Two more Minnesota turkey farms have been found infected with bird flu Thursday, bringing the total number of farms affected to five, according to the Minnesota Board of Animal Health.

The fifth and latest commercial turkey farm to be found with the H5N2 strain of avian influenza is in Stearns County; it's the second farm to be affected in that central Minnesota county. The USDA said avian flu was found in a flock of 71,000 turkeys.

The fourth farm to be found with the infection, in Nobles County, has three barns with one housing about 7,000 turkeys affected by the disease. However, a total of 21,000 turkeys either died or were killed to stop the disease from spreading.

Minnesota Board of Animal Health state veterinarian Bill Hartmann said backyard and commercial flocks within a 6-mile radius of the southwest Minnesota farm are being tested and quarantined. Others in a 12-mile radius — which extends into Iowa — are notified and advised to watch for unusual deaths.

Minnesota Board of Animal Health workers cleaned and discarded their protective gear at a turkey farm in Nobles County. Jackson Forderer for MPR News

He said it's unclear how the virus is getting into the barns.

"So far investigations haven't found an answer to that," Hartmann said. "But what we do know is there is no connection between these farms."

Avian flu investigators looking for the cause of the deadly disease think migrating ducks could be the key. Now they are looking for proof.

The Minnesota Department of Natural Resources is looking for waterfowl that may carry the highly pathogenic influenza disease that's been killing turkeys around the state.

Wildlife health program supervisor Michelle Carstensen said the DNR has collected 148 environmental samples from resident waterfowl in frozen and open water in Pope County, which had the first confirmed case of avian flu in a commercial turkey flock in March.

A Minnesota Board of Animal Health worker carried a barrel out of the back of a pickup truck at a turkey farm in Nobles County. Jackson Forderer for MPR News

Just two samples turned up positive for avian influenza, but they were found to be a low pathogenic strain, she said.

Pathogenicity refers to the ability of the virus to produce disease, according to the U.S. Department of Agriculture. Turkeys contract avian influenza from fecal matter passed on by waterfowl that carry the disease.

"At this point we haven't had any confirmation of high path (avian influenza) in wild birds in the Mississippi flyaway area," Carstensen said. "And we also haven't had any reports of dead raptors or wild turkeys in the state."

Wild birds can carry the disease without getting sick from it, Carstensen said. The DNR will continue to look for birds carrying the virus now that more birds are moving back up into the area as the weather warms up in Minnesota.

A turkey feather lay in a ditch near a farm infected with the Avian flu in Nobles County. Jackson Forderer for MPR News

"Birds moving in and intermixing that can be definitely a mechanism for spread and distribution of this virus," Carstensen said.

In addition to Nobles, Stearns and Pope Counties, avian influenza has turned up in a commercial turkey flock in Lac qui Parle County.

A total of 19 people have had direct contact with turkeys in the first four farms where the infection was found, according to the Minnesota Department of Health. The virus does not pose a public or food safety risk.

State Public Health Veterinarian Joni Scheftel said 14 of those workers were evaluated and advised to take anti-viral medication. Eight agreed.

"None of these individuals has developed any influenza like symptoms while they're being monitored," she said.

No human cases of avian influenza have been detected in the U.S., Scheftel said.