A highly infectious type of bird flu began spreading in dairy cows across the US earlier this year, providing the pathogen with one of its best opportunities to date to evolve and more easily infect people.
Infectious disease experts have been concerned about the virus, called H5N1, for decades. Of the more than 900 people worldwide who are known to have caught it since 2003, around half as a result.
For now, H5N1 is poorly adapted to infecting humans and, as far as we know, can’t pass between people. But each time a non-human animal transmits the virus to a person or another mammal, it has an opportunity to acquire mutations that could change that.
Advertisement
This is why public health experts were particularly alarmed when a dairy cow in Texas tested positive for H5N1 in March. It not only gave the virus an opportunity to better adapt to mammals, but also put it in close proximity to dairy workers.
Despite these risks, US officials have largely left H5N1 to spread without restrictions in cattle. As a result, more than 690 herds across 15 states had for the virus by early December.
“What is important to recognise about [H5N1] is that it has already been unprecedented,” says at Johns Hopkins University in Maryland, referring to a subtype of the virus that emerged several years ago, killing tens of millions of birds and thousands of mammals around the world, including foxes, bears and seals. “It has done things that highly pathogenic avian influenza viruses typically don’t do. So to have it enter the dairy cow population was really remarkable.”
Cows aren’t natural hosts for avian flu viruses, which is partly why public health officials were caught off guard by the outbreak, says at Emory University in Georgia. Unlike the chicken or pig industry, the dairy industry in the US doesn’t have a robust, centralised system for monitoring and containing flu viruses.
Developing such a system has been complicated by the fact that state governments have jurisdiction over how to respond to disease outbreaks in farm animals, leaving each state to implement its own surveillance method. For instance, Texas has largely taken a hands-off approach, recommending that farmers only test cows that appear sick. Colorado, on the other hand, tests milk tanks on individual farms, as sick cattle shed high amounts of the virus in raw milk.
“What we don’t know, which we should know, is how many herds are actually infected in the US – and even when we talk about a positive herd, we don’t know how many cows on that herd are infected,” says Lakdawala.
It is also unclear exactly how the virus is spreading among cattle, she says. While most evidence points towards milking equipment, which isn’t always sterilised between uses, it could also be transmitted via respiratory droplets, she says.
“I had thought after the covid-19 pandemic that, in the face of [another virus] emerging, we would be stronger in our response to prevent another pandemic,” says Lakdawala.
Getting to the bottom of how H5N1 is spreading among cows is crucial to prevent it infecting more people. in the US have tested positive for H5N1 this year. Canada also recorded its first case this year in a teenager in British Columbia who, as of late November, was in a critical condition.
The Canadian teenager, along with one case in Missouri and another in California, had no known contact with animals or raw milk, so it isn’t clear how they contracted the virus. Everyone else who has been infected so far has fully recovered and most have only experienced mild symptoms, such as eye redness. This death rate is very different from what has generally been recorded since 2003, which may be a reflection of different healthcare services around the world or how the virus enters the body. For instance, infections that arise via the eyes are thought to be more severe than those that start in the airways.
While no human fatalities have occurred so far in this outbreak, there are some indications that the virus is adapting to more readily infect mammals, says Lakdawala. For example, a genetic analysis of a sample collected from the person in Missouri led to the identification of a mutation that seems to diminish the ability of antibodies to recognise and neutralise the virus.
It is too early to know whether this and other mutations will make H5N1 more dangerous to people, but as long as the virus circulates freely among dairy cows, it may just be a matter of time before that is the case.
“At this point in time in the outbreak, we should have a better handle on some very basic fundamental questions,” says Lakdawala.
Article amended on 11 December 2024
This article has been changed to correct the recovery statuses of the people with known infections.
