In scientific experiments designed to assess the threat from H5N1 avian flu in the milk of infected cows, researchers today reported that the virus can bind to both avian and human-type cell receptors but doesn't easily spread through respiratory routes.
The research team, from the University of Wisconsin-Madison, two universities in Japan, and Texas A&M Veterinary Medical Diagnostic Laboratory, detailed its findings today in Nature.
So far, H5N1 has been confirmed on at least 140 dairy farms since March, with infections identified in four dairy workers. Since the virus first emerged in dairy cattle, scientists have been closely watching for changes in the virus that would signal a greater pandemic threat and conducting experiments to gauge infectivity and transmission.
New information on pathogenicity, receptor binding
In one set of experiments, researchers dripped milk from infected cows into the noses of mice and ferrets, which caused severe disease in the animals. Ferrets are often used to examine potential influenza A transmission patterns in people, because the animals show similar clinical symptoms and immune responses.
In the intranasal experiments, researchers compared the infectivity of three different viruses: H5N1 from cows, a Vietnamese H5N1 strain, and seasonal H1N1. Mice receiving the H5N1 viruses had high virus levels in respiratory and non-respiratory organs, including mammary tissues and muscles, but H1N1 was found only in respiratory tissues. The group also found that the virus can spread from mice mothers to their pups, likely via infected milk.
In ferrets, H5N1 was found only in the respiratory system.
"Together, our pathogenicity studies in mice and ferrets revealed that HPAI H5N1 derived from lactating dairy cattle may induce severe disease after oral ingestion or respiratory infection, and infection by either the oral or respiratory route can lead to systemic spread of virus to non-respiratory tissues including the eye, mammary gland, teat and/or muscle," the group wrote.
Our study demonstrates that bovine H5N1 viruses may differ from previously circulating HPAI [highly pathogenic avian influenza] H5N1 viruses by possessing dual human/avian-type receptor-binding specificity with limited respiratory droplet transmission in ferrets.
The group also found that mice can become sick after drinking even a small amount of raw milk from an infected cow, a finding that some of the same researchers had reported in a May research letter to the New England Journal of Medicine.
In yet another set of experiments, researchers mixed the H5N1 virus from cows with different types of receptors, which the virus uses to enter cells. They found that the virus can bind to receptors that can recognize both avian and human influenza viruses, which adds more evidence that the virus may be adapting to human hosts and that it may have the ability to bind to cells in the human upper respiratory tract.
The team wrote, "Collectively, our study demonstrates that bovine H5N1 viruses may differ from previously circulating HPAI [highly pathogenic avian influenza] H5N1 viruses by possessing dual human/avian-type receptor-binding specificity with limited respiratory droplet transmission in ferrets."
Ferret experiments didn't easily spread in air
Another big question about the virus is how well it can spread through the air. To gauge transmission, researchers placed H5N1-infected ferrets in cages that were near, but not touching, the cages of uninfected ferrets. None of the four exposed ferrets got sick or tested positive for the virus. However, follow-up tests found that one ferret had antibodies to H5N1.
When they did the same experiment with seasonal flu, they found efficient noncontact spread.
Yoshihiro Kawaoka, PhD, a group leader and professor of pathobiological sciences at the University of Wisconsin-Madison, said in a news release from the school, that the finding "suggests that the exposed ferret was infected, indicating some level of airborne transmissibility but not a substantial level."
Experts weigh in on new findings
Ian Brown, PhD, a group leader in avian virology with the UK-based Pirbright Institute, said data from mice doesn't always directly correlate to humans, but the new work on predicted cell binding offers new evidence for wider attachment, including cells lining the upper airway of humans. However, he added that more study is needed to understand the underlying factors.
"Overall the study findings are not unexpected but this report provides further science insight to an evolving situation, that emphasises the need for strong monitoring and surveillance in affected or exposed populations, both animals and humans to track future risk," he said.
Ed Hutchinson, PhD, senior lecturer at the MRC-University of Glasgow Centre for Virus Research, said it's notable that both H5N1 viruses can spread through the bodies of infected animals, including the mammary glands. "Now that we’re looking, it seems like spreading into the mammary glands is something that any of these highly pathogenic H5N1 viruses could do," he said. The results of the ferret respiratory transmission experiments are somewhat reassuring, but he said there is still reason for concern.
It seems like spreading into the mammary glands is something that any of these highly pathogenic H5N1 viruses could do.
When the researchers compared bovine H5N1 with the other H5N1 strain, they saw some evidence that the bovine strain had already started to gain some of the properties linked to the ability to spread through respiratory transmission in humans.
"Although it is good news that cow flu cannot yet do this, these findings reinforce the need for urgent and determined action to closely monitor this outbreak and to try and bring it under control as soon as possible," Hutchinson said.
