The Arizona Department of Health Services said there have been 64 cases of West Nile virus so far this year. There were 82 cases in 2023 and 60 in 2022.
New research from Arizona State University found that there are more mosquitoes in Arizona now, but climate change may have an impact on their population.
Kelsey Lyberger studies the ecology of mosquitoes as an assistant professor in the College of Integrative Sciences and Arts at Arizona State University. When she came by the studio to talk about her work, she said she was surprised by how much of a threat West Nile is in the desert.
Full conversation
KELSEY LYBERGER: Thinking about the distribution of some of these vectors and mosquitoes in general, they like it warm and they like it wet. But Phoenix is so hot in the summer, I was surprised they could persist through that heat.
MARK BRODIE: Well, so how are they doing it? Because as you say, Phoenix is pretty darn hot, especially in the summer.
LYBERGER: Yeah, and the interesting part about how they're doing that is that I think it depends on what species you're talking about. We have multiple species here in the area and, for some of them, they are finding these little microhabitats, often that we are creating, where it's a little bit cooler and it's wetter — whether that's from irrigation or as we just got some rainfall.
BRODIE: So, could this be the kind of thing where, like in somebody's backyard you have, you know, maybe a flower pot that's empty, that has some water and it's maybe in the shade, so it's out of direct sun. Is that the kind of microhabitat you're talking about?
LYBERGER: Yeah. So that would be a perfect habitat. For example, a species called Aedes aegypti, they love those human-made containers.
But we also have species like Culex, which are the main vectors of West Nile virus. And they're breeding in slightly more organically-rich water. And so they're finding storm drains that maybe have backed up a little bit or, for example, in this study that I've been working on over the summer, we went to a bunch of nurseries where they're ... healthily watering some trees.
And another big one here that I'm not sure I would have thought of beforehand is empty swimming pools. So people maybe take off for the hot summers and leave their pools, maybe drain them, but then they fill again if we get rainfall. And they love breeding in there. And you can imagine a swimming pool full of mosquitoes certainly breeds a lot.
BRODIE: Yeah, that's a lot of space. I mean, that could probably lead to a lot of mosquitoes and potentially a lot of illness.
LYBERGER: Yeah, and the swimming pool mosquitoes are one of those vectors for West Nile virus. ... The county vector control guys around here are doing their best and they have a huge investment in setting traps, monitoring the situation both in terms of mosquito abundance, but also that they test them — the mosquitoes themselves — to see if they're carrying the virus. And if, kind of, any thresholds are hit, they're going to go out and do something about it.
BRODIE: How big of a temperature difference do you find that the mosquitoes need from sort of, you know, maybe the high temperature at Sky Harbor, to where it is a better place for them to breed? Are we talking just a couple of degrees or does it have to be more than that to make it ideal conditions for them?
LYBERGER: Yeah. So a place like Sky Harbor can record temperatures of like 120, whereas the limits for these mosquitoes, they can't survive beyond something like 110. ... And so we're finding these mosquitoes are in places easily 10 to 15 degrees cooler than a weather station might report.
BRODIE: But even 110 degrees, I mean, during, you know, let's say July or August, a good part of the Valley, if not all of it, is routinely above 110. And yet, as you're finding, these mosquitoes are still breeding.
LYBERGER: Yeah. So that really touches on a couple of things that I want to do with my research, which is that right now, a lot of the kind of science of the temperature limits of mosquitoes is considered in terms of a single number: What is their thermal maximum? What's that one hot temperature? But really, survival at those temperatures is a function of time.
So we might hit 110 for a few hours, but then at night you're going to have reprieve from that. It's going to drop down. And so really, you touch on what I'm hoping to do in the lab next, which is test for how long can they withstand these really hot temperatures?
BRODIE: I mean, do you think it's possible — and not that anybody is hoping for Phoenix to get hotter — but if it does, could we kind of heat our way out of a disease like West Nile virus?
LYBERGER: That would be awesome if it happened, but I'm guessing not. I am guessing, for the most part, it's here to stay, because these mosquitoes are great at finding the habitats that we create for them to mitigate that heat. And it doesn't take much for them to successfully breed, emerge as these adults and females looking for a blood meal.
And to add on to that, which is that West Nile virus itself has to replicate inside the mosquito and that that process is temperature sensitive. So as you heat the outside air, the virus can replicate more quickly inside the mosquito, and that means transmission could happen even faster.
BRODIE: I wonder if that also means transmission could potentially happen later in the season here than it would maybe somewhere on the East Coast when it starts to cool off a little bit. And yet, we're still getting a little bit of rain, you know, there might be some sort of latent moisture around.
LYBERGER: Yeah, definitely. I could see warming leading to a longer West Nile virus season, and in fact, even on the East Coast, a recent study came out saying that's exactly what they're seeing, is a longer season with warmer temperatures.
BRODIE: How, ideally, would you like this research to be used — either by vector control folks or anybody else?
LYBERGER: Yeah ... part of the goal of this is to better understand the thermal limits of these mosquitoes. To answer questions about are going to be changes in seasonality, in the abundance of the mosquitoes, or year-to-year variation?And so, how vector control can use that and make different decisions about how they operate, I leave that to them. And I try to provide them the scientific basis on which they can make those decisions.
Same thing for public health agencies. They might be interested in knowing whether that potential season for West Nile virus is going to increase.
BRODIE: All right, that is Kelsey Lyberger, an assistant professor in ASU's College of Integrative Sciences and Arts. Kelsey, thanks so much. I appreciate it.
LYBERGER: Thank you.
EDITOR'S NOTE: Due an editing error, this story has been updated to correct the byline.
EDITOR’S NOTE: This story’s photo captions have been updated to correctly identify Alex Vela.
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