A monster that eats live deer, which we thought we defeated decades ago, is on the rise. Once chased all the way back down to South America, the New World screwworm fly is staging a breakout. In the last three years, this villain has reclaimed Central America and is now threatening to cross the Texas border into territory it hasn’t held for 60 years. The U.S. Department of Agriculture is scrambling to block the invasion.
The tiny New World screwworm fly (Cochliomyia hominivorax) doesn’t look so frightening until you learn its horrific life-cycle. The larvae of this particular fly don’t eat dead tissue like normal maggots. They burrow or “screw” into their victims and eat living flesh of deer, livestock, dogs, other small mammals, and even people in rare cases.
The latest active case of screwworm infection is 160 miles south of the Texas border, according to Blaise Korzekwa, White-tailed Deer Program Leader for the Texas Parks & Wildlife Department (TPWD). How did the screwworm fly escape from exile in South America and make it so far north again? What are agencies doing to stop their resurgence?
How a Screwworm Fly Kills Deer
New World screwworm flies lay their eggs in open wounds, mucous membranes or orifices of its victims. In spring and summer, newborn fawns are especially susceptible through their umbilical cords. In fall, adult bucks are susceptible through wounds on their heads and necks from fighting other bucks.
Once inside a wound or opening, Screwworm maggots eat the living tissue and burrow deeper. The wound attracts more flies, which lay additional eggs, and the infected wound grows. Without human intervention, secondary infection in the growing wound eventually kills the animal. A 2016 outbreak on Big Pine Key killed nearly 20% of Florida’s endangered Key deer population. Florida biologists found living deer with maggot-filled craters on their heads, necks, mouths, genitalia, between their hooves, and other locations.
A Wildlife Victory
Senior deer hunters of the Southeast like my dad remember the screwworm trouble of the 1950s, a serious threat to both deer and livestock. According to the Florida Fish & Wildlife Commission, “When present in the 1930s to 1950s, it is believed to have significantly reduced Florida’s deer population.”
The impact on livestock activated a response from the U.S. Department of Agriculture, and researchers came up with an incredible strategy to defeat the screwworm fly. They used radiation to sterilize male screwworm flies and released them by the millions in affected areas. Female flies mated fruitlessly with these sterile males, laid non-viable eggs, and fly populations crashed. The map below shows the timeline of the battle, which eradicated the screwworm fly from the Southeast in the late 1950s and eventually from the United States by 1966. USDA’s National Agricultural Library created an excellent history of this era in wildlife conservation.
From there, ongoing cooperative efforts of the USDA and Central American governments knocked the fly all the way out of Mexico in the 1980s and down the narrow Isthmus of Panama into South America by 2001. But like most movie villains, the screwworm fly is back for a sequel.
How Did the Screwworm Fly Escape?
With the screwworm fly eradicated from North and Central America, production of sterile flies declined. A small effort continued in Panama to ensure the fly could not escape South America, and the narrow Isthmus made a great natural barrier for keeping the pest bottled up. Experts stopped a 2016 outbreak in Florida’s isolated Key deer population by releasing sterile flies throughout Big Pine Key. Otherwise, the threat remained dormant.
Then came the COVID pandemic. Panama’s sterile fly production facility experienced disruptions and was not able to operate at full capacity. The fly took advantage of the lull in vigilance. In August 2022, it broke through the Darien Gap, where Panama meets Columbia.
“What’s concerning is that the fly made it this far north into Mexico in winter, which is supposed to be the less active time. Normally, cooler weather slows it down.”
Blaise korzekwa, deer program leader, texas Parks & Wildlife
“The Darien Gap used to be very dense, forested habitat, with no roads and no livestock,” said Blaise Korzekwa. “But recently they’ve been deforesting that area for livestock grazing, and with the introduction of livestock, that area is no longer a barrier. That’s another big factor that allowed the fly to come back north.”
Sterile fly production resumed, but it was too late. The fly reached Mexico by November 2024. In summer 2025, the USDA and the Mexican Department of Agriculture hoped to stop the fly at another bottleneck, the Isthmus of Tehuantepec, 600 miles from the Texas border. They didn’t.
At this report, active cases of screwworm fly are in cattle, pigs, horses and dogs in northeast Mexico, the closest within 160 miles of the Texas border (see map above). The Mexican Department of Agriculture has confirmed 1,404 cases, including 38 in the border state of Tamaulipas. The USDA is dispersing 100 million sterile flies per week in Mexico, focusing on areas with detected cases, and it is dispersing them 50 miles into south Texas as a precaution.
“What’s concerning is that the fly made it this far north into Mexico in winter, which is supposed to be the less active time,” said Blaise. “Normally, cooler weather slows it down.”
Buildup for Battle
The resurgence of the screwworm fly has caught the U.S. and Mexican governments without enough sterile fly production capacity. Though USDA is producing and releasing millions of sterile flies, the number is still short of an effective response level in a geographic area as large as northern Mexico.
Currently, one USDA facility in Panama is operational and producing sterile screwworm flies. A second facility in Metapa, Mexico is under construction. USDA recently announced a third at Moore Air Base in Texas, which already is home to a fly dispersal facility. That’s the race we are currently running: To scale up production of sterile flies in time to stop the pest at the Texas border.
There’s also the potential for the fly to jump directly from Central America to Florida. According to Florida FWC, the screwworm fly “could arrive in Florida via this expansion of range or through the movement of infested animals and people. If established, [the fly] may take years to eradicate. Control is directed at early detection and limiting the spread.”
The USDA’s 5-prong strategy covers surveillance to detect the fly’s presence, treatment of infected livestock, and controlling the movement of livestock, but the key to the effort is sterile fly production. Last year, NDA backed federal legislation to authorize funds for a new sterile fly production facility.
The USDA facility in Panama produces 100 million sterile flies per week. The new facility under construction in Mexico will add 60 to 100 million more per week when it is completed, hopefully this summer. That’s short of the more than 500 million per week it took to eradicate the fly in the 1960s. The facility at Moore Air Base in Texas will make up the difference with a capacity of 300 million flies per week, but that’s two years from completion.
“With the current fly production, unless something changes, it looks like it’s likely going to make its way into Texas at some point,” said Blaise. “They are dropping sterile flies in south Texas, and that’s been keeping them from going into that area. It’s working really well. It’s just that until we can produce more flies, we’re limited on how many locations we can drop them.”
What Hunters Can Do
“The biggest thing hunters can do is keep observing deer,” said Blaise. “Hunters aren’t handling many deer outside of hunting season, so keep those trail-cameras out in spring and summer. Keep an eye on the animals. If you see a suspected screwworm wound, contact your local biologist.”
Screwworms attack deer, rabbits, wild turkeys, raccoons, and other small mammals, including livestock, dogs and cats. If you see any of these with live maggots in a wound or opening, contact The Texas Parks and Wildlife Department. This document includes contact information for TPWD. Additional information from TPWD about the New World screwworm fly can be found here.
Hunters in Florida and other states along the Gulf coast should also remain vigilant until this new threat is suppressed. For the latest information and updates on the current status, the USDA maintains an excellent website.
On April 22, the USDA and U.S. Department of the Interior will jointly present a webinar on New World screwworm preparedness and response. The webinar is specifically targeted for the wildlife and hunting community. Register for the webinar here.