The U.S. bird-flu outbreak in dairy cattle is much larger than official figures suggest due to farmers' reluctance to test their animals and risk the economic consequences of a positive result, according to Reuters interviews with dairy experts, veterinarians, and farmers in six states with known cases.

The U.S. Department of Agriculture has counted bird flu in about 190 dairy herds in 13 states since March. The virus's jump from birds to cows heightened concerns that it could adapt to spread among humans. Scientists have warned that limited surveillance could weaken the U.S.' ability to respond to further human spread. Thirteen dairy and poultry farm workers have been infected with bird flu this year, according to the Centers for Disease Control and Prevention.

Reuters spoke with more than a dozen researchers, veterinarians, farmers, and livestock industry groups to understand whether the bird-flu spread in dairy cattle is being accurately tracked.

State animal and human health experts in three states who work closely with veterinarians and farmers said the government tally is likely an undercount because farmers are fearful of the economic hardship brought by a positive test, including being restricted from selling their milk or cattle for weeks. The virus reduces milk production in cattle. The U.S., the world's second-largest cheese producer after the European Union, is the only country with known infections in cows.

"While we have nine official positives, there are many, many, many more farms that are impacted or infected that are just not testing," said Joe Armstrong, a veterinarian and cattle expert at the University of Minnesota, who has spoken with farmers across the state. A more accurate cattle case count for Minnesota would be three to five times higher, Armstrong said.

A USDA spokesperson said the agency has encouraged testing by requiring negative tests for cows being shipped over state lines since April and offering a voluntary program for testing farmers' milk supplies weekly. Twenty-four dairy herds are participating in that program, of roughly 24,000 farms nationally that sell milk, according to agency data.

Six farmers, veterinarians, and other experts said farmers were reluctant to test because they did not believe the virus is a serious concern, or because government incentives to test did not offset their expected losses.

Colorado farmer Terry Dye, 78, said his two dairies were infected this summer and he did not notify the state because he wanted to handle it privately. State agriculture officials eventually heard about the infections and quarantined his animals, he said.

"Sometimes it's more convenient to not know," said Dye.

USDA offers to compensate farmers with infected animals for veterinary care and 90% of lost milk production. Forty-seven herds have signed up for agency financial assistance, though that total includes farms without infections that are seeking support for biosecurity costs.

USDA tests raw milk from cows to identify the virus in herds. The Food and Drug Administration has separately tested commercial milk supplies and says pasteurization kills the virus, so milk is safe to drink.

TOUGHER TESTING

Experts said ways to better track the spread include more states mandating raw-milk testing or higher compensation to farmers.

Michigan and Colorado have taken aggressive approaches to containing bird flu in cattle, though experts there still think cases are being missed.

Phil Durst, an educator with Michigan State University who has spoken with farmers whose herds contracted the virus, said Michigan's 27 positive herds are likely an undercount by at least a third.

Jenna Guthmiller, an assistant professor of immunology at the University of Colorado who has studied the virus, said Colorado's 63 positive herds are also likely an undercount.

After a series of outbreaks, Colorado on July 22 became the only state to require dairy farms to test bulk supplies of milk each week. The tests have uncovered 10 infected herds that have been quarantined.

"Once we better understand the scope and scale of the outbreak, we can put measures in place to mitigate further spread," said Maggie Baldwin, Colorado's state veterinarian.

Some farmers do not test because they distrust government officials or information about the risks of bird flu to cattle and humans, four sources said.

"There's plenty of dairy farms that I've heard about that just don't believe it," said Jason Schmidt, a dairy farmer in eastern Kansas.

In Oklahoma, a dairy that suspected it was infected in April did not submit stored milk samples to USDA for testing until July, according to the state. The herd had recovered by the time testing confirmed an outbreak, and Oklahoma has not had other reported cases, the state said.

In states with few or no infected cows, farmers and veterinarians are concerned that when the virus arrives or resurges, they won't be able to track it.

"The longstanding adage is that the cure for fever is don't take a temperature. So, if we don't test, then we're not positive," said Mark Hardesty, a dairy cattle veterinarian in Ohio, which reported one dairy herd infection in April.

Wisconsin, the No. 2 milk-producing and top cheese-producing state, has not reported any bird-flu cases in cattle. Dairy farmers likely would not test even if they suspected symptoms in their herds, said Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory.

"It's still cheaper to just go through a herd outbreak, recover, and move on down the road," Poulsen said.