H5N1 D1.1 Detected in Nevada Dairy Cows Raises Concern

H5N1 D1.1 spillover in Nevada dairy cows

A newly emerging form of highly pathogenic avian influenza has crossed from wild birds into dairy cattle in Nevada. US Department of Agriculture (USDA) officials and some researchers are concerned about a mutation detected in the virus.

Known as genotype D1.1, the viral variant differs from B3.13, the strain that first began infecting dairy cows in Texas in March 2023. B3.13 has so far caused disruption in nearly 1,000 US herds.

The USDA describes D1.1 as the "current predominant genotype in migratory wild birds". It has also been linked to several severe human cases of highly pathogenic avian influenza (HPAI), including a near-fatal infection in a teenager in Canada and North America's first, and so far only, bird-flu-related death: a Louisiana resident aged over 65.

Before this event, D1.1 had not crossed from birds into cattle.

"This is not what anyone wanted to see," University of Pennsylvania evolutionary biologist Louise Moncla told The New York Times journalists Emily Anthes and Apoorva Mandavilli.

"We need to now consider the possibility that cows are more broadly susceptible to these viruses than we initially thought."

Mutation identified in Nevada cattle

The virus found in Nevada dairy cattle is closely related to viruses circulating in migratory wild birds. However, it carries an important mutation that could allow it to replicate more readily within mammalian cells.

This adaptation is absent from the B3.13 genotype, although it has appeared in certain human bird flu cases.

"Investigations are ongoing to fully characterize this event," reads the USDA brief.

"The Nevada Department of Agriculture acted quickly, by first rapidly enrolling in the National Milk Testing Strategy to initiate active surveillance, and then to identify and quarantine the affected dairies before cattle movements could further transmit this virus beyond the local area."

Established by Federal Order in December 2024, the USDA's National Milk Testing Strategy monitors avian influenza among dairy cows nationwide. It is also intended to prevent infected milk and dairy cattle from being moved between states.

In Nevada, however, the cattle were not infected by a neighbouring herd from another state. The likely source of the threat was wild birds overhead.

Human risk and H5N1 monitoring

Although infected cattle can pass the virus to people, particularly through their milk, there is still no evidence that H5N1 can spread between humans. For the time being, US Centers for Disease Control and Prevention (CDC) officials consider the risk to the wider public to be low.

Nevertheless, scientists and public-health officials, including the World Health Organization (WHO), are treating the prospect of a future human pandemic seriously and closely tracking H5N1 mutations.

Some virologists have recently argued that the likelihood of an HPAI-driven human pandemic is increasing as the pathogen spends longer inside mammalian hosts.

Although D1.1 has been associated with more serious human infections than B3.13, Emory University influenza virologist Seema Lakdawala told Max Kozlov at Nature in late January that the sample sizes are too small to reveal much.

Since 2024, at least 66 human bird flu cases have been documented in the US. Scientists still lack sufficient understanding of the factors that make each viral variant more or less hazardous to humans overall.

CDC updates and USDA data sharing

The CDC last published a statement on the bird flu outbreak on 17 January 2025. Because the Trump administration has currently frozen communications from some federal health agencies, it is unclear whether, or when, the CDC will comment on the newly identified Nevada spillover.

The USDA says it will keep providing its data to the CDC and the National Center for Biotechnology Information within seven days of analysis.