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A dairy worker in Texas tested positive for avian influenza this week. This new human case of bird flu—the second ever reported in the United States—isn’t cause for panic. The individual’s illness was mild—an eye infection—and they are already recovering. There’s still no evidence that the virus is spreading person to person. The person who became infected in Texas likely picked the virus up from infected cows or poultry on the farm where he works.

But the rash of recent infections among livestock is unsettling. Last month, goats in Minnesota tested positive. And avian influenza has now been confirmed in dairy cows in Texas, Michigan, Kansas, New Mexico, and Idaho. In some of those cases, the virus appears to have spread between cows. This week, let’s take a look at what we know about this new outbreak and what people are doing to prepare for further spread.  

The strain of flu infecting dairy cows—H5N1—is a highly pathogenic avian influenza. Scientists have been watching these viruses closely since the 1990s because of their potential to spark a pandemic. In 1997, avian influenza sickened humans for the first time. Eighteen people in Hong Kong became infected, and six died. 

Small spillovers into mammals aren’t uncommon for these viruses, especially in recent years. Avian influenza has been reported in mink, skunks, raccoons, coyotes, seals, sea lions, and bears, to name a few. But having the virus in domesticated mammals that come into frequent contact with humans is new territory. “Exactly what happens when an avian flu virus replicates in a cow and potentially transmits from cow to cow, we actually don’t have any idea at all,” says Richard Webby, a virologist at St. Jude Children’s Research Hospital who studies avian influenza.

Here’s the good news: even though the virus is infecting dairy cows (and now one dairy worker), “this is still very much a bird virus,” Webby says. Genetic sequencing by the USDA and the Centers for Disease Control suggests that these new infections are caused by a strain of flu that’s nearly identical to the virus circulating in wild birds. Few of the changes they did identify would allow it to spread more easily in mammals.

The spread of bird flu in cows is worrisome, but not as worrisome as it would be if the infections were happening in pigs, which are an ideal mixing vessel for flu virus. Pigs are susceptible to swine flu, avian influenza, and human influenza. That’s how swine flu emerged back in 2009—multiple viruses infecting pigs swapped genes, eventually giving rise to a virus capable of human transmission. 

Mammalian infections with bird flu have mostly been one-offs, Webby says. A mammal gets infected by eating a dead bird or ingesting bird droppings, but the infection doesn’t spread. One notable exception occurred in 2022, when H5N1 popped up on a mink farm in Spain and quickly jumped from barn to barn. Scientists also suspect that in rare cases, the virus has spread among family members. 

Cow-to-cow transmission hasn’t been confirmed, but the fact that some cows became infected after the arrival of cows from affected herds suggests that it may be occurring. That transmission may not be via coughs and sneezes—the traditional way flu gets passed on. It could be indirect. “So an infected cow drinks from a trough of water and the next cow comes along and drinks from that same trough,” Webby says.

How can we curb the spread among animals? That’s an ongoing debate. Vaccination is an option, at least for poultry. That’s common practice in China, Mexico, and a handful of other countries. Immunization doesn’t prevent infection, but it does reduce symptoms. That might curb the impact on flocks, but some experts are concerned that vaccinated flocks might allow the virus to spread undetected. Vaccination also would likely affect trade. Countries don’t want to import birds that might be infected. France decided to begin vaccinating ducks last year, and the USDA promptly announced it would restrict poultry imports from France and its trading partners. In the US, the current practice is to cull infected flocks. But there are signs that vaccination isn’t off the table. Last year the USDA began testing four vaccine candidates against the particular strain of H5N1 driving the current outbreak that has affected poultry across the globe. 

As a longer-term solution, researchers have also been working on creating genetically engineered animals that are resistant to bird flu. Last year, researchers created such chickens by using CRISPR to alter a single gene. 

For cattle, the current options to curb transmission are limited. Culling cattle would be a much harder sell because they’re so much more valuable than chickens. And cow vaccines for avian influenza don’t yet exist, although they would be relatively easy to produce. 

Bird flu has been on public health officials’ radar for more than two decades, and it has yet to make a jump into humans. “I do think that this particular virus has some fairly high hurdles to overcome to become a human-transmissible virus,” Webby says. But just because it hasn’t happened doesn’t mean it won’t: “We can be a little bit reassured that it’s not easy, but not assured that it can’t do it at all.”

Luckily, even if the virus suddenly acquired the ability to spread in humans, it would be vastly easier to develop a vaccine than it was to create one for covid-19. A vaccine already exists against H5N1. Doses of that shot are sitting in the country’s national stockpile. “This is one case we’re a little luckier because it’s a pathogen that we know. We know what this is and what we have in the freezer, so to speak. We have a little bit of a leg up on at least getting started,” Paul Marks, the FDA’s top vaccine regulator, told a reporter at the World Vaccine Congress this week. 

It’s not clear how well those doses would work against the current strain of H5N1. But many companies are already working on improved vaccines. Moderna plans to test an mRNA vaccine against the H5N1 strain causing the current outbreak. mRNA technology has a major advantage over traditional production methods for influenza vaccines, which grow the virus in eggs. In the event of a bird flu pandemic, eggs could be in short supply. Even if enough eggs were available, it could take half a year to develop a vaccine. mRNA technology, however, could shorten that timeline dramatically. 

That’s good news. With avian influenza surging across the globe, there are more opportunities than ever before for the virus to hit on a combination of genes that gives it the ability to easily infect humans. 

Now read the rest of The Checkup

Read more from MIT Technology Review’s archive

In a previous issue of The Checkup, Jessica Hamzelou explained what it would take for bird flu to jump to humans and why we don’t need to panic. Not yet, anyway. 

Google Earth can help scientists visualize the movement of H5N1 and perhaps even improve our ability to predict where outbreaks might occur. Rachel Ross had the story. 

Dig deep into the archives and you’ll find that Tech Review has been asking if bird flu will jump to humans for nearly two decades. Emily Singer reported on efforts to answer this question in 2006.

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