The work, which hasn't yet been published, suggests these two types of birds may be key to the explosive outbreak of the new flu that erupted in China earlier this spring.
The research, done at the U.S. Department of Agriculture's Southeast Poultry Research Laboratory in Athens, Ga., also found something interesting about the way this virus acts in poultry.
In birds, influenza is an infection of the gut, not the respiratory tract. And when chickens or ducks or other infected birds shed flu viruses, it is normally via the cloaca, the anus. But the chickens and quails infected at the Athens laboratory shed H7N9 virus from their nasal passages. That suggests if people testing birds for the virus only swabbed cloacal passages, they might not find evidence of infection.
Dr. David Swayne, director of the laboratory, says the research makes it clear that chickens and quails played a key role in the spread of H7N9 virus in affected parts of China.
The work also suggests that pigeons, ducks and geese are probably not major players in the spread of H7N9, Swayne says. His scientists tried to experimentally infect those types of birds, and found that while the birds developed infections, they did not shed the virus to any significant degree. It is through shedding that infected birds would pass the virus to healthy birds.
"I think that kind of points the finger to say, hey, you know, what's really going to be important in the field is not thinking about ducks and geese as being the primary source of the virus growing and spreading, it's chickens and quail," Swayne said in an interview.
"The data we produced experimentally ... would suggest that chickens, possibly quail could be amplifiers of the virus and are causing a lot of environmental contamination. Pigeons appear not to be really ... perpetrators in spreading this virus around."
Swayne says the virus is very infectious for chickens, and infected birds give off a lot of virus through their mouths and nasal cavities. The virus grows even better in Japanese quail, another species tested by the Athens labs, and those birds too shed high amounts of virus.
"This virus is very well adapted for chickens and quail and it may be for other related poultry species, what we would call gallinaceous poultry," he says, a term that includes turkeys, chickens, pheasants, quail, partridge and grouse.
"And when they become infected, they put a lot of virus into the environment."
The H7N9 virus was unknown before the end of March, when China declared it had found three human infections caused by the newly spotted virus. Spread was rapid, climbing over 100 cases in less than a month. To date there have been 131 confirmed cases in China and one in Taiwan, in a man who had been working in China, and 38 of the infections have been fatal.
Since early May, however, few new infections have occurred. Experts put that down to a combination of two factors. China closed live poultry markets in areas where human cases occurred. And human infections with bird flu viruses tend to drop off during the spring and summer in affected countries — only to pick up again when temperatures fall in the autumn.
At this point it is impossible to know whether the virus has been contained or will return. That, in part, is because finding the source of the virus has proved to be unusually challenging.
H7N9 is what is known as a low pathogenicity, or low path, bird flu virus, which means it does not make poultry sick. As a result, in nature the virus has been virtually invisible, spotted only when it infects people. In contrast, the H5N1 bird flu virus is highly pathogenic, or high path, and wipes out huge numbers of birds as it spreads.
Chinese authorities have struggled to find where people were coming in contact with the virus, finding few positive hits in tens of thousands of tested birds.
The discovery that the virus is not shed from the cloaca in infected birds makes it tempting to think people sampling poultry in China may have been swabbing the wrong end. But Swayne, who was part of a mission of experts sent to China in April by the World Organization for Animal Health, suggests that probably isn't the problem.
He says in the United States, people testing birds for flu viruses would swab both ends of the bird; that's standard protocol. And in China? "They take both swabs," he says.
Swayne struggles to explain why so few poultry have tested positive for the virus. Given the number of human infections, he says he would have expected the Chinese would have found a lot of virus in the live markets and on farms that supply those markets. But so far, the virus has not been found to be running rampant in poultry farms.
"You can look at the data and say hey, one reasonable interpretation is this virus is not very common on farms," he says. "And occasionally ... chicken or quail are infected and they go to the market and it amplifies in the market and get more birds infected in the market. That's a very plausible hypothesis based on the data right now."