When the U.S. Department of Agriculture (USDA) reported the H5N1 avian flu virus first infecting dairy cows in Texas and Kansas a year ago, it was considered a rare event. However, 10 months later, infections occurred in dairy herds in Nevada and Arizona, indicating that cross-species transmission of the virus was not accidental.

Scientists are most concerned about how the virus infects cattle and how often it spreads. Experts point out that understanding these mechanisms is crucial to preventing and controlling future epidemics. If the virus continues to spread among cattle, it may gain the ability to spread among humans through mutation or recombination, and even trigger a pandemic. Some people believe that the United States needs to strengthen surveillance and consider vaccinating cattle, otherwise it will fall into a passive response.

H5N1 belongs to the highly pathogenic avian influenza strain clade 2.3.4.4b, which previously mainly infected poultry. Studies have found that H5N1 of different genotypes can infect dairy cow udders. But confusingly, no cattle infections have been reported in Europe, where bird flu has been circulating for longer. Analysts believe that large-scale interstate cow transportation in the United States may contribute to the spread of the epidemic, while Europe's intensive breeding model limits the spread.

How the virus infects cattle remains a mystery. Some experts speculate that it may be transmitted through the breast through contaminated instruments. However, it is difficult to trace the origin of the epidemic in the United States. The first case was discovered with a lag of several months. The case in Nevada tested positive for mixed stored milk, and it took several weeks to identify the source. Cases were quickly identified in Arizona due to single-farm testing, but follow-up investigations were insufficient.

Scientists warn that if it cannot be effectively controlled, H5N1 may continue to spread and evolve in cattle, threatening public health. The scientific community recommends establishing a farm milk sample bank to speed up traceability work. It is generally believed that surveillance and in-depth research must be strengthened, otherwise the continued spread of H5N1 in cattle may pose a major public health risk. This phenomenon of cross-species transmission of viruses not only affects the development of animal husbandry, but may also threaten human health and safety.