Texas is gearing up for war as a savage, flesh-eating fly appears poised for a US invasion and is expanding its range of victims.
On Friday, the Texas Department of Agriculture announced the debut of TDA Swormlure, a synthetic bait designed to attract the flies with a scent that mimics open flesh wounds, which are critical to the lifecycle of the fly, called the New World Screwworm. The parasite exploits any open wound or orifice on a wide range of warm-blooded animals to feed its ravenous spawn. Female flies lay hundreds of eggs in even the tiniest abrasion. From there, screw-shaped larvae—which give the flies their name—emerge to literally twist and bore into their victim, eating them alive and causing a putrid, life-threatening lesion.
The new lure for the flies is just one of several defense efforts in Texas, which stands to suffer heavy livestock losses from an invasion. Screwworms are a ferocious foe to many animals, but are particularly devastating to farm animals.
"When it comes to safeguarding Texas' $15 billion cattle industry, we need to focus on action rather than words. That's why I instructed my Biosecurity team to develop an effective screwworm lure," Texas Agriculture Commissioner Sid Miller said in the announcement.
But cattle industry losses aren't the only grave risk from the vicious parasites. In Colombia, endemic screwworms have apparently grown bolder in their taste for flesh. This week, biologists and researchers reported in the journal Emerging Infectious Diseases that the worms caused life-threatening infections in two wild mountain tapirs, an endangered species that lives in a protected area of the Central Andes. While there had been anecdotal reports of screwworm infestations in mountain tapirs, they were not previously considered a threat to the herbivorous mammals. The authors noted that it's unclear why the fly has suddenly emerged in mountain tapirs, which resemble wild pigs with trunks, but climate change and livestock movements could be causing the flies to expand their appetites.
Past success
As the flies' host and geographic range expand, pressure is intensifying to control the flies—something many countries have managed to do in the past.
Decades ago, screwworms were endemic throughout Central America and the southern US. However, governments across the regions used intensive, coordinated control efforts to push the flies southward. Screwworms were eliminated from the US around 1966, and were pushed downward through Mexico in the 1970s and 1980s. They were eventually declared eliminated from Panama in 2006, with the population held at bay by a biological barrier at the Darién Gap, at the border of Panama and Colombia. However, in 2022, the barrier was breached, and the flies began advancing northward, primarily through unmonitored livestock movements. The latest surveillance suggests the flies are now about 370 miles south of Texas.
The main method to wipe out screwworms is the sterile insect technique (SIT), which exploits a weakness in the fly's life cycle since they tend to only mate once. In the 1950s, researchers at the US Department of Agriculture figured out they could use gamma radiation to sterilize male flies without affecting their ability to find mates. They then bred massive amounts of male flies, sterilized them, and carpet-bombed infested areas with aerial releases, which tanked the population.
Panama, in partnership with the US, maintained the biological barrier at the Colombian border with continual sterile-fly bombings for years. But as the flies approached this year, the USDA shifted its aerial deliveries to Mexico. In June, the USDA announced plans to set up a new sterile fly facility in Texas for aerial deliveries to northern Mexico. And last month, the USDA halted livestock trade from southern entry points.
Miller said in the announcement today that SIT is no longer enough, and Texas is taking its own steps. Those include the new bait, insecticides, and new feed for livestock and deer laced with the anti-parasitic drug ivermectin. Miller also said that the state aims to develop a vaccine for cattle that could kill larvae, but such a shot is still in development.
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