17-05-2025
Parasitic fly halts livestock imports from Mexico; officials point fingers over outbreak
When Basilio Aja was 7 years old, he went out to ride on the Mogollon Rim with his family's foreman to check the sheep camps. Near Willow Creek Canyon, a stench stopped them.
"Es el gusano," the foreman told him — it's the worm.
They found a stumbling deer, blinded because its eyeballs were eaten out, stinking and lost in the forest. The foreman put him down. It was Aja's first lasting memory of the New World screwworm, he said.
The maggot of the New World screwworm, a fly that lays eggs on open wounds, burrows and feeds on the flesh of living animals, and creates gaping wounds that facilitate secondary infections, which, if untreated, can lead to death in a matter of weeks.
Through the 1960s, Aja's family treated thousands of infection cases on their sheep and cattle herds. The parasite was finally eradicated from the U.S. and Mexico about 60 years ago, and contained at Panama's Darien Gap thanks to international joint efforts.
In June 2023, it was detected again in the Central American country and made its way back some 2,000 miles north to the Mexican border with Guatemala, where the first infection in cattle was reported in November of last year. Cases in Mexico represent the first outbreak in nearly three decades.
There is no certainty that the parasitic fly will make it to the U.S., but the risk is high, and eradication efforts are urgent. Across Central America and in southern Mexico, infection cases by the parasitic fly, Cochliomyia hominivorax, are rising.
The latest infection case in the Mexican state of Oaxaca prompted the U.S. Department of Agriculture and Customs and Border Protection to suspend all live cattle, bison, and horse imports coming from Mexico for at least two weeks. The Arizona livestock industry is bracing for losses: When feeder cattle numbers go down, prices will go up.
"The feeder (cattle) market price is record high," said Aja, now 69 and an influential lobbyist and the executive director of the Arizona Cattle Feeders' Association, about the immediate impact of the second livestock import suspension, which came from Washington, D.C.
The biggest impact, however, will be on Mexican ranchers, whose major market for feeder cattle is the U.S., said Barbara Jackson, owner of Animal Health Express and a longtime cattle feeder.
Thousands of head of cattle are imported across the Arizona-Sonora border every week, about 300,000 every year. Over half are brokered and sold to Eastern feedlots, but the rest stay in the state. In good rain years, ranchers keep that cattle to take advantage of excess forage, said Aja, but due to drought, most are now sent directly to the feed yards. This year so far, 45% of all feeder cattle coming into the U.S. came through Arizona.
Many producers are frustrated about the halt of imports, but the key thing is that all efforts go to eradicate the parasite again, he added; "It doesn't matter if we have the border open or closed. If we're not using every tool in our box to eradicate them, then we're not doing our job."
Modern cases are not exclusive to Central America. In late 2016, there were two confirmed outbreaks of New World screwworm in Florida, which were contained.
Jackson says it's a broad health issue. "Unless you are over 60, a lot of people don't even realize what it is. It affects all mammals, wildlife, dogs, cats, and people," she said. "We've got to be aggressive about not letting it get up here."
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Mexico has strengthened checkpoints to inspect livestock, treated and quarantined cattle and is conducting a multi-state sweep from the Gulf of Mexico, renamed by the U.S. government as Gulf of America, to the Pacific, visiting every ranch for inspection, said Mexican Secretary of Agriculture and Rural Development Julio Berdegué, at a news conference on May 14.
The main tool for eradication is the aerial dumping of sterile flies of the parasite species to create sterile offspring. In coordination with Mexico, the U.S. Animal and Plant Health Inspection Service has released over 800 million sterile flies by airplane in southern Mexico since the outbreak crossed the Guatemalan border, and is ramping up production to increase the number of flights per week.
Binational collaboration has intensified, but both countries are pointing fingers. Berdegué said the U.S. had the responsibility to stop the spread in Panama, back when it was detected in June 2023, and that the Mexican authorities have asked for technical support to open a plant for sterile flies, with no response from the USDA.
In turn, U.S. authorities have said barriers and bureaucratic delays in Mexico, like restrictions on aircraft, permitting and onerous customs duties, have hampered eradication efforts.
The USDA did not immediately respond to The Republic's request for comment or to answer questions.
Ranching associations from both sides of the border and sanitary organizations from Mexico and the U.S. established two defense lines or buffer zones in Mexico to contain the northward movement of the parasite in the Mexican Isthmus, the narrowest area of the country. One case was detected beyond that line.
"This issue should be a national emergency," said Alvaro Bustillo, president of the Chihuahua Regional Livestock Union, to a Mexican news outlet. The parasite would devastate the livestock industry, but it is a threat to all animals — and to humans.
The Mexican National Confederation of Livestock Organizations demanded this week that the Mexican government "seal" the border with Guatemala to stop cattle trafficking, which they point to as a major driver for the advance of the parasite.
Environmental organizations like the Wildlife Conservation Society have also pointed out that cattle-smuggling routes mirror the hotspots for New World screwworm outbreaks, documented by InSight Crime investigations.
At Guatemala's Mayan Biosphere Reserve, organized crime groups have deforested hundreds of thousands of acres to illegally ranch cattle and then smuggle them to Mexico.
Demand and prices in Guatemala are low, so there is an incentive to move cattle across the border, Aja said. In his view, cattle trafficking might have given the parasite a "taxi" ride once in a while, but the main cause of the Central American outbreak is that U.S. eradication efforts were lessened.
"We just kind of got a little lazy on it," he said.
There is no direct evidence that the outbreak in southern Mexico was caused by illegal cattle trafficking, but it's the most plausible explanation, said Manuel Weber, an animal ecologist with the College of the Southern Border in Mexico, who specializes in disease ecology and wildlife health.
"It got (to Mexico) too fast for it to have been by lawful pathways," he told The Arizona Republic. There had been infection cases in the southern border of Guatemala-Honduras, which were being contained and monitored, and in a heartbeat, there were cases in Mexico.
Cattle that are being transported illegally are usually stressed, some injured, which makes screwworm infections easier, said Jeremy Radachowsky, the Wildlife Conservation Society's director for Mesoamerica and the Caribbean.
It's estimated that some 1 million head of cattle are crossed illegally from Central America to Mexico every year — the same amount that is legally exported to the U.S.
"We haven't seen the urgency (from government authorities) that I think was necessary," Radachowsky said.
"The efforts so far have focused completely on releasing millions of sterile flies from airplanes. The problem is that as long as you have illegal cattle being moved northward, essentially it's like fumigating the house but then leaving the doors open."
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Because the screwworm can infest any living mammal, the potential for its spread is huge; the fly has many hosts, including hard-to-track wildlife.
"In Central America, we have seen transmission from cattle to species like tapir, deer, probably to puma," said Weber, the Mexican wildlife and disease ecologist. It can also be transmitted to cats and dogs, and cases like that are prevalent in some countries.
Footage from camera traps in the Mayan Biosphere Reserve captured a mountain lion with a "large gaping wound on its side," said Radachowsky, with the Wildlife Conservation Society. While it's hard to tell if it's from the screwworm, if it is, that would mean a 95% chance of death for the animal in the next couple of weeks, he added.
A student of Weber's also interviewed a hunter in southern Mexico who harvested a deer that appeared to be infected with the fly's larvae.
"It's really devastating for wildlife. You will smell it before you'll see it," said Aja, recalling his childhood experience seeing the infected deer.
Wildlife could also play a role in maintaining the parasite within the ecosystem. However, studies of the parasite behavior and spread are complicated in the tropics, Weber said. There are low chances of identifying and tracking infected wildlife, and once the animal is dead, the carcass can last less than 48 hours after being eaten by other animals or decomposing.
Wildlife has also been noted as a vector by U.S. authorities. In its May 11 news release announcing the suspension of livestock imports, the USDA stated that 'the northward spread" of the parasite could take place through "natural wildlife movements, including wildlife that transits the border region without impediment.'
In theory, it is possible — though there isn't scientific evidence — that the screwworm can be carried by wildlife. Long-distance migrations of that scale, common in the U.S., are rare in Mexico, Weber said.
"I think it's pretty unlikely that wildlife movement per se brings the disease all the way up from where it is now in Oaxaca and Veracruz — a few species like pumas and jaguars can make movements of hundreds of miles, but these are pretty rare events," offered Jesse Alton, a faculty member at the University of Arizona who studies wildlife movement.
"If screwworm were to pop up in Sonora or Chihuahua, wildlife could definitely play a role in transporting screwworm across the border."
Arizona conservation advocate Myles Traphagen, with Wildlands Network, hopes the USDA message does not imply there is a need for more border wall to control the outbreak.
"The reality is that the vector is a fly, and it is far more likely that a fall hurricane in the western Caribbean Sea could blow flies north towards South Texas than be transmitted by native wildlife," he commented.
"A border wall isn't going to stop a fly."
Clara Migoya covers agriculture and water issues for The Arizona Republic and azcentral. Reach her at
This article originally appeared on Arizona Republic: New World screwworm outbreak halts livestock imports from Mexico
