoid wild overreactions if and when the bird flu arrives here.
Boyce fears that Americans will act on the same fearful impulse that led scores of Europeans to surrender their cats unnecessarily to overwhelmed animal shelters. Authorities had only ad- vised cat owners to keep their cats inside in regions where the bird flu had cropped up in wildfowl.
"What I hope will not happen is that people will abandon their animals because they're afraid of them,'' Boyce said.
Even if no risk of pet-to-human transmission ever proves real, cat owners can guard against the known risk that their animal companions could become infected by sick birds.
In the United States, owners who are already concerned about H5N1 exposure can maintain a cat's risk at zero by keeping it indoors. Many veterinarians already recommend this to protect cats from cars, fleas, fights with other animals, and germs other than the bird flu virus.
"Indoor cats live a lot longer than outdoor cats,'' said Heather Coburn, medical director of Tony La Russa's Animal Rescue Foundation in Walnut Creek.
Other owners may want to wait and watch the news. The avian virus has not yet appeared in this country, though surveillance teams are testing poultry and wildfowl to detect its arrival.
Sun, the state public health veterinarian, said there's no vaccine against the deadly H5N1 avian flu virus for dogs or cats. But owners can limit contact between wild birds and their cats, dogs and pet birds. Cat owners should wash their hands thoroughly after changing kitty litter, Sun said.
Keep away from feral or unknown cats and minimize your cat's contact with them, he said.
If your cat is sick, call the vet. Cats infected with H5N1 virus may become weak, inactive and feverish. Their breathing may become labored, and they can die suddenly.
If your cat dies, avoid direct contact with the body. Call the local animal control agency for disposal instructions, Sun advises.
At some point, if the bird flu risk seems high in a certain region, local health agencies might take measures to control feral cat populations, Sun said. But no such moves are contemplated now. "Most people don't have intimate contact with feral cats,'' said Sun.
Chomel of UC Davis said people shouldn't be kissing their cats, or allowing them on the kitchen counters. He said he was appalled at a recent survey reporting that nearly half of pet owners slept with their animals. "That should not be done,'' he said.
Cat owners in rural areas may be more likely to face the need to confine their animals indoors, Chomel said, because bird flu outbreaks are more likely to arise there. But the danger may pass, allowing the cat to go out again.
Pet owners should stay updated on the issue, but avoid panic, said Chomel.
"We are right to be concerned, but we should not be overly alarmed,'' he said.
E-mail Bernadette Tansey at btansey@sfchronicle.com
