CDC calls for expanded testing for avian flu after increasing number of infections among farmworkers

Federal health officials on Thursday called for more testing of farm workers for bird flu after a new study found that some dairy workers showed signs of infection even if they did not report feeling ill.

Farm workers who have close contact with infected animals should be tested and offered treatment, even if they don’t have symptoms, said Dr. Nirav Shah, chief director of the U.S. Centers for Disease Control and Prevention.

The new guidance comes after blood tests of 115 farmworkers in Michigan and Colorado showed that eight workers — or 7% — had antibodies indicating previous infection with the virus known as influenza A H5N1.

“The purpose of these actions is to ensure the safety of workers, limit the transmission of H5 to people and reduce the likelihood of the virus changing,” Shah told reporters.

The CDC study provides the biggest insight to date into how the avian virus, first discovered in dairy cows in March, can spread to humans. That suggests the virus has infected more people than the 46 farmworkers identified in the U.S. as of Thursday. Almost all had contact with infected dairy cows or infected poultry.

Notably, outside experts have prompted the CDC to take new action. Previous guidance called for testing and treating workers only if they have symptoms.

“This is a significant step toward assessing that these H5N1 viruses pose a greater risk than the CDC previously thought,” said Dr. Gregory Gray, an infectious disease researcher at the University of Texas Medical Branch at Galveston.

Each additional infection in animals or people gives the virus an opportunity to change in potentially dangerous ways, says Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada.

“This once again shows that we are not effectively responding to the H5N1 outbreak in cattle in humans and animals, and if we continue to allow this virus to spread and jump from species to species, our luck will eventually run out,” Rasmussen said in an email letter.

The CDC study involved 45 workers in Michigan and 70 in Colorado, tested between June and August. Of the eight workers with positive blood tests, four reported no symptoms. All eight milking parlors were cleaned and none were using respiratory protection such as face masks. Three said they used eye protection.

High levels of the virus were found in the milk of infected cows, increasing the risk of infection, the researchers said.

Efforts to monitor dairy worker illnesses are hampered by several barriers, including reluctance by farm owners and farmworkers to allow testing, the researchers said.

Rasmussen and others have criticized the federal response to the outbreak as too slow and “lackluster.”

“These studies should have been done months ago and should have been a priority,” she said.

The virus has been confirmed in at least 446 cattle herds in 15 states. Last week, the Department of Agriculture reported that a pig on an Oregon farm was confirmed to have avian influenza, the first time the virus has been detected in pigs in the United States.