By Deena Beasley and Leah Douglas

(Reuters) – Farm workers who have been exposed to animals with bird flu should be tested for the virus even if they do not have symptoms, the U.S. Centers for Disease Control and Prevention said on Thursday.

The change to the agency’s testing recommendation comes as the U.S. Department of Agriculture is also expanding its testing of milk for bird flu, signaling concern by both agencies about the ongoing spread of the virus on dairy and poultry farms.

Bird flu has infected nearly 450 dairy farms in 15 states since March, according to USDA data.

The CDC is not seeing mutations in the virus that would lead to easier transmission or evidence of person-to-person spread, agency officials said on a press call.

The CDC has identified 46 human cases of bird flu since April, though farm worker groups have suggested the figure is an undercount as workers avoid testing for fear of economic repercussions from quarantining or because their symptoms are too mild to warrant concern.

Disease experts have warned that as the virus circulates, the risk of it more easily infecting humans increases, which could lead to a pandemic.

“There may be individuals who were infected with H5 but who do not recall having symptoms. That means that we in public health need to cast a wider net in terms of who is offered a test,” Nirav Shah, CDC’s principal deputy director, said on the call with reporters.

Recent testing of 115 dairy farm workers in Michigan and Colorado who were exposed to cows infected with H5N1 bird flu found that 7% of them had evidence of prior infection, but only half remembered having symptoms, according to a CDC study released on Thursday. 

The June through August serological study found evidence of antibodies to the virus in eight workers involved in milking and cleaning milking parlors.    

The CDC is also recommending offering the antiviral drug Tamiflu to workers with high risk exposures to sick animals and widening its guidance for worker protective equipment, including eye protection.

The USDA last week said it would begin testing bulk milk for bird flu after a push from industry and veterinary groups concerned about the virus’ spread. 

A pig on an Oregon farm tested positive for bird flu for the first time in the U.S. on Oct. 30, concerning virologists because pigs have been a source of prior human pandemics. The USDA said on Nov. 6 that a second pig on the farm had also tested positive.