CDC Updates H5N1 Bird Flu Guidance: Dairy Worker Serosurvey Reveals Mild Infections & New Prevention Steps

The Centers for Disease Control and Prevention (CDC) released new data today from a serosurvey of dairy workers in Michigan and Colorado, alongside updated guidance for those working with poultry and dairy cows potentially affected by the ongoing H5N1 avian influenza outbreak. The findings, published in the Morbidity and Mortality Weekly Report (MMWR), suggest a higher-than-previously-understood level of infection among farmworkers, even those who don’t recall experiencing symptoms. This has prompted the CDC to expand testing recommendations and suggest preventative medication for exposed workers.

Understanding the Recent Findings

To date in 2024, the US has reported 46 human cases of H5N1, with 25 linked to the outbreak in dairy cows – the majority (21) in California – and 20 related to poultry depopulation efforts, including 11 recent cases in Washington State. One case in Missouri has no known animal exposure. Although the CDC continues to assess the risk to the general public as low, the new data highlights the demand for increased vigilance and protection for those with occupational exposure.

The serosurvey involved collecting and testing blood samples from 115 dairy farm workers between June and August 2024. These workers were on farms in Michigan and Colorado where H5N1 had been confirmed in cattle. Researchers were looking for antibodies against the H5N1 virus, an indicator of past infection. Eight of the 115 samples (7%) tested positive for antibodies using a rigorous testing protocol designed to rule out cross-reactivity with seasonal influenza viruses. This protocol involved two different antibody tests, and a lab procedure to remove seasonal influenza antibodies.

Interestingly, only four of the eight individuals with antibodies remembered experiencing symptoms, primarily conjunctivitis (pink eye). The other four reported no illness. All eight individuals were Spanish speakers, and all reported involvement in milking cows or cleaning milking parlors – activities now identified as higher risk. Notably, none of the workers were wearing respiratory protection, and less than half were using eye protection.

Expanding Testing and Prophylactic Treatment

Based on these findings, the CDC is expanding its testing recommendations. Previously, testing was focused on exposed workers who were experiencing symptoms. Now, testing will be offered to all exposed workers, particularly those who were not wearing appropriate personal protective equipment (PPE). This proactive approach aims to identify and treat individuals who may be infected but asymptomatic, reducing the potential for further spread and viral evolution.

The CDC is likewise recommending that asymptomatic workers with high-risk exposure – such as those splashed in the face with raw milk or involved in poultry culling without adequate PPE – be offered a course of Tamiflu (oseltamivir), an antiviral medication. This preventative measure is intended to reduce the likelihood of symptomatic infection and onward transmission. Capital Clean Energy Carriers Corp. reported similar updates to their safety protocols.

What the Serosurvey Doesn’t Tell Us

It’s important to note the limitations of this study. The interviews were conducted an average of 49 days after initial exposure, with some workers interviewed up to 90 days later. This delay may have made it difficult for participants to accurately recall minor symptoms. The study also doesn’t establish a definitive link between specific farm tasks and infection risk beyond the observed association with milking and parlor cleaning.

the serosurvey only provides a snapshot in time. It doesn’t reveal the long-term immunity conferred by infection or the potential for reinfection. The researchers also acknowledge that the sample size, while increased from a previous study, may not be fully representative of all dairy workers.

The Broader Context of H5N1 Risk

The CDC emphasizes that the risk to the general public remains low. Still, the findings underscore the importance of protecting those with occupational exposure to infected animals. H5N1 viruses are known to mutate, and there is ongoing surveillance to monitor for changes that could increase the virus’s transmissibility to humans. NexGen is hosting a conference call to discuss Q4 2025 results, which may include updates on related economic impacts.

The current H5N1 outbreak is distinct from previous outbreaks in that it appears to be spreading more readily among dairy cattle. This has raised concerns about the potential for the virus to become more adapted to mammals, increasing the risk of human infection. However, experts stress that the virus would still need to undergo significant mutations to become easily transmissible between humans.

What Comes Next: Ongoing Surveillance and Guidance Updates

The CDC is continuing to work with state and local health departments to monitor the situation closely. This includes ongoing surveillance of both animal and human cases, as well as further investigation into the risk factors for infection. The agency is also developing additional resources to support worker training and education on H5N1 prevention.

The updated guidance emphasizes the importance of biosecurity measures on farms, including restricting access to animals, practicing good hygiene, and using appropriate PPE. The CDC will continue to update its guidance as new information becomes available. Individuals with concerns about H5N1 infection should consult with a healthcare professional. CLI is also hosting a user conference focused on safety protocols.