The first documented case of New World screwworm in the U.S. in at least 10 years was recently discovered in Florida, according to USDA.
The announcement earlier this week stated that a horse from Argentina, presented for routine inspection at an import quarantine facility in Florida, was found by Animal and Plant Health Inspection Service inspectors to have an open wound with larvae, a common visible sign of a potential screwworm infection.
Samples were shipped to the National Veterinary Services Laboratories in Iowa, and it was confirmed the larvae were New World screwworm. According to USDA, the animal will remain in quarantine until it has been reexamined and determined to be free of the pest.
USDA also announced a new dispersal area for sterile flies going as far as 50 miles into Texas to stop the insect’s northern spread.
Screwworm flies can be deadly to cattle. Female flies lay their eggs in open wounds or body orifices of cows. When the eggs hatch, they burrow deep into the skin — hence the name screwworm — to feed on tissue and cause larger wounds.
Eradication of the insect has been successful in the past by dispersing sterile males to mate with females, which only mate once.
Requests for comment from the National Cattlemen’s Beef Association (NCBA) were not returned by publication time.
Sid Miller, Texas ag commissioner, said in a statement that the detection is proof of the importance of USDA’s import inspection and quarantine protocols.
“I want to set the record straight on the recent New World screwworm detection in Florida," he wrote. "This detection did not constitute evidence of a U.S. outbreak or a domestic New World screwworm infestation. It was thankfully caught during a routine inspection of an imported horse arriving from a country south of the Darién Gap. The detection demonstrates that our federal inspection system is working exactly as designed and is an additional biosecurity tool that will protect our industry."
New World screwworm was largely eradicated from the U.S. in the 1960s. But there have been occasional outbreaks. The last major outbreak was in 2016, which affected Key deer in the Florida Keys.
Outbreak in inevitable
Speaking at the recent Lancaster Cattle Feeders Day, Colin Woodall, CEO of NCBA, stated that it was not a question of if, but when the New World screwworm would appear again the U.S.
“Are you worried about it? You should be worried about it because no doubt there will be a market impact,” he said. “It will send shock waves through the system. There will be some panic like we see with most any animal disease or pest issue. So, it will have a ripple effect everywhere.”
The insect has come back with a vengeance since a spike of cases was reported in 2023 by Panamanian authorities. Since then, it has continued a quick march north through Mexico after being found in the southern state of Chiapas, bordering Guatemala, just before Thanksgiving 2024.
Recent cases have been in Tamaulipas, where USDA reports at least eight active cases, but the pest also been identified just 70 miles south of the Texas border in Nuevo Leon state.
Southern ports along the U.S.-Mexico border have been closed to livestock trade since last July. According to Agriculture Dive, the U.S. relies on live cattle imports from Canada and Mexico to fill in the gaps from years of herd declines. In 2023, for example, Mexican cattle imports were 1.25 million head following a record-low year. That’s about 3.7% of the U.S. calf crop.
USDA and Panama’s Ministry of Agriculture Development have jointly managed and funded a sterile fly production in Panama that produces and disperses about 100 million sterile flies per week. An additional production facility is planned to open later this year in Metapa, Mexico.
Two additional fly dispersal facilities in Mexico — one in Tuxtla Gutierrez, southern Mexico, and another in Tampico, northern Mexico — are in operation. And construction is continuing a sterile fly dispersal facility at Moore Air Force Base in Edinburg, Texas, that will be capable of dispersing up to 100 million sterile flies per week.
Still, Woodall said beef producers should be prepared for an outbreak.
“If they can do their job well enough, hopefully we can contain it to as few Southern states as possible,” he said. “As I said, Texas is going to get it. That's a guarantee at this point in time. It's a matter of how quickly we can get the New World screwworm sterile insect facility in Texas up and running.
“Once that's up and running, and it's producing about 300 million sterile flies a week, we will be able to eradicate it. And we're seeing more products come on the market to help us control the larvae, to be able to heal up the cattle. We know that there will be more innovation coming forward.
“You're never ready for something like this, but we are prepared. We are prepared once it gets here, and we will eradicate it once again.”
Texas bracing for the worst
Miller does not dispute Woodall’s prediction. In October, his state’s agriculture department began fly trapping operations intended to detect the presence of screwworms.
Through Jan. 27, state officials had captured 6,675 blow flies, but no New World screwworms. Still, it seems it may only be a matter of time before the pests migrate north of the Rio Grande.
Miller has watched warily as screwworms have spread from an outbreak in southern Mexico to isolated cases only 70 miles from his state’s border. He characterizes the fight against screwworms over the past year as “static.” This, despite the release of more than 5 billion sterile flies in southern Mexico intended to stop the screwworm’s spread.
“We’re still somewhere between 800 and 1,200 cases, so we haven’t eliminated it,” Miller says. “It didn’t get a lot worse, but it didn’t get a lot better.”
USDA had hoped the release of sterile flies would halt the spread of screwworms in southern Mexico, and then out of the country completely. Despite those efforts, screwworm cases continue to be reported in central and northern Mexico.
To prepare for the potential fly invasion, the Texas Department of Agriculture has placed several hundred traps along the Rio Grande River and all the state’s livestock export facilities. That will allow Texas officials to detect the presence of screwworms and determine where they are headed.
Miller also ordered traps placed near cruise ship terminals in Galveston and Corpus Christi. He said one of his biggest fears is a cruise ship passenger returning from areas known to have a heavy screwworm presence. Hitching a ride is how the pests got so close to Texas in the first place.
“We probably had some flies hitchhike their way up 700 miles north, and now they're 187 miles from the Texas border,” Miller says.
Indeed, the earliest reported cases of New World screwworm in northern Mexico were attributed to infected cattle transported from the south. More recent cases indicate screwworms are now infecting cattle in the border states of Tamaulipas and Nuevo León.
Also concerning, according to Miller, is the fact that New World screwworms have been more active this January than they typically are in winter months.
Per data from USDA and its Mexican counterpart, SENASICA, there have now been 20 animal cases of New World screwworm in Tamaulipas and three in Nuevo León. Eight of the Tamaulipas cases were still active as of Jan. 29.
How bad could the impact be?
Any spread of New World screwworm could have a devastating impact on the Texas cattle industry. But it’s not just cattle producers who should be worried. Miller says screwworms can infest any of the 200 mammal species in Texas. That’s bad news for pet owners, not to mention the state’s multibillion-dollar wildlife industry.
“It’s hard to rope and doctor a deer," Miller said. “It’s a little tough to do that, so [screwworm] could wipe out big populations of our deer, turkeys, feral hogs.”
Miller jokingly conceded reducing the number of feral hogs might not be a bad thing
Not even people are immune from screwworm infections. As of Jan. 29, there had been 1,190 cases of New World screwworm myiasis reported in Mexico and Central America. Seven of those people died.
On Jan. 20, the Centers for Disease Control and Prevention issued a health advisory due to the ongoing screwworm outbreak and its potential to spread.
How far north can it go? What to look for
Since it is native to tropical and subtropical regions, and the larvae can’t survive freezing temperatures, the chances are low that the pest will eventually spread far north into the U.S.
Still, it’s a good idea to be vigilant. Penn State Cooperative Extension lists several things livestock producers should look out for:
- wounds with visible larvae, often foul-smelling, in areas such as navels of newborns; ear tags and brands; scratches from fencing; and tick bites
- excessive head shaking
- lethargy or restlessness
- excessive licking of wounds
It’s also important to keep the following factors in mind, according to Extension:
International travel. Any farm visitors who have traveled internationally to regions where the screwworms are present should wait at least five days after returning before visiting livestock operations.
Livestock sourcing. Be extra cautious about sourcing livestock from Southern states or any animals with recent travel history to affected areas. Purchased animals should be inspected before travel and monitored before integration.
Show animals. Animals returning from shows or events near areas with screwworm could pose biosecurity threats. These animals should be quarantined and monitored before travel and before they are integrated back into the herd.