New World screwworm parasite confirmed in United States

The U.S. Department of Agriculture says has confirmed the parasitic fly larva in a three-week-old calf in south Texas.

The U.S. Department of Agriculture has confirmed a New World screwworm case in south Texas, after a more than yearlong attempt to contain it in Mexico.

The USDA said in a statement that the parasitic fly larva was found in a 3-week-old calf in Zavala County Texas, a few hours north of the border city of Laredo.

Authorities in the United States and Mexico have been trying since late 2024 to contain the parasite south of the border, after a case was discovered in southern Mexico.

The United States officially eradicated the parasite in the 1960s. The fly larvae can burrow into the flesh of warm-blooded animals and be deadly.

The parasite has not been detected in Sonora, the Mexican state south of Arizona, but the entire border has been closed to cattle for nearly a year.

Hear Nina Kravinsky on The Show with host Mark Brodie

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Full conversation on The Show

MARK BRODIE: ... Federal officials have confirmed New World screwworm has been found in cattle in the US. Up until now, it had been confined to cows south of the border. This is not the first time the flesh-eating insect has been detected in this country. In the 1950s, the US started a multi-decade effort to eradicate it. That effort was largely successful until fairly recently. With me now to talk about the current situation is KJZZ’s Nina Kravinsky from our Hermosillo bureau. Nina, good morning.

NINA KRAVINSKY: Good morning.

MARK BRODIE: So, what is the situation right now?

NINA KRAVINSKY: Yeah. So, so far, the USDA has confirmed four cases of the New World screwworm in the U.S., two of them just this morning. You know, three of those four cases have been in calves in South Texas, not too far from the Mexico border. But one of the cases that they announced this morning was a dog much further west, not far from the New Mexico border, but still in Texas.

And all of these animals have cases, like you said, of the New World screwworm. So this is a flesh-eating parasite that can burrow into the flesh of warm-blooded animals and can be deadly. And like you said earlier, just a few bloody just a few seconds ago, the U.S. officially eradicated that parasite in the 1960s, but now it’s back. And that has a lot of people on high alert. You know, the governor of Texas issued a statewide disaster declaration to free up resources to fight the New World screwworm in Texas. And the USDA has also been on the ground there to try to stop the spread of the parasite and identify these cases as they come in.

MARK BRODIE: Yeah, this has been pretty much what folks in this country did not want to have happen, right? Like, there had been cases in in Central America and Mexico, and they were trying desperately to avoid it getting into the U.S. But now that it is here, what does that mean?

NINA KRAVINSKY: Yeah. So, the parasite spreads when flies travel from animal to animal to lay their eggs, and then those eggs turn into larvae, which is the New World screwworm. So, unless you or your pets spend a lot of time around cattle and like have an open wound, you can pretty much count yourself safe. The beef that you buy at the grocery store is also OK. The screwworm doesn’t infest meat, so the USDA says that the U.S.’s food supply is safe. But the way that this is more likely going to show up for listeners and and for most people is that beef prices may go up. You know, beef prices have already been pretty high. The US cattle herd is already at like these record lows, and that’s creating this supply-and-demand issue. And the screwworm threatens to diminish that even further, diminish that U.S cattle supply. So that can drive up prices for, you know, steaks and hamburger meat at the grocery store.

MARK BRODIE: So, Nina, how exactly did we get here?

NINA KRAVINSKY: Yeah, so, you know, Mark, I kind of feel like I’ve been watching the dominoes like fall in this story in slow motion. In late November 2024, we saw the first case of a cow with screwworm in Mexico. It was right near the Guatemala border in the very south of Mexico, you know, far from the U.S.-Mexico border. But it meant that this parasite had breached containment in Central America and was now in Mexico. And since then, U.S. and Mexican authorities have been working to try to stop the spread of the New World screwworm.

But even so, you know, cases have been creeping up through Mexico over the past year and a half. That’s include confirmed cases in Mexican states that border the U.S. Although it’s important to note that there have been no reports of the screwworm here in Sonora, which is where I live in Mexico, south of Arizona.

MARK BRODIE: Right. Well, so how have authorities been trying to contain this? Like, we’ve talked about how they’ve really not wanted it to first get to Mexico, which it did, then get to the U.S, which it now has. So ... what are folks doing to try to prevent it from spreading further?

NINA KRAVINSKY: Yeah, totally. So over the past year and a half, there have been a couple of different methods that they’ve used to try to stop the spread. One way is by closing the entire U.S.-Mexico border to cattle. In normal years, hundreds of millions of dollars worth of live cattle cross from where I am in Sonora into Arizona. And cattle ranching is a big industry. You know, it’s been designed in northern Mexico a for a lot of ranchers for this export market. But much of last year and for all of this year, that trade has been halted to protect U.S. herds against the New World screwworm.

And authorities have also been ramping up this method of sterile fly production. That’s what scientists have told me is the most effective way to contain the spread and eventually eradicate this parasite. And so sterile fly production basically means that they’re releasing large amounts of sterile flies that they breed in these big like factory-like laboratories. They release them into the environment, and then those sterile flies mate with regular screwworm flies, and eventually that kind of dwindles the population.

That’s actually one of the ways that the USDA has been combating the parasite now in Texas, and they’ve been using this this method in Mexico and Central America for a while. But, you know, Mark, all of that takes time. ... And so, the parasite has had time to spread into Texas despite those efforts.

MARK BRODIE: So, Nina, you mentioned that, you know, hundreds of millions of dollars worth of cattle in a typical year cross from Sonora into Arizona. So, how are ranchers doing where you are?

NINA KRAVINSKY: Yeah, so, you know, this export market is kind of the bread and butter of a lot of ranchers in Sonora. Last fall I visited a rancher in his 70s, a few hours south of the Arizona-Sonora border, and his family has been in this industry for a few generations. And he’s historically bred his cattle to sell to feedlots in Texas, so they start their lives in Mexico and Sonora, and then they go to be fattened in the feedlot in Texas.

And and when I visited him, the border closure meant that he hadn’t been able to do that in months, export his live cattle over the border. And he was at that point already working on selling his cattle domestically in Mexico for a much lower price.

You know, that was months ago, and the border hasn’t reopened even though there haven’t been any cases, like I said, of the screwworm here in Sonora. So northern Mexican states have been working on kind of building out their own infrastructure to make the border closure less costly for these ranchers. But being locked out of higher-paying markets in the U.S. has been tough for ranchers in Sonora, es- especially these smaller operations where the margins are already really tight.

MARK BRODIE: Yeah, sounds like things probably won’t be changing that much anytime soon, either. All right. That is KJZZ’s Nina Kravinsky from the Hermosillo bureau. Nina, thanks as always.

NINA KRAVINSKY: Yeah, thanks, Mark.