Farm workers exposed to infected dairy cows found to have avian influenza antibodies
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There may be more human cases of avian influenza in the United States than previously thought. Health officials in the two states conducted blood tests on workers at dairy farms known to have received infected cattle and found that about 7% of them had antibodies to the disease. This included people who had never experienced any flu symptoms.
Since March, an avian influenza virus known as H5N1 has been circulating among dairy cows across the United States. So far, 446 cattle in 15 states have tested positive for the virus. Since April, 44 people in the United States have tested positive for H5, the influenza subtype that includes H5N1. All but one of these cases occurred in poultry or dairy farm workers infected with H5N1.
To better understand how many farmworkers may have been exposed to the virus, the Centers for Disease Control and Prevention (CDC) is working with state health departments in Colorado and Michigan to raise cattle infected with H5N1. Blood samples were collected from 115 people working on dairy farms. All samples were taken between 15 and 19 days after a cow on the farm tested positive for the virus.
Nirav Shah and his colleagues at the CDC then removed seasonal influenza antibodies from the samples before testing them for H5N1 antibodies. H5N1 antibodies were detected in eight of the samples, or about 7%, suggesting that eight of the workers may have been unknowingly infected with the virus. Furthermore, four of the employees did not remember having symptoms.
“This is important because previous (H5N1) testing recommendations have focused primarily on symptomatic workers,” said Megan Davis of Johns Hopkins University in Maryland. “If workers are unaware that they are infected, they may inadvertently expose others in the community to infection.”
H5N1 is poorly adapted to infecting humans and is not known to be transmitted between humans. Still, more than 900 people worldwide have been infected with the virus since 2003, and about half of them are reported to have died from the virus. Each of these infections gives the virus a chance to mutate, potentially making it more dangerous to people.
“Those of us in public health need to cast a wider net in who we provide testing to,” Shah said at a press conference today. “Going forward, CDC will expand its testing recommendations to include workers who have been exposed (to H5N1) but have no symptoms.”
The agency also recommends providing antiviral drugs to asymptomatic workers at high risk of infection, such as dairy farm workers who may get raw milk on their faces. That way, even if you do get infected with the virus, the amount of virus circulating in your body will be reduced and the risk of spreading the virus to others will be lower. “The less room we give this virus to spread, the less chance it has of changing,” Shah said.
The data also highlights that many H5N1 infections remain undetected, a concern public health officials have long suspected to be true. But until more data is available, “we can’t estimate how many unconfirmed cases there are,” Shah said.
The CDC is currently analyzing an additional 150 blood samples taken from veterinarians who work with cattle. Once these results are available, Shah said it should give a clearer picture of how many cases are slipping through the cracks.
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