The H5N1 bird flu virus spreading among cows is infecting more farm workers than experts thought.
The CDC tested 115 workers at farms suffering from outbreaks and found that eight of them tested positive for the virus, a 7 percent infection rate.
Many of them had no symptoms, which officials fear indicates H5N1 is spreading much more than official numbers show.
46 farmworkers have been officially diagnosed with the virus, but hundreds of livestock farms in dozens of states are experiencing outbreaks.
The CDC now recommends that anyone exposed to infected animals get tested, not just those who get sick.
The agency said it sees no mutations in the virus that would facilitate transmission or evidence of person-to-person spread.
But disease experts have warned that as the virus circulates, the chances of it evolving increase, which could lead to a pandemic.
A pig on an Oregon farm tested positive for H5N1 for the first time in the U.S. last week, causing alarm because pandemic pigs have been a source of previous human pandemics.
Dead cows are piling up in California as dairy farmers battle H5N1 bird flu, which made landfall in the state in August.
The CDC’s change in testing guidance comes after an investigation of 115 farm workers in Michigan and Colorado who had been exposed to H5N1.
Of the eight workers with positive blood tests, four reported no symptoms.
All eight cleaned the milking parlors and none of them used respiratory protection such as masks. Three said they wore eye protection.
High levels of the virus have been found in the milk of infected cows, increasing the risk of exposure and infection, the researchers said.
The United States Department of Agriculture (USDA) has said it will begin testing bulk milk for bird flu, signaling concern from both agencies about the continued spread of the virus on dairy and poultry farms.
The CDC also recommends offering the antiviral drug Tamiflu to workers with high-risk exposure to sick animals and expanding its guidelines for worker protective equipment, including eye protection.
Agency researchers said efforts to monitor dairy workers for illness have been hampered by several barriers, including the reluctance of farm owners and workers to allow testing.
Bird flu has infected nearly 450 dairy farms in 15 states since March, according to USDA data.
Nirav Shah, principal deputy director of the CDC, said: “The purpose of these actions is to keep workers safe, limit transmission of H5 to humans, and reduce the chance that the virus changes.”
Outside experts said it’s notable that the study prompted the CDC to take new action.
“This is a significant step toward assessing that these H5N1 viruses pose a greater risk than the CDC had previously estimated,” said Dr. Gregory Gray, an infectious disease researcher at the University of Texas School of Medicine at Galveston.
Each additional infection in animals or humans gives the virus a chance to change in potentially dangerous ways, said Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada.
“This shows once again that we are not responding effectively to the H5N1 outbreak in human or animal livestock and if we continue to allow this virus to spread and jump from one species to another, our luck will eventually run out,” Rasmussen said.
Rasmussen and others have criticized the federal response to the outbreak as too slow and “mediocre.”
“These studies should have been done months ago and should have been prioritized,” he said.