Intramammary infection of bovine H5N1 influenza virus in ferrets leads to transmission and mortality in suckling neonates

Abstract

The spread of H5N1 clade 2.3.4.4b in dairy herds raises concerns about zoonotic transmission due to the high viral load in milk, a key point of contact between livestock and humans. H5N1 clade 2.3.4.4b exhibits tropism for the mammary gland, with milk from infected cows containing high levels of viral RNA and infectious virus. The virus's ability to infect the mammary gland suggests potential risks of transmission to offspring through breastfeeding. Here, we show that mammary gland infection with bovine H5N1 influenza in lactating ferrets transmits virus and disease to suckling kits. Viral RNA titers significantly increased in milk over time and remained high in mammary gland tissue. Suckling kits had significantly increased viral RNA in the oral and nasal cavity, as well as in feces. However, viral detection in the dam nasal cavities was delayed, with only minimal virus present in the oral cavity. Lung viral RNA levels were low in dams but high in a sampled suckling kit. Additionally, we detected the expression of the H5N1 receptor, α2,3 sialic acid, in mammary tissue from both ferrets and humans. These data demonstrate that H5N1 clade 2.3.4.4b infection in lactating dams leads to mastitis-related disease and transmits to suckling pups, resulting in 100% mortality among the neonates.