Study shows fatal spread of H5N1 virus from ferrets to suckling kits

During 2020, the highly pathogenic avian influenza A(H5N1) virus of clade 2.3.4.4b emerged and rapidly spread across many parts of Africa, Asia, and Europe. In 2021-2022, the 2.3.4.4b clade of H5N1 first spread to North America and then to South America where it caused large-scale mortality of seabirds. Except for causing episodic mortality of pinnipeds and cetaceans in Europe and North America, the virus had not spread to marine mammals. But that changed when the virus spread along the coast of Peru and Chile in 2022-2023 infecting and killing 30,000 sea lions; the virus also spread to dolphins, otters, and other marine mammals.

In Argentina, the virus spread to and killed over 17,000 elephant seals, including almost 97% of the pups. Till the outbreak in different mammals, H5N1 viruses were considered to rarely infect mammals and not likely to spread among mammals.

By the end of March this year, the virus was first reported in cattle in Texas. The first case of a farm worker in the U.S. was reported on April 1.

The patient developed symptoms on March 27, days after the first case in cattle was reported. The patient had a history of exposure to dairy cattle presumed to be infected with the H5N1 virus. As of November 22, 2024, 58 H5N1 cases in people have been reported in the U.S., and the virus has spread to 616 dairy herds across 15 States, with 402 herds in California alone.

The 2.3.4.4b clade of the H5N1 virus is typically found in the mammary glands of cows, with milk from infected cows containing high levels of viral RNA and infectious virus. The virus infecting the mammary gland raises concerns about milk as a possible interface for virus transmission to offspring through breastfeeding, and even to humans. Since studying H5N1 infection in the mammary gland through controlled studies in cattle is challenging and labour-intensive, researchers turned to ferrets to study the H5N1 virus of 2.3.4.4b clade infection and transmission in the mammary gland via milk. The researchers found that lactating ferrets with mammary gland infection with the H5N1 influenza virus (2.3.4.4b clade) are capable of spreading and causing disease in suckling ferret pups. It also caused 100% fatality in lactating ferrets and ferret pups. The results have been posted on a preprint server; preprints are yet to be peer-reviewed.

In another study published in Nature on October 28, ferrets infected with the H5N1 virus (clade 2.3.4.4b) had severe disease and 100% mortality. The study found the virus spreading efficiently between ferrets in direct contact but failed to spread efficiently between ferrets via respiratory droplets or fomites.

To study H5N1 virus 2.3.4.4b clade infection from lactating ferrets to infants, the researchers of the preprint study inoculated the virus into the mammary glands of lactating ferrets at 2.5 weeks postpartum. The preprint study notes that viral RNA titers significantly increased in milk over time and remained high in the mammary gland tissue. While suckling kits had significantly increased viral RNA in the oral and nasal cavities, the virus in the nasal cavities of lactating ferrets was delayed, with only minimal virus present in the oral cavity. Viral RNA levels in the lungs were low in the lactating ferrets but high in the suckling kits. This demonstrates that H5N1 2.3.4.4b clade infection in lactating ferrets leads to mastitis-related disease and virus spread to suckling pups, resulting in 100% mortality among the pups, the study notes.

They found intramammary H5N1 virus (clade 2.3.4.4b) infection resulted in 100% mortality of kits four days post-inoculation and six days post-inoculation in the case of the lactating ferrets. Milk collected from lactating ferrets showed viral RNA levels increased compared to baseline. “The presence of virus in the mammary gland suggests active viral replication within the gland,” they note.

Significant increase

While there was no difference in viral RNA in the case of the oral cavity of the lactating ferrets, there was a significant increase in viral RNA in the oral cavity of the suckling kits. RNA levels in the pups’ oral cavity peaked at four days post-inoculation, which is consistent with viral transmission in milk, they write. Though the lactating ferrets did not show any viral RNA in the nasal area even on day two post-inoculation, prolonged contact with the suckling kits led to an increase in viral RNA in the lactating ferret.

“Viral kinetics in the oral and nasal cavities suggest that kits developed respiratory infections directly from H5N1-positive milk, leading to respiratory transmission to the mother. These findings indicate that intramammary H5N1 infection causes more severe disease in lactating ferrets and their kits than H1N1,” the researchers write.