The question what causes the current outbreaks in Minnesota is heavily discussed, especially the role of migratory birds. USDA APHIS tends to the opinion that migratory birds play an important role. In that case, H5N2 will likely remain a threat to U.S. poultry for three to five years, Olson said, citing information from wildlife experts. That is how long it will take wild birds to develop immunity to the disease. Since the beginning of the year, the flu, which can kill nearly an entire poultry flock within 48 hours, has also been found in birds from Oregon to Arkansas. The discoveries have prompted major overseas buyers such as Mexico and Canada to limit imports of U.S. poultry and companies such as Tyson Foods Inc to strengthen measures to keep the disease off farms.

The number of infections is climbing as migratory ducks, which are believed to be spreading the disease, return to Minnesota to breed after spending the winter farther south, said Beth Thompson, assistant director of the Minnesota Board of Animal Health. The larger number of ducks likely increases the risk for wild birds to transmit the virus.

Farm workers are probably infecting turkeys by tracking the virus into barns after stepping in contaminated duck feces, said John Glisson, vice president of research for the U.S. Poultry and Egg Association. Chicken flocks are also vulnerable. “Minnesota is a real hotbed for returning waterfowl,” Glisson said. The USDA has said it believes migratory ducks are spreading the flu and sent a team to Minnesota to determine how it is moving into poultry flocks. So far, efforts to stop the spread by controlling human and vehicle traffic on farms have not worked.

The number of infections may continue to rise through mid-May, when spring migration ends, said Steve Olson, executive director of the Minnesota Turkey Growers Association. New cases may accelerate again in the autumn when recently hatched ducks, which have never been exposed to the virus, begin migrating south, he added.

In the absence of reliable epidemiological data about the spreading under wildfowl population, the source of infection will continue to spark speculations about the role of migratory birds.

Despite all evidence, there are scientists who have reason to doubt the thesis that migratory birds are to blame, like Professor David Stallknecht from University of Georgia’s College of Veterinary Medicine. David is openly questioning the USDA’s conclusion. David Stallknecht called the notion that avian flu originated in wild birds “pure speculation” in a story posted by the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP).

David Stallknecht: “It is based on circumstantial evidence that is rapidly becoming accepted dogma. The avian flu saga dates back to late last year, when H5N2 struck several farms in British Columbia. That sparked surveillance programs in the U.S., which turned up cases of pintail duck virus in Washington and a related strain in a captive gyrfalcon nearby. Since then, cases of H5N2 have shown up in poultry in several states, including Idaho, Minnesota and Arkansas.”

In order to stop large-scale outbreaks, more efforts have to be undertaken to protect commercial flocks and to predict future outbreaks. Even more reasons to increase the active surveillance efforts, including the crucial active surveillance programs under wild birds, like Stallknecht suggests.