Debate begins over whether a bird flu virus that infects cows could be better adapted to infect humans

A study published this week provides new evidence that the H5N1 virus currently causing an outbreak of bird flu in U.S. dairy cattle may be better suited to infecting humans than other strains of the virus in circulation. The discovery is causing controversy among the world’s leading flu researchers, according to Stat.

Influenza viruses circulate constantly in different animal species around the world. One factor that determines what type of animal an influenza virus can infect is the type of receptor present in the tissues the virus comes into contact with. Influenza viruses that commonly infect birds have an affinity for a specific receptor that is common in the intestines of birds. Human influenza viruses, instead, prefer a receptor located in the upper respiratory tract.

The study was published in the journal Nature

A new study published in the journal Nature found that the bovine H5N1 virus can bind to both types of receptors. “There is an ability to bind to human-type receptors,” lead author Yoshihiro Kawaoka told Stat. However, he cautioned that it is too early to say whether this ability represents a newly emerged bovine branch. The H5N1 evolutionary tree has an increased potential to become a significant human pathogen. “Binding to human receptors is not the only factor required for avian influenza virus to replicate well in humans,” added Kawaoka, a leading virologist at the University of Wisconsin-Madison who has studied H5N1 for decades.

Ian Brown, former head of virology at the UK Animal and Plant Health Agency and now group leader at the Pirbright Institute, said in a statement that the findings were not unexpected but provided new scientific insight into a developing situation that highlights the need to monitor affected or exposed populations, both animals and humans, to monitor future risks.

Different results against H5N1 virus

The result is sure to fuel concerns that the H5N1 virus currently circulating in dairy cows has adapted to spread more efficiently among people. But other scientists have found different results by examining the same molecules that the bovine H5N1 virus uses to infect cells.

James Paulson of the Scripps Research Institute and Scott Hensley of the University of Pennsylvania said their data suggest the bovine H5 molecule binds weakly to human receptors, highlighting the need to determine why the different results are observed.

Kawaoka’s team used a method that involves coating plastic sheets with synthetic versions of the different receptor subunits, mixing them with the H5N1 virus, and measuring the amount of virus that adhered. Ron Fouchier, a virologist at Erasmus Medical Center in the Netherlands, noted that the method was easy to implement and interpret, but there are other methods available that would give a clearer picture of binding specificity.

Other parts of the study show that the H5N1 virus is not very effective at infecting mammals through the respiratory tract, but it has an affinity for mammary tissue and can be transmitted effectively through contaminated milk. In previous experiments, Kawaoka’s team showed that female mice fed milk from cows infected with H5N1 became severely ill, and the virus spread throughout their bodies. In the most recent studies, the scientists confirmed that mice are susceptible to infection even with small amounts of contaminated milk.

Vertical transmission of the virus

They also showed for the first time that vertical transmission was possible; infected mice could pass the virus to their offspring through their milk. They also infected ferrets intranasally and found that although the animals became sick, they were unable to effectively transmit the virus to other nearby ferrets.

The findings are consistent with a study by the U.S. Centers for Disease Control and Prevention in May that found that the H5N1 virus isolated from the first human case linked to an outbreak among dairy cows in Texas was easily transmitted between ferrets living in the same cage, but not between cages.

“Continuous surveillance is still necessary,” Kawaoka said, concluding that we should be concerned about the evolution and spread of the virus.