U.S. Health officials announced expanded testing and treatment recommendations Friday following a serological survey of dairy farm workers exposed to the H5N1 avian influenza virus. The Centers for Disease Control and Prevention (CDC) revealed that eight of 115 workers surveyed in Michigan and Colorado tested positive for antibodies against the virus, indicating prior infection.
The findings, published in the Morbidity and Mortality Weekly Report (MMWR), prompted the CDC to broaden its testing criteria to include asymptomatic workers who had exposure to infected animals without wearing appropriate personal protective equipment (PPE). Previously, testing was primarily focused on symptomatic individuals with known exposure. The CDC also now recommends offering the antiviral drug Tamiflu to asymptomatic, high-risk exposed workers, even in the absence of symptoms.
“This is an example of CDC using data to drive action,” said Dr. Nirav Shah, Principal Deputy Director of the CDC, during a press briefing. “We will continue working with trusted partners to reach farm workers and their employers to communicate these prevention strategies as widely as possible.”
The serosurvey, conducted between June and August 2024, involved workers on dairy farms where H5N1 had been confirmed in cows. Of the eight workers who tested positive for H5N1 antibodies, four reported experiencing symptoms, primarily conjunctivitis, while the other four were asymptomatic. All eight individuals were Spanish speakers, and all reported involvement in milking cows or cleaning milking parlors – activities identified as higher risk. Notably, none of the seropositive workers reported direct contact with known infected cows, and less than half used eye protection, while none wore respiratory protection.
The CDC emphasized that the serological results are not being counted as confirmed human cases of H5N1, but rather represent retrospective data used to better understand the extent of infection and inform preventative measures. To date, there have been 46 confirmed human cases of H5N1 in the U.S., with 25 linked to the dairy cow outbreak and 20 associated with poultry depopulation efforts, including 11 recent cases in Washington state. One case in Missouri has no known animal or animal product exposure, according to Dr. Demetre C. Daskalakis, Director of the National Center for Immunization and Respiratory Diseases.
Officials stated that the expanded recommendations are intended to limit the transmission of H5N1 to humans and reduce the possibility of the virus changing. The CDC is updating its PPE guidance for farm workers based on risk levels, with a focus on prioritizing protection during high-risk tasks like culling operations and working in milking parlors with potentially infected animals. Additional training resources are being developed to support worker education on PPE use.
The CDC is also collaborating with the National Institutes of Health (NIH) to evaluate existing rapid diagnostic tests for their ability to detect the current circulating strain of H5N1. A pilot program is planned to assess the performance of a near point-of-care test against laboratory-based testing.
The agency continues to assess the risk to the general population as low, but stressed the importance of protecting individuals with exposure to animals or animal products affected by H5N1. The CDC anticipates publishing a spotlight document detailing the updated recommendations.
