The Fall and Rise of Screwworm

Lifecycle of the Screwworm

From Construction Physics:

The Fall and Rise of Screwworm by Brian Potter

On June 3 of this year, a flesh-eating parasite, the screwworm, was found in a three-week-old calf near the Texas town of La Pryor. Since then, dozens more cases have been discovered in Texas and New Mexico. Outside of a screwworm outbreak in the Florida Keys in 2016 (which was contained), this marks the first screwworm infestation in the US since the 1980s.

Until now, the US has been free of screwworm not due to luck, but because of a decades-long program to eradicate the parasite by breeding it out of existence. By dropping millions of sterile male screwworm flies in an infested area, agricultural agencies can overwhelm the native, fertile male screwworms. Female screwworm flies, who only mate once in their life, will mostly mate with sterile males, producing no living offspring. Drop sterile flies for long enough, and eventually there will be no viable offspring at all, and the pest will be eliminated.

Over the course of several decades, this “sterile male technique” was used by the USDA to eliminate screwworm from the US, Mexico, and Central America. Since the early 2000s a joint US-Panamanian organization, COPEG, has maintained a “screwworm barrier” at the Darien Gap between Colombia and Panama. Every week, millions of sterile male screwworm flies are dropped over the gap, preventing the screwworms from spreading north from South America (where it remains endemic).

Sometime around 2023, the barrier at Panama failed, and for the last several years screwworm has been marching north. It’s now reached the US. Efforts are underway to eliminate screwworm from North and Central America once again, but it will likely be years before they succeed.

The screwworm eradication program was so effective at eliminating the parasite that we’ve collectively forgotten what an enormous problem it used to be. It’s worth understanding the costs inflicted by screwworm prior to its elimination, how a program emerged for eliminating it, and how control was allowed to lapse. 

Previous posts on the subject:

• Screw You, Screwworm
• Screw You, Worm