The Fall and Rise of Screwworm

Every 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 ProblemScrewworms completely dominated our life. - T.A. Kincaid Jr., Texas rancher, quoted in The Peaceful Atom and the Deadly FlyOn 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.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.Via the USDA 1938Since at least the early 19th century, and likely much earlier, screwworm was a miserable fact of life for raising livestock in the Southwestern US. The screwworm fly would find and lay her eggs on even the smallest open wound, and animals had to be inspected for worms constantly. One author notes that “[p]eople would not leave home for more than a day for fear of finding their animals had been eaten alive while they were away.

Anyone who didn’t check their animals at least every two days — or have someone do it for them — knew that they would pay a heavy toll in damaged or dead animals.” When worms were inevitably found, they had to be dealt with, typically by applying various insect-killing chemicals, though occasionally by manually removing the worms, a task described as “disgusting and sickening.” Often animals couldn’t be saved — an infection in a horse, for instance, was described in the late 19th century as “typically fatal.” The amount of time and effort required was such that ranchers employed ranchhands whose entire jobs were inspecting animals for screwworm.Despite efforts to prevent infection, such as by covering animals’ open wounds with tar, screwworm infestations occurred by the millions — in cattle, sheep, goats, horses, and occasionally humans (the first documented screwworm infection in a person in the US dates to the 1830s). One history of the scourge noted that by the early 20th century, infestations had reached “such a disastrous level that it was becoming unprofitable to raise livestock. Producers were forced to hire more and more cowboys to check and treat animals.” A 1935 USDA survey found more than 1.2 million infections and 180,000 dead livestock in Texas alone, with actual totals likely being far higher. A year earlier the USDA estimated that screwworm had killed 1.3 million animals across the Southeast. Some estimates suggested that in the 1930s and 40s, 60% to 80% of white tail deer in Texas were killed by screwworm infections.1By the 1930s, the situation appeared grim. Screwworms had, by then, found their way into Florida, a climate warm enough that they could survive in the southern portion of the state year-round, spreading up through the southeast during the spring. Prospects for dealing with the pest appeared bleak:Even with the best efforts of scientists and the use of the latest insecticides, repellents, and liver-baited traps, US livestock producers were losing the war. After more than one hundred years of struggle, the screwworm had spread out of control — and there was no solution in sight. -

History of the Mexico-United States Screwworm Eradication ProgramThe toll inflicted by screwworm inspired vigorous efforts to try and combat it. In 1929 the USDA began a screwworm research program at Menard, Texas. This program initially focused on developing better chemical treatments for infections, and would ultimately generate the research that would banish the screwworm from the US.The first breakthrough occurred thanks to USDA entomologist Emory Cushing. At the time the primary methods to battle screwworm were insecticides to treat infections and traps to capture the flies. After several years of research at Menard, Cushing became convinced that these measures wouldn’t be enough. Believing that something was missing from “the screwworm puzzle,” Cushing applied for a grant to study entomology at the University of Liverpool with Walter Scott Patton, a world expert on flies. Working under Patton, Cushing closely studied the internal structure of different flies, determining for the first time that the screwworm was a unique species with a unique lifecycle. Prior to this, the screwworm fly had often been confused with other, similar flies whose larvae fed on decaying, rather than living, flesh. The identification of screwworm as a distinct species, one whose larvae ate only living flesh, was critical: previous eradication efforts had included using traps baited with carrion to attract flies, but Cushing’s work showed that these efforts were useless. Following Cushing’s discovery, research efforts shifted focus to understanding the screwworm specifically.In 1931, shortly before Cushing left for England another entomologist, Edward Knipling, joined the USDA. Initially posted at the Menard laboratory in Texas, Knipling was moved to a series of other labs, before eventually arriving at a newly formed lab in Valdosta, Georgia, established in 1934 to help combat the worsening screwworm problem in the Southeast. By then, thanks to Cushing’s research, it had become clear that the screwworm was a unique species, and Knipling and other USDA researchers began studying it more closely to understand and characterize its lifecycle and behavior. This meant watching the flies closely; at one point, Knipling spent every waking hour for a week straight doing nothing but watching screwworms all day.

From “The Peaceful Atom and the Deadly Fly:”He placed a wounded goat in the open. From daylight till dark for a week he watched the goat. Upon seeing a female screwworm fly deposit eggs on the wound in the goat, Knipling would mark her with fingernail polish. He would then watch the marked female and plotted all her activities for the rest of the week if she remained in the area. Day after day, fly after fly, Knipling observed. What time were flies most active? How many eggs were laid? Dozens of questions were asked. Knipling and his fellow workers tried to pry into the very basic life functions of the screwworm.These studies revealed two important facts about the screwworm. One was that compared to other flies, the number of screwworms in a given area was surprisingly low: later studies eventually put the figure at around 100 flies per square mile. The other was that screwworms would not infect dead animals, only living ones.In 1935 another young entomologist, Raymond Bushland, joined the USDA and was soon tasked with studying the effects of pesticides on screwworms. This required a steady supply of fresh screwworms, which could only be created by deliberately wounding caged animals allowing them to be infected. Bushland spent his days extracting screwworm pupae from the carcasses of dead rabbits, a task so repulsive that Bushland spent every morning of the first few weeks on the job vomiting.Desperate to find a way to grow screwworms that didn’t require live animals, Bushland was, after months of work, eventually able to create an artificial growth medium consisting of “hamburger, blood, water, and a little formaldehyde to delay putrefaction.” The disgusting mixture made it much easier to grow screwworms, and before long Bushland was harvesting them by the thousands.In 1937, Knipling, who had continued to move between various USDA locations, arrived at Bushland’s lab in Texas. Knipling was astounded at the huge number of flies that could be grown in the lab, compared to the relatively small number of flies that seemed to exist in the wild, and he began to ponder whether there might be some way to take advantage of the fact.