Widespread H5N1 in US Dairy
A new study published in Emerging Infectious Diseases highlights the critical role of retail milk monitoring in tracking the unprecedented outbreak of highly pathogenic avian influenza (HPAI) A(H5N1) in US dairy cattle. The research suggests that while the virus spread extensively and silently in early 2024, subsequent federal interventions have significantly improved detection and control.
When H5N1 was first reported in dairy cows in March 2024, initial identification was delayed because cows were not considered typical hosts for the virus. Early surveillance relied on "passive" reporting, testing only clinically ill animals, which led to a massive underestimation of the virus's reach.
To assess the true extent of the spread, researchers conducted retail milk surveillance at two key timeframes:
- Period 1 (April–May 2024): The study found influenza A virus (IAV) RNA in 36.3% of pasteurized retail milk samples. Remarkably, the virus was detected in five states (Arkansas, Indiana, Minnesota, Missouri, and Oklahoma) where no outbreaks in dairy cows had been reported at the time.
- Period 2 (December 2024–January 2025): After the implementation of federal testing mandates, the prevalence in retail milk dropped to 6.9%. Crucially, during this period, positive samples were found exclusively in California, aligning closely with the high number of reported infected herds in that state at the time.
On April 24, 2024, the USDA mandated testing before the interstate movement of lactating cattle, followed by a National Milk Testing Strategy in December 2024. These measures transitioned the industry from passive observation to active surveillance, allowing authorities to better "map" and mitigate the virus.
Although control measures have improved, the virus has not been eliminated.
It should be noted that retail milk monitoring is an imperfect solution, as it provides limited geographic resolution due to the transportation of milk across state lines before it undergoes processing. The study’s findings highlight a significant gap in the initial diagnostic and response systems for the H5N1 virus in the United States. This delay in detection contributed to a widespread outbreak among dairy herds and wild birds, leading to a loss of control over a virus characterized by complex and rapid evolutionary trajectories.
Tarbuck, Natalie N., et al. Retail Milk Monitoring of Influenza A (H5N1) in Dairy Cattle, United States, 2024-2025. Emerging infectious diseases, 32.2.
