The recent spread of avian influenza in dairy cattle in the U.S. has startled even some scientists who've tracked a global outbreak of the virus over the last few years.
"There's a heap of unknowns right now," says Richard Webby, a virologist at St Jude Children's Research Hospital.
How widespread is the virus in dairy cattle? What could this mean for humans? None of this is clear yet.
The first cases of this H5N1 bird flu strain emerged in North America among wild migratory birds in late 2021 and soon spread to poultry farms. It's now showing up among dairy cows and at a major egg producer and one person who had close contact with cows has been infected.
"This particular version of the H5N1 virus is teaching us that some of the things we thought we knew about flu were wrong," Webby says.
The current outbreak has affected many new wild bird species and persisted for longer than previous ones. The virus has also popped up more often in mammals, both in the wild and on farms, and at times led to a wave of infections and death.
"We are in fairly unprecedented, uncharted territory, globally in relationship to avian influenza," says Dr. Peter Rabinowitz, director of the UW Center for One Health Research.
But federal officials and scientists stress the risk to the public still remains low.
So far, the virus does not appear to have mutated in a way that would make it significantly more dangerous. While concerning, the one human case, they say, is consistent with how people usually catch these viruses, through direct exposure to a sick animal.
But scientists are watching this outbreak closely. Here's more of what they are learning.
1. Genetic sequencing shows 'minor' changes in the virus, nothing alarming
While it's still early days, Webby says the genetic sequencing collected from infected cattle hasn't turned up anything that "immediately screams, this virus has changed, and that's why these cows are getting infected."
"It just seems to be fairly typical of the viruses that have been detected in birds in various regions," he says.
Sequencing of the virus in the Texas patient did show "minor changes," includung one mutation associated with viral adaptation to mammals that's appeared in other human cases, according to a report from the Centers for Disease Control and Prevention.
However, there's no indication from those previous infections that this mutation makes the virus more likely to spread among humans.
This change does seem to have happened when the virus jumped from a cow to the person, but there's nothing altogether alarming about it, says Angie Rasmussen.
"It doesn't look like there's any indication that this has adapted itself to spread efficiently between humans, and to routinely cause severe disease," says Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan.
But she says this human case and the infections in dairy cattle are clear warnings: "The less human or cow transmission we have, the fewer of these mutations the virus can acquire."
2. Human-to-human spread of bird flu remains extremely rare
It's generally rare for people to catch any type of bird flu and human-to-human spread is even rarer.
During the current outbreak, this version of H5N1 has only been detected in a handful of humans in the last few years globally, and there aren't anydocumeneted cases of human-to-human transmission.
In the Texas case, the person's only symptom was eye redness after being exposed to cattle. It's the second known H5N1 infection of a human in the U.S. In 2022, a poultry worker in Colorado was exposed to sick chickens and developed a mild illness.
Some recent human infections have led to severe illness in other countries, including Ecuador, Chile and China. "This is a virus that doesn't infect humans very well, but can — not all the time — cause very significant disease when it does."
Historically, human infections with avian influenza were often traced back to close contact with birds, specifically in markets or on farms.
"If you're exposed to bird feces, if you're exposed to dead birds, if you're around a lot of live birds, you're going to be exposed to more of that," says Rasmussen.
Unlike the seasonal influenza viruses that infects humans, H5N1 doesn't have the ability to easily attack our upper respiratory tract, so it doesn't tend to spread among humans.
However, the virus can bind to receptors in the lower respiratory tract. This may be one reason that people who develop respiratory infections with bird flu "can get very, very sick with severe pneumonia because those receptors are located deep in the lungs," says Rasmussen.
Of course, scientists are looking out for any signs that the virus has adapted to better target our upper respiratory tract.
The recent case in Texas also raises the possibility of "mucosal exposure," meaning the person may have come in contact with the virus and then touched their eyes, although the details and what that might mean for tranmission are unclear, she says.
3. Cows may be spreading it to one another, but it's not proving deadly
A central question for scientists right now is whether there's significant transmission of the virus between dairy cattle.
Cases have been detected in herds in Texas, Michigan, Kansas and New Mexico, and are suspected in other states, as well.
Evidence suggests infected wild birds may have been the initial source of the infection, but "it's hard to explain exactly what's going on without some degree of mammal-to-mammal spread," says Webby.
Others tracking the outbreak agree.
"The way people are telling me it gets on their farm and moves, I'd be very surprised if this was not being spread from cow to cow," says Joe Armstrong, a veterinarian at the University of Minnesota.
In early February, Armstrong started tracking reports of a mysterious illness in dairy cattle on the Texas panhandle and other states. He says just how many of these cases can be attributed to avian influenza still remains fuzzy.
"If we have wild birds involved and other wildlife, it's almost impossible to keep livestock and wildlife separate completely," he says.
While cows are falling sick, it's not proving to be deadly, says Armstrong. And so far, federal officials emphasize the commercial milk supply is not at risk because products are pasteurized.
4. Sustained spread between mammals could potentially lead to more problematic mutations
Currently it's not clear exactly how bird flu is spreading among mammals, and to what extent infections are mostly happening after some kind of contact with infected birds.
But scientists worry about sustained mammal to mammal transmission of avian influenza because that gives the virus more opportunities to adapt to that host and acquire mutations that could make it better suited to mammals.
There have been large die offs of marine mammals in South America and a particularly alarming outbreak on a mink farm in Spain.
In both of those examples, the virus had evolved a couple of "mammal adaptive mutations" that haven't yet been seen in cows, says Louise Moncla, a virologist at the University of Pennsylvania.
Cows are typically infected by a different type of influenza, so that makes it hard to speculate about the risk posed to humans.
"It's just simply very unusual and very odd," says Moncla.
Unlike pigs — known to be intermediary hosts for human and bird viruses — there is no data that show that cows are an important intermediary host for these viruses, she says.
5. An existing bird flu vaccine could be tapped and adapted in case of human spread
An ongoing outbreak in livestock not only threatens the industry, but also makes it more likely that other animals will be exposed, or the workers themselves.
"In general, we have not paid a lot of attention to these workers, even though they've often been sort of like the canary in the coal mine, the first evidence of a transmission event," says Rabinowitz.
Federal health officials stress that they are taking the situation seriously.
'The United States has been preparing for avian flu outbreaks for more than 20 years," CDC director Dr. Mandy Cohen told NPR's All Things Considered. "It's very different from what we experienced, for example, at the beginning of COVID, when we were seeing a brand new novel virus where we didn't have tests, we didn't have treatment and we didn't have vaccine."
The U.S has a limited stockpile of vaccines that were developed for early strains of H5N1 that could be tapped if there's any human-to-human spread.
Immune-stimulating ingredients, known as adjuvants, can be added to these older vaccines in order to broaden the immune response so that it better covers mismatched strains. In addition, mRNA technology could be leveraged to produce new vaccines, says Dr. Wilbur Chen, at the University of Maryland's School of Medicine.
"All of those can be used in a pandemic response," he says,
Chen says part of the ongoing preparation for the possibility of more human cases could include manufacturing limited quantities of vaccines.
Dr. Ashish Jha, dean of Brown University's school of public health, says he would not be surprised if there are more cases in humans, most likely other farm workers.
But he says it's not yet time to start mobilizing a larger pandemic response — say, pumping out millions of vaccines — because the chances that will be needed are very very low.
"If you are seeing it widespread in farm workers, you want to think about vaccinating farm workers. If you start seeing it in non farm workers with evidence of human-to-human transmission, that's when you start wanting to think about vaccinating a much broader set of the population," he says.
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