Understanding the Emerging Threat of H5N1 Bird Flu Transmission from Cows to Humans

H5N1 bird flu is evolving and expanding its reach. Recent findings from the Centers for Disease Control and Prevention (CDC) reveal alarming evidence that a second variant of H5N1 has very likely transitioned from dairy cows to humans. This new information raises serious concerns about the virus’s ability to adapt and infect different species, including humans.

The CDC, along with health officials in Nevada, reported on Monday the state’s first confirmed case of highly pathogenic avian influenza H5N1. The individual affected is a farm worker who was presumably infected by dairy cows carrying the D1.1 genotype of H5N1, a variant that is distinct from the more prevalent strain typically found in cows since 2024. Fortunately, the worker only experienced mild symptoms of conjunctivitis, and as of now, no additional human cases linked to this incident have been reported.

After years of circulation in wild birds, the H5N1 virus made a significant leap across species to infect dairy cows in early 2024. Since then, outbreaks have been documented in 16 states, affecting not just cows but also other mammals like cats and humans. Additionally, the virus has continued to cause devastating die-offs among wild bird populations and domestic poultry, further illustrating its impact on wildlife and livestock alike.

The original genotype of H5N1 that transitioned from birds to cows is known as B3.13. This strain has been linked to relatively mild illness in both cows and humans. However, federal and local health officials recently confirmed the presence of D1.1 on a dairy farm in Churchill County, Nevada. In the case involving the farm worker, he had direct exposure to sick cows prior to his symptoms developing, and laboratory tests confirmed that his infection was due to the D1.1 strain. This occurrence marks the 68th confirmed case of H5N1 in humans across the U.S. since the beginning of 2024.

The rise of the D1.1 strain has quickly established it as the dominant variant of H5N1 circulating among wild birds, and there have been sporadic reports of human infections associated with contact with infected birds in recent months. For example, in January, health officials in Louisiana reported a case involving a resident who had exposure to both infected backyard chickens and wild birds. Tragically, this individual succumbed to the infection—the first reported human fatality linked to H5N1 during this ongoing outbreak in the U.S. Additionally, Canada reported its inaugural human case of H5N1 last November, involving a teenage girl who likely contracted the D1.1 strain from wild birds. Although she was hospitalized in critical condition, she appears to have recovered.

These latest cases involving both cows and humans raise significant alarms for several reasons. Firstly, they suggest that H5N1 has successfully crossed the species barrier from birds to cows multiple times, and that the D1.1 strain has the capacity to transmit from cows to humans. Furthermore, USDA officials have identified a specific mutation in the cow-derived strains of D1.1 that is linked to adaptations that may facilitate infection in mammals—in essence, this mutation could make it easier for H5N1 to infect and potentially harm mammals.

Fortunately, at this stage, neither the D1.1 strain nor any other H5N1 variant has shown evidence of sustained human-to-human transmission. However, the current situation is precarious. As long as these flu viruses continue to circulate freely among birds, cows, felines, humans, and various other mammals, the risk of a new strain emerging that is well-adapted to humans increases. This scenario presents the potential for the next major epidemic or pandemic to arise, which could have widespread implications for public health.