Distribution of sialic acids in the bovine mammary gland across different physiological states

Highly pathogenic avian influenza (HPAI) A(H5N1) virus emerged in lactating dairy cattle in March 2024, causing mastitis-related disease and infections in other farm animals and workers. Recent work identified α2,6 and α2,3-linked sialic acids (SIA), which serve as influenza virus receptors, in the lactating bovine mammary gland; however, their distribution across stages of mammary growth and development remains unknown. We compared the distribution of sialylation in mammary glands of prepubertal dairy calves, primigravid dairy heifers, and lactating cows. Mammary glands at all physiological stages displayed both α2,6 SIA, the preferred receptor linkage for human influenza viruses, and α2,3 SIA, the preferred receptor linkage for avian influenza viruses. Importantly, in mammary glands of pregnant dairy heifers, α2,3 SIA was most abundant, distributed in both mammary tissue and alveolar lumens. Our results indicate that the mammary glands of pregnant dairy heifers, like those of lactating dairy cows, display receptors that could support H5N1 binding and entry. Further studies are needed to determine whether developing mammary glands can be a site of influenza virus replication.