The avian influenza virus is transmitted between mammals through contaminated milk.

The H5N1 avian influenza virus can be transmitted between mammals, according to a study published in ‘Nature’. The research team isolated the virus in the milk of an infected cow in New Mexico (USA) and found that it spreads in mice. and ferrets, reaching the mammary glands of both animals. In addition, the virus was also transmitted from infected suckling mice to their offspring.

The detection of bovine influenza on a US dairy farm in spring 2024 is the first documented outbreak of highly pathogenic H5N1 avian influenza in cattle. Since then, viral spread among herds and infection in other mammals, including humans, has been documented, indicating an increased risk for public healthMammary gland infection and contaminated milking equipment are speculated to be involved in transmission between cows, and viruses have been detected in the milk of infected cows, but the basic characteristics of bovine H5N1 are unknown.

Yoshihiro Kawaoka and colleagues at the University of Wisconsin-Madison (USA) characterized an H5N1 virus isolated from the milk of an infected dairy cow in New Mexico, USA.

They tested how the virus replicates and causes disease in mice and ferrets (two common animal models for studying influenza in mammals) and showed that the virus spreads systemically, including to the mammary glands of both animals.

The authors also showed that the same occurred with an earlier version of the H5N1 avian influenza virus, which has not been detected in cows, indicating that infection of the Mammary glands may have been a previously overlooked feature in mammalian infection with these avian viruses.Transmission of the virus from infected suckling mice to their offspring was observed.

According to the authors of this experimental study, carried out on mice and ferrets, the ability to transmit via milk derives from the virus’s capacity to proliferate in the mammary gland. This new ability of the virus could have been produced by its capacity to use the appropriate cellular receptors as a result of its adaptation to mammals.

For the virologist Aitor Nogales Gonzálezfrom the Animal Health Research Centre (CISA, INIA-CSIC), some of these results are similar to those already obtained with other H5N1 viruses in the past. However, “the researchers also point out that the virus isolated in the outbreak that occurred in cattle has characteristics that can facilitate infection and transmission in mammals, including humans. This conclusion is due to the ability of the virus to bind to cellular receptors present in the upper respiratory tract of humans.”

Transmission capacity

“This is a qualitative leap in the ability of these highly pathogenic H5N1 viruses to transmit to other non-avian animal species,” he told Science Media Centre.Gustavo del Real Soldevillaa researcher at the Center for Research on Influenza Pathogenesis and Transmission (CRIPT) at the Icahn School of Medicine at Mount Sinai, New York (USA).

Until now, he continues, “cases of transmission of this type of virus to various species of mammals have been reported, especially cats, mustelids, seals, elephants and sea lions, as well as a few hundred isolated human cases. Fortunately, subsequent sustained transmission between individuals of these species is not very efficient, except for those that have occurred in recent months in marine mammals on the coasts of South America and in the Antarctic region.”

The presence of the virus in the milk of infected cows facilitates a new route of transmission of the virus both to their offspring and, potentially, to the human species.

The novelty of this new episode, highlights Real Soldevilla, “apart from affecting the bovine species for the first time, is the presence of the virus in the milk of infected cows, which facilitates a new route of transmission of the virus both to their offspring, and, potentially, to the human species – only in the case that the milk is not adequately sanitized.”

Given the increase in cases in mammals that have been infected by avian influenza of the H5N1 subtype and the characteristic evolutionary capacity and adaptation of the influenza virus, says Nogales, “the situation must continue to be actively monitored. In addition, it is important to Evaluate the evolution of the virus and the functional significance of changes in its genome to identify possible adaptations to mammals, including humans».

Although the risk to the general population is considered low, Nogales points out that “this situation could change in the future and we must remain alert. We must remember that the most recent flu pandemics have had avian flu as a key player.”

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