CDC calls for expanding bird flu virus testing

In a press briefing on Nov. 7, the US Centers for Disease Control and Prevention (CDC) announced its recommendation to test farmworkers who were exposed to animals infected with bird flu regardless of whether these individuals develop symptoms. This move revises the agency’s previous recommendation to test only symptomatic people exposed to H5N1-infected animals. The CDC had issued that advice in April, when multiple states first reported an outbreak of bird flu in dairy cows.

The update is the result of a new study conducted by the CDC that found evidence of recent H5N1 infection among farmworkers who don’t recall experiencing any disease symptoms (Morb. Mortal. Wkly. Rep. 2024, DOI: 10.15585/mmwr.mm7344a3).

For the study, CDC researchers teamed up with state officials to collect blood samples from individuals working on dairy farms in Michigan and Colorado that had infected cows. Between June and August 2024, 115 farmworkers volunteered to have their blood drawn. The scientists scanned these samples for antibodies against the H5N1 virus. Eight individuals—all involved in milking cows or cleaning the milking parlor—tested positive for H5N1 antibodies, indicating a recent infection. Half those workers recalled feeling unwell. While the other half didn’t recall feeling ill, it’s possible they were asymptomatic or their symptoms were mild and went unnoticed.

“That means that we in public health need to cast a wider net in terms of who is offered a test,” Nirav Shah, the CDC’s principal deputy director, said on the press call.

The new guidance expands testing to include farmworkers who were exposed even if they show no disease symptoms. This recommendation especially applies to those individuals who weren’t wearing adequate personal protective equipment (PPE) while milking cows or culling infected poultry. The federal agency also recommends offering Tamiflu to such individuals. The use of this antiviral will lower the likelihood of an asymptomatic case becoming symptomatic, Shah said, and limit the transmission of H5N1 to other humans. “This is fundamentally a respiratory virus,” he added, “and although we have not seen changes to the virus that would suggest the capacity for it to spread from person to person, we want to keep that risk as low as where it is right now.”

The CDC is also advising exposed farmworkers—particularly those performing higher-risk activities such as culling sick poultry or working in the milking parlor with infected cows—to wear PPE. “Having tailored, targeted, nuanced PPE guidance is definitely a step in the right direction,” says Natasha Bagdasarian, chief medical executive for the State of Michigan. But Bagdasarian acknowledges that it’s challenging to work with animals while wearing coveralls, hair and shoe covers, an N95 mask, and a face shield. Face shields and goggles can fog up, and people can get hot and uncomfortable in coveralls, especially during the summer. Many declined free PPE.

Another challenge is pushback from dairy farm owners or workers to conduct H5N1 testing. “If you think about it from a worker’s perspective, it makes sense,” says Elizabeth Strater, national vice president for United Farm Workers. If workers test positive, they can’t afford an unpaid 10-day isolation period and risk losing their jobs. Farm owners, on the other hand, need even more help to care for infected or sick animals. The dairy cows—irrespective of their H5N1 status—must be milked twice a day, Strater says, but sick animals also need supportive care. “Who’s going to do that work?” she asks.

While public health agencies continue to invest in creating awareness about H5N1 and building trust with farm owners and workers, compensating for loss of workers’ wages could help expand testing, Strater adds.