Screwworm Update - NWS Update Shared, Implications, Preparedness And Response Plan

Screwworm is resurgent in Mexico. Can we keep it out of the U.S.?

NOTE: this article was originally distributed by email by Livestock Weekly on November 19, 2025. It was written by  Colleen Schreiber .

KINGSVILLE — In early November 2024, three New World screwworm cases were reported in Chiapas, Mexico, in the very southern part of the country. Since then, in a little over a year, there have been just over 9000 confirmed cases of screwworm infestations in Mexico.

As of November 5, Senasica, Mexico’s counterpart to USDA, reports that there are 757 active NWS cases in Mexico.

That was some of the information shared in an update offered by Dr. Jason Sawyer, chief scientific officer with East Foundation, at the close of the recent King Ranch Institute for Ranch Management’s annual Holt-Cat symposium.

“We went from three cases to a little over 2000 reported cases over seven months, and during that time the screwworm fly migrated about 300 miles north,” Sawyer told participants.

In June, NWS was confirmed as far north as Oaxaca and Veracruz, 375 miles from the Texas border, and in September, a case was confirmed in Nuevo Leon near Sabinas Hidalgo, just 70 miles from the South Texas border with Mexico.
The East Foundation has been following the NWS issue for some time, given the footprint of their ranches, all in South Texas. The San Antonio Vejo, their headquarters, is essentially 30 miles west of the border, and the El Sauz is about 48 miles.

As Sawyer reminded, NWS are blowflies that lay their eggs in living animals and then the larvae consume the flesh of those living animals. He also pointed out that NWS has always existed in South America, and they existed in the U.S. until the 1960s, when, with the use of the sterile fly program, they were eradicated and pushed into Mexico and eventually back to Panama, south of the Darien Gap. With the COPEG facility in Panama producing some 100 million flies a day, that biological barrier held south of the Darien Gap until 2022.

“Screwworms escaped their bondage, so to speak, and started to make their way up through Panama,” said Sawyer.

Once NWS got north of the Darien Gap, which is a very narrow strip of land, they rapidly made their way up into Central America, where there are large livestock and wildlife populations, and then from there on into southern Mexico.
Sawyer also told participants that there have been three cases of screwworms in Nuevo Leon, but two were not included in the active case report because they were resolved within a day or two of discovery. More importantly, those cases were not the result of an expansion of the area where the fly now lives but rather were the result of animals being shipped from southern Mexico to grow yard facilities in Nuevo Leon.

“The first of those was an active infestation, one animal in a load of 100 head, with larvae in a wound,” said Sawyer. “That animal was identified upon unloading and treated, and they immediately started to disperse sterile flies in that area.”
A week or 10 days later, a second case was reported about 70 miles south of the first. Again, a grow yard received a shipment of cattle, and one animal had a lesion.
“Upon inspection, it was determined that while there were larvae present, those larvae were dead,” said Sawyer.

That meant the animal had been treated. He then further explained that Mexico currently has what they call an infested zone, considered to be the area where the flies have reestablished themselves. North of that zone is the buffer area where active cases have been found, but those cases are considered “episodes or incursions”. Specifically, in the buffer zone, it’s believed that there is not a resident or stable fly population, thus the active cases there are termed incursions.

“They’re trying to maintain the buffer zone and prevent the establishment of resident populations of screwworm flies by dumping all 100 million of the sterile flies produced in the world at the COPEG facility in this zone,” said Sawyer.

He acknowledged, however, that the ability to get timely and consistent reporting from Senasica about the status of cases has been difficult, though he also acknowledged it has improved a lot over the last month.

“We're reliant on this information coming out of Senasica, and we believe that the credibility of the information that they've produced over this last, say, 60 days or so, is better than it was, say, 180 days ago, but we're still only able to act on information that we have,” said Sawyer.

He made a chart plotting the cases as the information became available, and the accumulation of cases and the time between the different cases to get an idea of the growth rate. During the seven-to-10-day lifetime of a female screwworm fly, that female can lay as many as 1000 eggs, Sawyer reminded.

“We’re anxious to see if these cases are going up exponentially or if they are slowing down,” said Sawyer.

The metric he used was growth rate per 10 days.

“Obviously, going from three to 2000 cases is a pretty big growth rate every 10 days, but the rate of growth has declined,” he told participants. “That means that the expansion of case load in Mexico appears to be tamped down. We view that as a positive.”

While the growth rate appears to be stable or flat, he reminded that we’re now into the time of the year where the weather is less conducive to NWS. He also reminded that come spring, that could all change very quickly.

“We can't forget that as long as the flies are somewhere in Mexico, they are a threat to the United States. That's just a reality,” said Sawyer.

Thus, while the fly population in Mexico may be stable for now, there remains the need to step up efforts to fortify sterile fly production. He shared some of the history of the sterile fly program, but the take home was that even after the mid-1960s when the U.S. had no more stable screwworm populations, stable populations remained south of the border. Consequently, every spring, with favorable weather conditions, the screwworm flies moved back into Texas. Thus, the U.S. made the commitment to not only eradicate them from the U.S. but also from Mexico. That eradication effort got serious beginning in about 1972, and it took until 1991 to push the flies off the Mexican border down to about Veracruz and another decade to get them out of Central America down to Panama to the Darien Gap, he said.

NWS is a foreign animal disease, and in Texas, by law, the state agency that's responsible for incursions of foreign animal diseases in livestock is the Texas Animal Health Commission. Texas Parks and Wildlife Department is responsible for the native wildlife, which are also susceptible and are a significant host for the pest.

The U.S. Department of Agriculture has revised the plan for NWS. The last time it was revised was in 2018 after the NWS outbreak in the Florida Keys, in the endangered Key deer population. That revised response playbook was released in early October and is now available for public comment.

One critical piece of particular interest is the movement control portion of the response plan.

“Movement has actually been the reason for the rapid expansion from Central America into southern Mexico,” said Sawyer. “The flies hitched a ride, so controlling livestock movement becomes a particularly important part of the response plan, but it also becomes an impediment to business.”

Currently, what USDA proposes is once an active case is confirmed, an infested zone of 310,000 acres, essentially 12.4 miles out in every direction from the infected case, would be established, from which there would be movement controls. An additional larger surveillance zone encompassing another million or so acres around the infection zone, about 24.8 miles out from the infected case, would be set up.

Animal movement wouldn’t be restricted in this zone. However, active surveillance for additional flies extending out to cover 31 million acres, about 124 miles from the infected case, would be established.

“I'm not saying whether I think it's a great plan or a bad plan, or anything like that,” Sawyer told the group. “However, all of us need to recognize that there will be some movement control requirements.”

In Mexico, their plan requires that any cattle leaving the infested zone be treated and held for three days, then reinspected before they can get on a truck and move within Mexico.

“I would anticipate something similar here, a treat and move on inspection and certification,” said Sawyer. “The commitment from the agencies involved is that it will be a minimum impediment to commerce, but not zero impediment to commerce.”

Speaking from experience as an operation that has been dealing with movement restrictions because of the cattle fever tick eradication program, Sawyer told participants that while movement restrictions are an impediment, an operation can manage through it.

“It’s not going to wreck you so long as there's some clear indication about what the actual requirements are,” he said.

Still, he reiterated that the long-term solution is more sterile flies. A new facility in Mexico that is to produce an additional 150-350 million flies is under development. USDA has also committed $750 million to construct a new facility in South Texas that would theoretically provide a total capacity of around 500-600 million flies per week for distribution and eradication. The plant is now in the design phase.

Sawyer pointed again to the timeline for pushing the flies out of Mexico the last time, reminding that it took the weekly dispersal of 500 to 700 million flies for 20 years.

“It would be easy for some of us to say, ‘hey, these things are way down there in southern Mexico; they're far away. They're not getting closer right now. Forget about it.’ If we don't construct these new facilities and fulfill that total eradication plan, they will be in Texas,” Sawyer told the group. “It might not be this year; it might not be next year, but they will be in Texas, and they will have a significant impact. So, we want this to get done so that we can succeed at getting them pushed back out of Mexico and perhaps even get them out of South America.”

He wrapped up by reminding that everyone has a role to play. He shared the East Foundation’s framework for identifying strategies and solutions to mitigate the potential risk of a reinvasion of NWS in the U.S.

He outlined the basic steps in their risk assessment plan. At the top is hazard identification. The second step is exposure risk, the third is attack rate and the final is the consequential outcome. All of those considered together result in a risk adjusted outcome.

“What we've tried to do is figure out which of these things can we actually do something about,” said Sawyer. “I can't do anything about exposure risk. That depends on Mexico and how flies fly, and whether the government drops sterile insects. The attack rate and the consequential outcome rate, or the case fatality rate, are the only two things that we, as managers, can do something about, so that's where we've been concentrating our energy.”

He further explained that as of today, East Foundation’s exposure risk is low. If the cases in Nuevo Leon were not just incursions, Sawyer said, they would have expected to have NWS cases on their ranches within two months.

“That’s the difference between knowing whether those cases were treated and resolved and there are no resident flies, versus there are resident flies south of Laredo,” he explained.

If the latter were true, then it’s a matter of changing the timing of things to mitigate or reduce the attack rate risk, he said. Because of the risk to calves, for example, that may mean changing an enterprise by having fewer cows and more yearlings.

“That has a cost, so we’re trying hard to understand what these risks are and how big they are so that we can make cost-based decisions,” said Sawyer.
As for treatment, borrowing from TAHC’s Dr. TR Lansford, he told the group that the best thing anyone can do is to put eyes on your animals.

“We’ve already increased our observation and surveillance measures, particularly with wildlife,” he said. “We can’t catch them, but we can observe them.”

Wrapping up, Sawyer encouraged anyone who does not yet have a beef quality plan, specifically anyone who does not have a valid veterinarian-client-patient relationship, to get one.

“If you don’t have your veterinarian’s phone number, that means you don’t have a valid veterinarian-client-patient relationship,” Sawyer said.

He suggested getting advice from the veterinarian as to what vaccines can be used and how to use them. One of the considerations that East Foundation has already been thinking about is how to alter their current parasite control programs to perhaps reduce the attack rate risk.

He talked briefly about resistance in some of the current products being used. For example, Avermectins are effective against NWS, and South America has been using them for the last 30 years. The issue is, in Brazil, for example, an animal might get a shot of Ivermectin or a similar product every 28 days for a minimum of eight months out of the year, and usually it's 12 months out of the year, said Sawyer. The problem is that with chronic use, the efficacy of a product declines. Thus, it’s unknown how effective those products might be in the U.S. for the management of NWS.

"They’re already part of our parasite control program, so we’re going to consider them, but we want to use them judiciously,” Sawyer told the group.

He also noted that the USDA has urged pharmaceutical manufacturers, the FDA and the EPA to consider conditional approvals and emergency use authorizations for both treatment and preventive use. The first of those for livestock use was announced on September 30. Dectomax CA-1 was given conditional approval for the treatment and control of NWS in cattle.

Finally, Sawyer said that while their risk assessment framework may not work for others, it has been a useful tool for them to build internal written protocols to have them already in place should they be needed.

“Preparedness beats panic,” he reminded. “I hope they don’t get here, but if they do, we at least know the first things that we will do to mitigate the risk.”

He ended by telling participants that at no time have they considered liquidating their cattle inventory or eliminating a wildlife population to be a suitable response for NWS.

“We don't want it to happen. It's going to be burdensome, it's going to be expensive, it's going to be a problem, but we're confident we can manage through it and wait for the day that the sterile insect airplanes come and get rid of it,” Sawyer concluded.