GAINESVILLE, Ga. — As 2026 continues, the poultry industry is going into its fourth year of the current outbreak of highly pathogenic avian influenza. As of right now, Pennsylvania and Indiana are being hit the hardest by the virus, but every state has had its fair share of HPAI woes.
A new development during this outbreak has been the detection of HPAI in species besides birds, such as dairy cattle and humans. Since March 2024, there have been 1,088 confirmed cases of HPAI in dairy cattle in 19 states in the country, according to USDA data.
As for humans, the numbers are a lot lower but still alarming, nonetheless. Since 2024, there have been 71 human cases of HPAI and two deaths with the cause suspected to be from the virus, according to CDC data. The most recent human case was in November 2025 in Grays Harbor County, Wash.
The only positive when it comes to humans contracting bird flu is that, since the first human case reported in 1997, there has been limited evidence to suggest that the disease can spread from human to human. However, one of the many negatives is that the presence of the virus in milk from infected cows has likely played a role in the virus spreading and makes transmission to humans a greater risk.
Additionally, backyard farms throughout the country don’t separate different animals, which can lead to interspecies transmission and possible mutation if a virus enters the farm.
With all this in mind, the spread of the virus to other species raises an important question: why are humans and livestock being infected with bird flu? A recent study by a team of researchers from Iowa State University tackled this question.
“The main thing we wanted to understand in this study is whether there is potential for transmission among these other domestic mammals and humans, and it looks like there is,” Rahul Nelli, the study’s lead author and a research assistant professor of veterinary diagnostics and production animal science with Iowa State, said in a release.
After gathering and testing samples from other production animals, such as pigs, goats, sheep, beef cattle, and alpacas, during a time when they were actively lactating, the researchers found that the mammary glands of these animals are biologically suitable to harbor avian influenza, due to high levels of sialic acids.
Sialic acids, sugar molecules found on the surface of many types of animal cells, are important for cell communication, immune response, and pathogen binding. Additionally, sialic acids are one of the major components of milk and play an important role in human nutrition, particularly in brain development, Nelli et al. (2025) said in the study.
While sialic acids are important, Nelli et al. (2025) explained that Influenza A viruses (IAV) utilize host sialic acids for initial attachment and cell entry, which is a key factor in the susceptibility to HPAI H5N1 infection. The presence of sialic acids is actively contributing to virus attachment and replication.
Also, the presence of sialic acid receptors along the mammary tissue suggests that both mammalian and avian influenza A viruses have the potential to bind, Nelli et al. (2025) added.
“The potential for HPAI H5N1 to bind to this tissue and possibly replicate could lead to viral mastitis, complications due to unknown etiology, breastfeeding complications, inflammation, pain, and discomfort, compromising both maternal and neonatal health,” Nelli et al. (2025) said. “Hence, there is an immediate urgency to understand the consequences of HPAI H5N1 in human breast tissues.”
Not only can the presence of these receptors cause harm to mothers and newborns, but it can also be harmful to farm workers and consumers of unpasteurized dairy products.
“Pasteurization kills influenza viruses, so store-bought milk is safe,” Nelli said. “But concerns about raw milk should extend to other mammalian livestock. Some people do consume the raw milk of these other animals.”
While further investigation is needed to determine the possible sustainability of Influenza A replication in the mammary glands of cattle, sheep, goats, alpacas, pigs and humans in experimental and natural environments, Nelli et al. (2025) suggest that IAV have the potential to bind to the mammary glands of these animals.
To conclude the study, Nelli et al. (2025) explained that routine milk or milk product testing from these species, together with continued surveillance of the human agricultural interface, will aid in containing the virus spread, establishing mitigation strategies, and understanding evolutionary changes in IAV and their interactions with different species.
“We need to try to stay ahead of this, so it doesn’t have a chance to continue to replicate and potentially evolve into something even more troublesome,” Dr. Todd Bell, Iowa State University professor of veterinary pathology and study co-author, said.
