Second H5N1 strain detected in US dairy herds
2025
On 5 February, the US Department of Agriculture confirmed the infection of dairy cattle in Nevada with HPAIV H5N1 genotype D1.1. This genotype is currently dominant in wild birds in the US and is also causing outbreaks in poultry. This is only the second initial introduction of HPAIV H5N1 clade 2.3.4.4b into dairy herds worldwide. The fact that both events occurred in the USA within one year is critical and points to the possibility of such spillover. As with the first entry in spring 2024, the cause of entry is unclear. The spread of the virus from the first entry in dairy cattle in the United States, caused by HPAI H5N1 of clade 2.3.4.4b, genotype B3.13, has not yet been definitively stopped. It is therefore all the more important to learn from last year's events and to contain the spread of genotype D1.1 by taking immediate, nationwide and consistent measures. In Germany, there is still no evidence of a situation similar to that in the USA. However, FLI reiterates the need to maintain biosecurity measures in cattle herds due to the re-entry into the USA. Diagnostic tests should be carried out in cases of unclear mastitis.
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