Migratory waterfowl, according to Texas Agriculture Commissioner Sid Miller, are responsible for the widening avian flu outbreaks affecting Texas cows and poultry.
He stated on Tuesday that wild birds carrying the virus are likely to be heading north soon.
Since last week, the U.S. government has reported cases of the disease in seven dairy herds in Texas, along with one person who had contact with cows.
This has made Texas the state most affected by the country's first-ever outbreaks in cattle, given that it is the biggest U.S. cattle producer.
The cases in dairy cattle and the second human case in two years in the United States have raised concerns about the virus, which has been infecting poultry flocks and a growing number of other species globally since 2022.
A positive test at a Texas egg farm led egg producer Cal-Maine to cull 1.6 million laying hens, the company said on Tuesday.
According to Miller, Texas had never before experienced such a major outbreak at a commercial poultry facility.
"This is spread by waterfowl," Miller explained in an interview, noting that it's migratory season.
The U.S. Department of Agriculture (USDA) first reported on March 25 that a cow and milk from two dairies in Texas tested positive for bird flu, along with milk from two dairies in Kansas.
The agency later confirmed positive tests in additional dairy herds in Texas, New Mexico, Michigan, and Idaho.
The strain of the virus found in the subsequent states is very similar to the strain confirmed in the initial cases in Texas and Kansas, suggesting that it was introduced by wild birds, the USDA said. "We're ready for the ducks to head north to their nesting grounds," Miller said.
"We think within a week or a little longer they'll all be out of Texas, and we'll be out of the woods."
USDA stated that transmission of the disease between cattle cannot be ruled out.
The U.S. Centers for Disease Control and Prevention considers the risk of bird flu for humans to be low. The Texas patient's only symptom was eye inflammation, according to the state's health department.
Richard Webby, a virologist at St. Jude Children's Research Hospital in Memphis, highlighted that testing for flu in cattle is not routine and emphasized the importance of establishing the connection between the sickness in cows, and ducks, and cats on the farms.
"With these cases, people will start looking for similar events in Europe and Asia," said Webby, who is also the director of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds.
The Texas outbreak may have started about a month ago when a mysterious illness affected about 40% of the state's dairy herds, Miller said.
He now suspects it was bird flu, though officials did not know it at the time and can't confirm it because the animals recovered.
"We were testing for every cattle disease we could think of, and then somebody said, 'What are all these dead birds doing around the dairies?'" Miller recalled.