While the presenters offered two hours of useful information and advice, the bottom line is no one really knows if the flesh-eating parasite will make its way north to the United States border. After being eradicated in Mexico and the U.S. in the 1960s, with the occasional isolated flareup in places like the Florida Keys, NWS has again gained a foothold in Mexico long after it had been contained at the Darien Gap in Panama, the isthmus that makes up the only land bridge between Central and South America.
Containment and encroachment
Battling the NWS, which is a species of blowfly, back that far south took an international effort, millions of dollars, and a struggle that continues into the foreseeable future. The screwworm flies are attracted to wounds on warm-blooded animals; each is capable of laying about 200 to 300 eggs in the wound — eggs which take just 10 to 12 hours to hatch, said Swiger. Once the maggots emerge, they dig into and eat the living flesh of their host. This distinct behavior — most blowfly species are attracted to dead, not living, tissue — is what makes the species so destructive. In fact, an infestation can prove deadly to the host animal in a week or so if left untreated.
While Swiger said the NWS is most comfortable in temperate zones, preferring weather between 64.4°F and 91°F, that doesn’t mean they aren’t able to survive suboptimal conditions. And one of the more difficult aspects of determining their presence or absence lies in their uneven distribution within a habitat. “They spend most of their time in forests and woods,” Swiger noted, “but will seek out host animals in pastures and fields.” And while livestock is naturally the primary concern of farmers, the presenters also focused on wildlife on the perimeters and deeper into undeveloped areas of farms and ranches.
“When it’s deadly, it tends to be because the area of the initial infestation allowed the eating of the flesh to get to a vital area,” Swiger said. She pointed out that it is not a contagious condition, but that the pupae, after feeding for about five days, will go into the ground to pupate and emerge five to seven days later. “They can travel unaided by humans up to 12.4 miles to find a suitable host, if need be,” she told attendees, “but the bigger concern is human movement of infested livestock.”
With this in mind, on May 11, U.S. Secretary of Agriculture Brooke Rollins announced the suspension of live cattle, horse, and bison imports from Mexico. At that point, the frontlines of the battle were about 700 miles south of the U.S. border, with maps from earlier 2025 Animal and Plant Health Inspection Service (APHIS) showing a slow northward and outward dispersal. The latest available map from APHIS can be found here.
Containment efforts have been ramped up, including sterile male fly production and release. This method, said Phillip Kaufman, is one of a multi-pronged approach to eradication. “We use integrated pest management throughout entomology. It entails multiple techniques to reduce a population while reducing the chance of (pesticide) resistance development,” he said. Sterile insect technique works for NWS because the females only mate once, and when the irradiated male specimens are released in infested areas, the subsequent population will drop.
“The flies are currently produced at one plant in Panama,” Kaufman said, “but others may come online.” Typical production at the Panama plant is about 20 million flies a week, but currently they’ve increased capacity to five times that. “However,” cautioned Kaufman, “only half of those are males.” But because the NWS is relatively rare in nature, targeted aerial or land dispersal can overwhelm the wild population. The sterile male program works best, he said, in conjunction with other efforts which include surveillance of potential or suspected areas. APHIS and other groups use traps for adult flies, and the information gleaned from those programs can help target sterile fly drops where they are most needed.
Surveillance was also a theme of Thomas Hairgrove’s portion of the webinar, but he focused on the producers’ end of things. “If you suspect infestation, contact your veterinarian or the Texas Health Animal Commission hotline. Don’t delay.” He suggested monitoring both livestock and keeping an eye on the wildlife population, “Be vigilant and report anything out of the ordinary.” Proactively monitoring wounds, including those from surgical procedures, and addressing tick and other external parasite issues, are good places to start, Hairgrove said, adding that injury prevention via good equipment maintenance is helpful as well. More details from the webinar can be found here.
NWS in the news
Other recent developments on the screwworm front:
On May 28, Missouri Attorney General Andrew Bailey requested that state agencies including the Livestock and Farm Protection Task Force launch an investigation into a false report that NWS had been confirmed in the state. The report was received and disseminated by a radio station on May 27, but its veracity quickly came into question. The investigation is ongoing, spurred in part due to the subsequent selloff in the U.S. cattle futures market, according to the Missouri Department of Agriculture. More information can be found here.
On May 30, the Ranchers-Cattleman Action Legal Fund United Stockgrowers of America (R-CALF USA) sent a letter to the FDA’s Center for Veterinary Medicine requesting approval for oral feed-through ivermectin. Stressing the “potential impact on the United States livestock markets” if cases were found in the nation, the letter noted that: “This program has successfully been utilized by the United States Department of Agriculture to control the northern movement of Cattle Fever Tick. All the necessary assessments have been completed for this program to approve the feeding of ivermectin to both livestock and wildlife in the southern counties of Texas.”
On June 6, the Mexican Agriculture Ministry announced that three people — two pilots and an agronomist — were killed when their plane crashed near the Mexican-Guatemalan border during a flight to release a batch of sterile flies.
