The H5N1 bird flu virus spreading among cows is infecting more farm workers than experts realized.
The CDC tested 115 workers at farms suffering outbreaks and found eight of them tested positive for the virus – an infection rate of seven percent.
Many of them had no symptoms, which officials fear indicates H5N1 is spreading much more widely than the official numbers show.
Forty-six farm workers have been officially diagnosed with the virus but hundreds of cattle farms in dozens of states are experiencing outbreaks.
The CDC is now recommending anyone exposed to infected animals get tested, not just those who become sick.
The agency said it is not seeing mutations in the virus that would lead to easier transmission or evidence of person-to-person spread.
But disease experts have warned that as the virus circulates, the chance of evolving increases, which could lead to a pandemic.
A pig on an Oregon farm tested positive for H5N1 for the first time in the US last week which caused alarm because pigs have been a source of prior 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 to testing guidance comes after an investigation into 115 farmworkers 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 milking parlors and none used respiratory protection such as face masks. Three said they used 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 US Department of Agriculture (USDA) has said it will begin testing bulk milk for bird flu, signaling concern by both agencies about the ongoing spread of the virus on dairy and poultry farms.
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.
But agency researchers said efforts to monitor dairy workers for illness have been hindered by several barriers, including the reluctance of farm owners and farmworkers to allow testing.
Nirav Shah, CDC’s principal deputy director, said: ‘The purpose of these actions is to keep workers safe, to limit the transmission of H5 to humans and to reduce the possibility of the virus changing.’
Bird flu has infected nearly 450 dairy farms in 15 states since March, according to USDA data.
Outside experts said it’s notable the study prompted the CDC to take new action.
‘This is a significant move towards the assessment that these H5N1 viruses are a greater risk than the CDC estimated before,’ said Dr Gregory Gray, an infectious disease researcher at the University of Texas Medical Branch in Galveston.
Every additional infection in animals or humans gives the virus the chance to change in potentially dangerous ways, added Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada.
‘It shows yet again that we are not responding effectively to the H5N1 cattle outbreak in humans or animals and if we continue to let this virus spread and jump from species to species, our luck will eventually run out,’ she said.
Rasmussen and others have criticized the federal response to the outbreak as too slow and ‘lackluster,’ adding the studies ‘should have been performed months ago and should have been prioritized.’
