Avian Influenza Outbreak in the U.S.: H5N1 Strain Detected in Dairy Cows

Introduction: In 2024, the U.S. witnessed an unprecedented outbreak of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b in dairy cattle, sparking significant concern across the agriculture and public health sectors.

Background: Initially detected in wild birds and poultry, the H5N1 strain spread to dairy farms across 14 states, affecting milk production and livestock health.

Incident Details: The U.S. Department of Agriculture's National Veterinary Services Laboratories confirmed the virus's presence in a Texas dairy cow. By September, over 200 cases spanned 14 states, marking a unique instance of avian influenza progressively infecting mammalian species.

Impact on Dairy Industry: Infected dairy farms noted a significant decrease in milk yield, with afflicted cows exhibiting symptoms such as thick yellow milk and reduced rumination. The virus's spread was majorly attributed to the movement of undiagnosed, subclinical cows among dairy farms.

Human Health Concerns: Though some human infections were reported, most cases associated with this dairy outbreak were mild. However, the virus's potential adaptation to mammals is worrying, presenting possible future transmission risks.

Research and Analysis: Investigations revealed the virus’s genomic stability among cattle, suggesting limited evolutionary changes. This stability, alongside a lack of reassortment, pointed to a single introduction event from avian sources into cattle populations.

Experimental Findings: Research into experimental infection routes showed significant replication of the virus in the mammary glands, with persistent shedding in milk. This highlights the mammary gland as a critical vector in the virus's spread between cattle.