Screwworm Invasion Threatens Texas: Strategies to Combat the Flesh-Eating Parasite

The Danger of Screwworms and Their Devastating Effects

Texas is bracing for an imminent invasion of New World screwworm flies, a fierce flesh-eating parasitic insect that is expanding its range of victims. To counter this threat, the Texas Department of Agriculture (TDA) announced the launch of an artificial bait called "TDA Swormlure," designed to attract the flies by mimicking the scent of open wounds, which are essential breeding grounds for this parasitic worm. The female screwworm fly exploits any open wound or orifice in a wide range of warm-blooded animals to lay hundreds of eggs, even in the smallest scratches. Subsequently, the larvae, which are spiral-shaped – hence their name – hatch and begin to burrow and penetrate the flesh of their victim, feeding on it while alive, causing a painful, putrid, and potentially fatal infection.

This new bait is part of a comprehensive defensive strategy adopted by Texas to counter the severe economic losses that this invasion could cause to the livestock industry. Screwworms are a dangerous enemy to many animals, but they pose a particularly devastating threat to livestock. In this context, Texas Agriculture Commissioner, Sid Miller, stated: "When it comes to protecting Texas's $15 billion livestock industry, we must focus on actions, not words. That's why I directed my biosecurity team to develop an effective screwworm bait."

Expanding Victim Range and Climate Change Implications

However, losses in the livestock sector are not the only danger posed by these fierce parasites. In Colombia, the endemic screwworm has shown increasing audacity in targeting new species. Researchers in the journal Emerging Infectious Diseases reported that the worms caused serious injuries to two mountain tapirs, an endangered species living in a reserve in the Central Andes. Despite previous unconfirmed reports, these herbivorous mammals were not considered a primary target for the screwworm. Researchers indicated that the reasons for this shift are not entirely clear, but they suggest that climate change and the movement of infected livestock may have contributed to the expansion of the fly's appetite.

History of Control and the Return of the Threat

With the expanding list of the fly's victims and its geographical range, pressure to control it is increasing, something many countries have successfully done before. Decades ago, the screwworm was prevalent in Central America and the Southern United States. But thanks to coordinated control efforts, governments managed to push the flies south. The worm was officially eradicated from the United States around 1966, then from Mexico during the 1970s and 1980s. In 2006, it was declared eradicated in Panama, where a biological barrier was established in the Darién Gap on the border with Colombia to control its numbers. However, this barrier was breached in 2022, and the flies began to advance north again, primarily driven by uncontrolled livestock movement. The latest data indicates that the flies are now approximately 370 miles south of Texas.

Sterile Insect Technique (SIT) and Additional Texas Efforts

Sterile Insect Technique (SIT) is the primary method for eradicating screwworms, exploiting a biological weakness: female flies mate only once in their lives. USDA researchers discovered in the 1950s that gamma rays could sterilize males without affecting their mating competitiveness. Based on this, they bred massive numbers of males, sterilized them, and then released them by air over infested areas, leading to a sharp decline in fly populations.

Panama, in partnership with the United States, has maintained the biological barrier on its border with Colombia through continuous releases of sterile flies for years. But with the danger approaching this year, the USDA shifted its aerial shipments to Mexico. In June, it announced plans to establish a new sterile fly rearing facility in Texas to support releases in northern Mexico. Last month, the USDA also halted livestock movement from southern entry points as a precautionary measure.

However, Miller stated that the Sterile Insect Technique alone is no longer sufficient, and Texas is taking its own measures. These measures include the new bait, the use of insecticides, and the provision of new feed for cattle and deer containing the antiparasitic drug ivermectin. Miller added that the state is also working on developing a vaccine for livestock that aims to kill larvae, but it is still in the development phase.