Understanding the Alarming Spread of H5N1 Clade 2.3.4.4b in Dairy Cows: What You Need to Know

In a shocking development, the highly pathogenic avian influenza virus (HPAIV) clade 2.3.4.4b H5N1 was first reported in dairy cows in Texas, USA, in March 2024. This has led to a rapid outbreak, affecting over 190 farms across 13 states within a short timeframe. Researchers have conducted two key studies to assess the susceptibility and transmission dynamics of this virus in calves and lactating cows.

Calves Show Moderate Response to Infection

In the first study focused on calves, researchers analyzed the oronasal susceptibility and transmission following infection with a specific H5N1 bovine isolate known as genotype B3.13. The results showed that while calves experienced moderate replication of the virus in their nasal passages and shedding, they did not exhibit severe clinical symptoms or transmit the virus to sentinel calves. This suggests that while calves can be infected, their role in spreading the virus may be limited.

Dairy Cows Hit Hard: A Disturbing Turn

The second study highlighted a more alarming scenario in lactating cows. Upon direct inoculation of the mammary glands with either the H5N1 B3.13 strain or a European wild bird isolate known as genotype euDG, researchers noted no nasal viral shedding. However, the consequences were severe: the cows developed acute infections in the mammary gland, diagnosed as necrotizing mastitis, alongside high fever. Tragically, milk production dropped dramatically, and the overall health of the cows was severely jeopardized.

One of the more concerning findings was that the virus reached staggering levels in the milk, with titers peaking at 108 TCID50/mL, yet the infection did not progress to systemic levels within the cows themselves. This indicates that while the virus can thrive in the udder, it may not spread throughout the animal's system.

Key Mutation Detected: Implications for the Future

Furthermore, during the replication of the H5N1 euDG strain, an adaptive mutation known as PB2 E627K emerged, raising questions about the virus’s ability to evolve and potentially become more virulent. This mutation could have profound implications for both animal health and agricultural practices.

Research suggests that H5N1 strains, including B3.13 and possibly others, can replicate in the udder of cows. It appears that the primary mode of transmission among cattle could be milk and milking procedures rather than respiratory pathways.

What Does This Mean for Farmers?

Farmers and veterinarians are urged to be vigilant about dairy cow health and to monitor for signs of influenza, especially in light of these findings. Enhanced biosecurity measures may be necessary to prevent further outbreaks and to protect livestock from infection. This situation serves as a critical reminder of how rapidly zoonotic pathogens can spread, posing risks not just to animal health but potentially to human health too.

Stay tuned for ongoing updates as scientists continue to research the implications of H5N1 clade 2.3.4.4b in cattle and its overall impact on agriculture and food production.