Avian influenza has been tearing through poultry, wild birds and even mammals without seriously affecting many people in the current outbreak. But in recent weeks, the virus has begun infecting U.S. dairy cows: the Centers for Disease Control and Prevention has reported sick animals in at least 34 herds as of April 26. And a human case involving a dairy worker in Texas has ramped up concerns that the virus, known as H5N1, could spread more widely in people. The U.S. Department of Agriculture has announced it will test ground beef in states with outbreaks as a precaution, although the virus is thought to spread through close contact with infected animals.
With COVID’s rapid development into a lethal pandemic still fresh in many people’s mind, it’s difficult to know how worried we should be about another infectious respiratory disease gaining ground. So Scientific American spoke with Katelyn Jetelina, an independent epidemiologist who writes a popular public health newsletter, about how the outbreak has gone so far and how the U.S. could strengthen its management—which she says began as “a very uncoordinated, messy response.”
[An edited transcript of the interview follows.]
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As an epidemiologist who followed COVID closely, what has it been like for you to watch the bird flu situation unfold?
There’s been a lot going on and pretty quickly, which I think is making us epidemiologists and virologists, as well as veterinarians, a bit nervous. H5N1 has been really heating up in the past two or three years, but what makes the past couple of months unique is that this is spreading among cows.
The mantra we always work on is that spillovers are common, but pandemics are rare because so many unlucky things have to happen in sequence. But pandemics are a game of roulette: they rely on luck, and they can get dramatic pretty fast.
Are we repeating the mistakes of the early days of COVID?
There are things we’ve learned and things we haven’t learned. One of the biggest challenges with this is: it involves humans and animals, so there are a lot of players—we have multiple agencies working in their lane. I’ve been impressed with how the CDC has been updating the public, and I hope that’s because they learned their lesson during COVID. Other players haven’t necessarily learned those lessons because they weren’t in the COVID trenches.
One key difference [with bird flu] is: this hasn’t seen human-to-human spread. We’re not talking about January through March 2020. I do think that we are watching this unfold in real time better than we did with COVID, which seemed to be coming out of nowhere, so that gives me a little more hope.
How have scientists been tracking the spread of bird flu in cows?
We don’t have a good sense of the spread because testing is voluntary and certainly not being done in a systematic way. We’re pretty much flying blind with the testing aspect.
But we have had two clues that this virus has spread very widely: The Food and Drug Administration found viral fragments in milk. And genomic surveillance estimates that spillover to dairy cows started back in December 2023, if not earlier, which means this has been spreading under our noses for a much longer time than we originally thought.
That means our surveillance systems are not great. It also means we haven’t been testing enough. We don’t think that this means the virus is being spread to humans, but it may mean that H5N1 is starting to find a new host in cows, which would not be great news.
Should people be worried about getting the H5N1 virus from milk?
It sounds scary, and I understand the concern. One thing that has made me calm down is knowing that we’ve had, like, 100 years of data around pasteurization, and it works really well. We also got confirmation that scientists were not able to grow active virus from the milk samples, which were pasteurized. That means these virus fragments detected in milk were broken pieces that cannot replicate and thus cannot harm humans. This is a good sign.
Do we know how the virus jumped into cows or how it’s spreading?
From the genomic surveillance, it looks like the spread to cows was likely from one source—probably a dead bird. One hypothesis is that the virus got into cow feed, and then cows ate it, and then it started spreading from cow to cow through the milk machines. But a lot of this is still up in the air.
Veterinarians swabbing cows have found the most virus in the udders, but they have found other positive swabs from, for example, lung tissue. So how is this actually spreading? We think it’s the milk machines spreading the virus from cow to cow, but we need to make sure that it’s not via respiratory transmission as well.
A huge new puzzle is that there is also transmission from cows to poultry. That is concerning; it gives this virus even more opportunity to adapt because it keeps jumping from species to species, and we don’t know how. Is it through humans? Is it through rats? Is it through feed? We don’t know. Trying to get a really good understanding of epidemiology here has been challenging just because the data are limited, and the communication is suboptimal.
The CDC says there has only been one case in humans from this outbreak. How plausible is that?
I’m a little skeptical because we haven’t been doing asymptomatic testing, and symptomatic testing is voluntary. There have been reports this week that there are other humans who had symptoms who didn’t test.
Until we start seeing data, I don’t think we should come to sweeping conclusions that it’s only one human case. The good news is that it doesn’t seem like anyone’s died yet, and we’re not seeing huge clusters [of sick people] in emergency departments. So if there are more human cases, I’m pretty confident they’re from direct contact with cows rather than human-to-human transmission. But we need more data and more communication around this.
Many people who have close contact with cows are migrant workers, some of whom are likely undocumented. Does that make them less likely to report any flu symptoms?
Yeah, there are so many layers to this. If they test positive, they’re not going to be able to work, and that has huge implications for their family. The other two big challenges that the CDC brought up publicly are that there’s a language barrier and that some of these people are undocumented workers with a lack of trust in government institutions.
Are we monitoring the virus in wastewater like we did for the virus that causes COVID?
One huge lesson we learned during the COVID pandemic is the value of wastewater. This is a perfect case to leverage this massive network that we built to try and figure out how and where H5N1 is spreading, given the limitations and biases of testing right now. What’s been disappointing is that the government has not officially shared any [flu data from] wastewater. To monitor human infections, the CDC said that they’re utilizing emergency department data, which is great. But once an outbreak hits emergency departments among humans, it’s already kind of out of control, so I would love to see wastewater surveillance leveraged a whole lot more.
If we need vaccines and treatments for bird flu, how prepared are we?
The government has confirmed that tools such as Tamiflu [an antiviral medication] would work to a certain extent against H5N1. Of course, Tamiflu is not very effective even against our seasonal flu. We also have stockpiled H5N1 vaccines that are predicted to have efficacy if this does move to humans, which is great news. [Editor’s Note: These vaccines require two doses per person. The U.S. government says it has some 10 million doses available and could make about 125 million or so within four months, which would total enough to cover about one fifth of the country.]
But there are still a lot of unanswered questions: What about manufacturing and supply? What about the rest of the globe? What about vaccine hesitancy and declining trust and access problems? Fully relying on vaccines and overconfidence were some of the biggest mistakes of the COVID emergency, and I think it’s best to face these threats and questions with a level of humility and determination.
When should people who work with livestock get access to those vaccines?
I don’t know what that line is, when we’re like, “This is an ‘oh, shit’ moment” versus a “watch and see moment.” Getting that line incorrect has massive implications. Certainly if we start seeing any trace of human-to-human transmission, we would really need to start ramping up. If we start seeing transmission in pigs, that would make me incredibly nervous. Pigs are a really great mixing vessel for influenza, so it would be incredibly easy for this virus to start mutating in a way that would have implications for humans. Veterinarians are continuing to test pigs for H5N1, and they have so far been negative, so I’m really happy we’re on top of that.
What’s missing from the U.S.’s H5N1 response right now?
We need a coordinated response from our government. I get that there are multiple players who have their own priorities and legal authorities, but honest, frequent, direct communication earns the public’s trust and confidence. When communities are starved for good information during outbreaks, it leads to unnecessary anxiety, confusion and frustration. The winds may be changing in this area, but we need to get better faster.
On the ground, we really need to get a handle on where H5N1 is spreading and how. That comes through asymptomatic testing of animals and people, sharing genomic surveillance data with the global community and understanding wastewater trends. So certainly, a lot more work needs to be done.
How worried should people be right now?
This is not a concern for the average American unless they’re working with a livestock. What I tell my mom, for example, is that the general public doesn’t have to worry. This is very low-risk to them at the moment. But that could change.
I think the most we can do is to know that this is going on and to pay attention to the status of the situation by finding trusted sources of information. There is so much misinformation and disinformation that finding and sticking to those credible sources is going to be incredibly important if this situation continues to evolve, which we expect it to.