GoKawiilEvery spring, as sure as the seasons, and for generations unknown, screwworms began their annual march northward from their overwintering sanctuaries in Mexico and South Texas. Pushed by an unknown force as inexorable as gravity, screwworms moved north — ever moving, ever spreading, ever multiplying, ever destroying. No army ever advanced any more surely or methodically. No army was ever more destructive. Attacking, killing, maiming, and destroying, screwworms literally ate their way north. Reaching upper South Texas, they fanned east and west - all the while moving north - dotting the countryside with the dead carcasses of hapless wildlife, cattle, sheep, and goats, filling the “wormy” pens of farmers and ranchers. - C.G. Scruggs, The Origin of the Screwworm Control Problem
Screwworms completely dominated our life. - T.A. Kincaid Jr., Texas rancher, quoted in The Peaceful Atom and the Deadly Fly
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.
Screwworm cases as of July 1st, via the USDA .
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.
History of screwworm
New World Screwworm (scientific name Cochliomyia hominivorax) is a species of fly native to the Western Hemisphere. While the larvae (maggots) of most flies feed on dead or decaying tissue, screwworm is unique in that its larvae feed on living tissue. The grisly cycle begins when a screwworm fly lays its eggs on the open wound of an animal. The eggs soon hatch into wriggling white worms, which can grow up to two-thirds of an inch long. These worms burrow into the flesh as they eat their way into the animal, making the wound even worse and attracting even more flies to lay their eggs. After a few days of eating, the worms transform into shell-covered pupae, falling out of the animal and emerging as fully grown flies about a week later. Untreated, a screwworm infestation in an animal is typically fatal.
... continue reading