Bird flu strain that just jumped to cows infects dairy worker in Nevada

The worker is said to have had pink eye and is recovering from the infection.

A dairy worker in Nevada has been infected with a strain of H5N1 bird flu—genotype D1.1—that has newly spilled over to cows, the Centers for Disease Control and Prevention has confirmed.

The worker experienced conjunctivitis (pink eye) as the only symptom and is recovering, according to a separate press release by the Central Nevada Health District Monday.

The bird flu strain H5N1 clade 2.3.4.4b, genotype D1.1 is the predominant strain currently circulating in wild birds in North America and was confirmed for the first time in cows in Nevada last week. According to the US Department of Agriculture, the new spillover was initially detected on January 31 via bulk milk testing. Until this point, the outbreak of H5N1 in dairy cows—which was declared in March 2024—was entirely caused by H5N1 clade 2.3.4.4b, genotype B3.13. The outbreak was thought to have been caused by a single spillover event from wild birds to cows in Texas in late 2023 or early 2024.

Since then, the CDC has confirmed 68 human cases of H5N1 in the US, 41 of which were in dairy workers. That total includes the new D1.1 case in Nevada. The remainder includes 23 poultry workers, one from a backyard/wild bird exposure in Louisiana, and three with unclear sources of infection. Nearly all of the cases have been mild.

Genetic findings

In a statement emailed to Ars, the CDC noted that the Nevada case is not the first human case with D1.1 amid the outbreak. Based on available genetic data, the D1.1 genotype likely infected a total of 15 people across Iowa, Louisiana, Oregon, Washington, and Wisconsin during 2024, the agency said.

However, the new Nevada case is notable because it marks the first time D1.1 is known to have jumped from birds to cows to a person. Moreover, D1.1 has proven dangerous. The genotype is behind the country's only severe and ultimately fatal case of H5N1 so far in the outbreak. The death in the Louisiana case linked to wild and backyard birds was reported last month. The CDC's statement added that the person had "prolonged, unprotected" exposure to the birds. The D1.1. genotype was also behind a severe H5N1 infection that put a Canadian teenager in intensive care late last year.

In a February 7 analysis, the USDA reported finding that the D1.1 strain infecting cows in Nevada has a notable mutation known to help the bird-adapted virus replicate in mammals more efficiently (PB2 D701N). To date, this mutation has not been seen in D1.1 strains spreading in wild birds nor has it been seen in the B3.13 genotype circulating in dairy cows. However, it was seen before in a 2023 human case in Chile. The CDC said it has confirmed that the strain of D1.1 infecting the person in Nevada also contains the PB2 D701N mutation.

The USDA and CDC both reported that no other concerning mutations were found, including one that has been consistently identified in the B3.13 strain in cows. The CDC said it does not expect any changes to how the virus will interact with human immune responses or to antivirals.

Most importantly, to date, there has been no evidence of human-to-human transmission, which would mark a dangerous turn for the virus's ability to spark an outbreak. For all these reasons, the CDC considers the risk to the public low, though people with exposure to poultry, dairy cows, and birds are at higher risk and should take precautions.

To date, 967 herds across 16 states have been infected with H5N1 bird flu, and nearly 158 million commercial birds have been affected since 2022.

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