U.S. Health officials are updating guidance for workers potentially exposed to avian influenza, commonly known as bird flu, following new data suggesting a higher rate of infection than previously understood. The Centers for Disease Control and Prevention (CDC) announced the changes on , based on findings from a recent study involving dairy farm workers in Michigan and Colorado.
Understanding the New Findings
The study, published in the CDC’s Morbidity and Mortality Weekly Report (MMWR), involved analyzing blood samples from 115 dairy farm workers between June and August 2024. Researchers were looking for antibodies against the H5N1 virus, an indicator of past infection. Notably, eight of the 115 workers (7%) tested positive for H5N1 antibodies, despite only four recalling experiencing symptoms, primarily conjunctivitis, or “pink eye.” The other four individuals showed evidence of recent infection without remembering feeling ill.
This finding is significant because it suggests that some individuals can be infected with H5N1 without exhibiting noticeable symptoms. “There may be individuals infected but who do not recall having symptoms,” explained Dr. Nirav Shah, the CDC’s principal deputy director, during a press briefing. “That means we need to cast a wider net in terms of who is offered a test.”
Current Situation with H5N1
As of , the CDC reports 46 human cases of H5N1 in the United States in 2024. Twenty-five of these cases are linked to the ongoing outbreak among dairy cows, with 21 occurring in California. Twenty cases are associated with poultry depopulation efforts, including 11 recent cases in Washington State. One case in Missouri had no known animal or animal product exposure.
The H5N1 strain of highly pathogenic avian influenza (HPAI) is currently circulating in wild birds worldwide and has caused outbreaks in both domestic birds and dairy cattle. The U.S. Department of Agriculture (USDA) is investing $1 billion to combat the outbreaks and support producers, with financial assistance available to dairy farmers to enhance biosecurity measures.
Updated CDC Recommendations
In response to the study findings, the CDC is implementing three key changes to its guidance:
- Expanded Testing: The CDC is now recommending testing for H5N1 not only for exposed workers who are experiencing symptoms, but also for those who were exposed and do not recall feeling sick, particularly if they were not wearing appropriate personal protective equipment (PPE).
- Prophylactic Treatment: The CDC now recommends offering antiviral medication (Tamiflu) to asymptomatic workers who have had high-risk exposure to infected animals without wearing adequate PPE. High-risk exposure includes situations like being splashed in the face with raw milk.
- Refined PPE Guidance: The CDC is updating its recommendations for PPE based on the specific tasks performed on farms. Workers involved in higher-risk activities, such as milking cows or working in the milking parlor with sick animals, will be advised to use more protective equipment.
“The purpose of these actions is to keep workers safe, to limit the transmission of H5N1 to humans and reduce the possibility of the virus changing,” Dr. Shah stated. The goal is to identify and treat infections early, even in individuals without noticeable symptoms, to prevent further spread.
What the Data Suggests About Risk
The study also revealed some patterns regarding risk factors. All eight workers who tested positive for H5N1 antibodies were Spanish speakers and reported either milking cows or cleaning the milking parlor. None of them were wearing respiratory protection, and less than half were wearing eye protection. This highlights the importance of providing adequate PPE and training to farm workers.
Researchers noted a limitation in the study: interviews were conducted an average of 49 days after initial exposure, with some occurring up to 90 days later. This delay could have made it difficult for workers to accurately recall minor symptoms. However, the data still provides valuable insights into the potential for asymptomatic infections and the need for improved worker protection.
Looking Ahead
The CDC continues to emphasize that the risk to the general public remains low. However, the agency is closely monitoring the situation and working with state and industry partners to strengthen prevention efforts. Additional serology studies are planned, including one in Ohio involving bovine practitioners, to further assess the extent of H5N1 exposure and infection rates. The CDC is also collaborating with the National Institutes of Health (NIH) to evaluate the effectiveness of existing rapid diagnostic tests for H5N1 and explore the development of new tests.
Health officials stress the importance of continued vigilance and adherence to recommended biosecurity measures to protect both animal and human health. The CDC will continue to update its guidance as new information becomes available.
