What needs to happen for an avian flu strain to jump from a duck to a human? Before 1997, researchers believed genes from a bird strain and a human strain had to mingle inside an intermediate host, such as a pig (antigenic shift graphic). Although that’s one way such a jump occurs, they now know it’s more complicated than that.
“The molecular features that allow a flu strain to jump species are poorly understood,” says Daniel Perez, Ph.D., assistant professor of veterinary medicine at the University of Maryland, College Park. “Unfortunately, we’ve barely scratched the surface.”
The NIAID-supported researcher is studying how one small, formerly overlooked bird could be playing a big role in the spread of flu in Southeast Asia from one species to another.
“We’ve discovered that quail can carry any one of 14 different strains of the flu, yet they show no signs of the disease,” says Dr. Perez. This is especially worrisome because, as quail show no symptoms of flu, they have a longer period in which to spread the disease. In addition, the virus multiplies primarily in their respiratory tract—same as humans—and can spread in the form of aerosol droplets in the air. (Other birds spread the flu virus mainly through feces.)
Dr. Perez and his research team are currently studying what it takes for several strains of influenza with pandemic potential—H2, H7, and H9—to jump from wild aquatic birds to quail. In one study, they are investigating how an H2 strain in mallards has adapted so that it can be transmitted to quail. They have found that the quail-adapted strain will cause illness in chickens while the mallard strain will not. Chickens can spread certain flu viruses directly to people.
In another experiment, they are comparing the infectivity of a mallard H7 strain, created through reverse genetics, with a quail-adapted version of the same strain by infecting mammalian cells growing in a nutrient-rich gel. The researchers have found that small holes called “plaques” (less than 1 millimeter in diameter) will form in the gel around cells infected with the mallard strain, indicating areas where the virus spreads. However, much larger plaques (2–3 millimeters in diameter) will form in the gel around the quail-adapted strain, suggesting that the quail-adapted strain has gained molecular features more compatible with growth in a mammalian host.
back to top
Last Updated December 05, 2006