The spread of avian influenza has prompted a fourth state, Nebraska, to declare a state of emergency a few days after the virus was discovered at a commercial egg farm housing 1.7 million chickens.

Declared by Nebraska Gov. Pete Ricketts, the state of emergency allows officials to tap into extra funding to combat the flu. Nebraska joins Wisconsin, Minnesota and Iowa in declaring a state of emergency.

The U.S. Department of Agriculture’s website says that more than 33.5 million birds in 15 states have been affected by the flu following 162 individual detections since mid-December, when the influenza first was detected in a backyard flock in Oregon. But the agency says the risk to humans remains low.

“No human cases of these [avian flu] viruses have been detected in the United States, Canada or internationally,” the department’s website says.

The news didn’t seem to affect poultry stocks either. Hormel Foods Corp. HRL, -1.53% was unchanged at $56.72 while Tyson Foods Inc. TSN, -2.69% tacked on 1% to $42.12. Sanderson Farms Inc. SAFM, -2.24% was up marginally to $82.25 and Pilgrims Pride Corp. PPC, -2.45% dipped nearly 1% to $25.48.

Nebraska reported the virus to agriculture officials on Monday, and it was the only one listed on the department’s site, though the number of birds affected was relatively large compared with other reports. The virus hit a farm in Dixon County, bordering Iowa, a state where dozens of cases have been reported since December.

A heavy concentration of these cases has been reported in and around Iowa and Minnesota, with some in western Wisconsin near the Minnesota border. Since the Nebraska finding, strains of the virus were reported in southern South Dakota near the Nebraska border on Wednesday, affecting flocks of roughly 140,600 commercial turkeys.

Also on the heels of the Nebraska discovery, several other cases in Minnesota and Iowa were reported, affecting mostly chickens but some turkeys. Officials from two poultry organizations didn't return messages seeking comment on how this flu strain would affect chicken supplies. Numbers show that the bulk of the 33.5 million birds are chickens.

Keith Williams, spokesman for the National Turkey Federation, said that while a number of Midwest turkey farms have been hit hard, less than 2.5% of the nation’s 240 million of the Thanksgiving centerpieces have been affected by the strain.

“They evolve in different places, depending on where they are geographically,” he said.

Williams added that the bulk of turkeys cooked for Thanksgiving are female hens, and noted that the virus is largely affecting the male, or tom, turkeys.