July 11, 2024


New study highlights potential human adaptation of H5N1 virus in US dairy cattle



A study presents new evidence suggesting that the H5N1 virus currently causing an outbreak of bird flu in US dairy cattle may be more adapted to infecting humans than other circulating strains of the virus, sparking debate among leading flu researchers, Stat reported.


Influenza viruses circulate globally in various animals. The type of receptors present on the tissues they contact largely determines the kind of animal a flu virus can infect. Bird flu viruses typically bind to receptors found in the guts of avian species, while human flu viruses prefer receptors lining our upper respiratory tracts.


The new research, published in Nature, indicates that the bovine H5N1 virus can bind to both types of receptors.


"There is an ability to bind to human-type receptors," said Yoshihiro Kawaoka, the study's lead author and an influenza virologist at the University of Wisconsin-Madison. However, he cautioned that this does not necessarily mean the virus is more likely to become a significant human pathogen. "Binding to human-type receptors is not the only factor required for an avian flu virus to replicate well in humans," he added.


Ian Brown, former head of virology at the UK's Animal and Plant Health Agency and now a group leader at the Pirbright Institute, commented that while the study offers new insights, further research is needed to understand the underlying factors. "Overall, the findings are not unexpected but provide further scientific insight into an evolving situation, emphasising the need for strong monitoring and surveillance in affected populations, both animals and humans, to track future risks."


The study's results may raise concerns that the H5N1 virus now present in dairy cows could be adapting to spread more efficiently among humans. However, other scientists have reported different findings when examining the same molecular interactions used by the bovine H5N1 virus to infect cells.


James Paulson, chair of Chemistry in the Department of Molecular Medicine at the Scripps Research Institute, stated that his lab, in collaboration with other groups, found "no suggestion of increased 'human type' specificity" in the bovine H5N1 virus. Scott Hensley, a microbiology professor at the University of Pennsylvania, also reported differing results, indicating that the bovine H5 molecule binds poorly to human receptors. "It will be important to determine why we are seeing different results," he said.


Kawaoka acknowledged the conflicting data, attributing the differences to variations in experimental design. His team used a method involving synthetic receptor subunits, while other groups used the entire receptor molecule naturally found on human cells.


Ron Fouchier, a flu virologist at Erasmus Medical Centre in the Netherlands, noted that while Kawaoka's method is easy to perform and interpret, other methods might provide a clearer picture of binding specificity. "The dual receptor binding is interesting, but I do not find these results very unsettling," he said. He called for further analysis to identify which mutations are driving the virus's ability to bind to different receptors.


Additional components of the study indicate that the H5N1 virus does not efficiently infect mammals through the respiratory route but has an affinity for mammary tissue and can transmit through contaminated milk. In previous experiments, Kawaoka's team showed that female lab mice fed milk from H5N1-infected cows became very ill. The latest research confirmed that even small doses of infected milk could infect mice, and that vertical transmission from mother to pups through milk is possible.


Experiments involving ferrets, which are commonly used to study respiratory virus transmission, showed that while the animals fell ill, the virus did not spread efficiently to other ferrets in nearby cages. These findings align with a May study by the US Centres for Disease Control and Prevention, which found limited transmission among ferrets.


"It's not zero transmission; there is some transmission but it's very limited," Kawaoka said, suggesting that the virus has not yet acquired the ability to easily spread through the air. However, with the virus expanding its presence and opportunities to adapt to human biology, continued surveillance is crucial.


-      Stat