US Closes Border to Livestock Over “Man-Eater” Worm
- Johnny Sheng
- May 29
- 7 min read
Somewhere in Mexico right now, a newly dehorned cow is grazing on a patch of grass with an open wound on its head. A small fly lands on the patch of exposed flesh, laying hundreds of eggs in a matter of minutes. Within days, these eggs will hatch hundreds of fly larvae that burrow into the unprotected wound, consuming flesh and blood to nourish their development. As the larvae grow, the affected animal will sustain massive tissue damage. Without a systemic insecticide like ivermectin for treatment, the cow may be dead in a week.
In that time, the larvae will have fallen out of the infected animal, burrowing into the ground, where they finish developing into adult flies. These flies will not live long; they typically live for less than three weeks—just long enough to mate and inject thousands of eggs into the next warm-blooded animal, restarting the cycle. If climate conditions are favorable, new flies can spread their progeny from Central America to Canada, traveling about a hundred kilometers with each generation.
This is the life cycle of the New World screwworm or Cochliomyia hominivorax—Latin for man-eater screw fly. For centuries, farmers in the Americas have dealt with the parasite, which can infect a wide range of animals, including livestock, pets, birds, and even humans, causing painful and sometimes fatal screwworm myiasis. They exploit any openings they can find—insect bites, scratches, the navels of newborn animals—and cannot be rooted out once inside. Many animals can survive a screwworm infestation but suffer permanent and crippling disease afterward, worsening the parasite’s economic impact for farmers.
More disturbingly, the worm can also jump from animal to person. Symptoms in people include non-healing wounds, pain and bleeding from sores, a foul odor, and secondary infections that cause fever and chills.
The New World screwworm’s lethality, extensive host range, and short life cycle allow it to cause rapid and devastating outbreaks similar to a virus or bacteria. It doesn’t help that human practices increase its spread. Tightly packed conditions on farms create dense concentrations of vulnerable hosts, and agricultural practices that leave open wounds on livestock, like dehorning, create more openings for screwworms to enter their hosts. The presence of pets and people in close proximity to farm animals then allows for interspecies spread.
Eradication
Today, the New World screwworm almost never infects animals or humans in the United States, but it was not always this way.
For centuries, screwworm infections have been recognized as a serious threat to all types of animals in the United States. Then, in 1933, a publication by Doctors Emory Cushing and Walter Patton in Liverpool, England set the stage for the parasite’s eradication.
Before 1933, some treatments for screwworms had emerged, ranging from insecticides to tar oil. However, this failed to address the issue in full, as farmers had to regularly check and treat animals, making onward spread difficult to prevent.
Cushing and Patton’s publication found that the cause of the screwworm infestation was a unique parasite distinct from blowflies that commonly feast on dead animals. Meanwhile, Dr. Edward F. Knipling began to work on a notable characteristic of newly distinguished parasites—female screwworm flies mated once. As such, Knipling reasoned that flooding screwworm populations with sterile males would theoretically prevent the fertilization of females, driving them to extinction. This technique was dubbed Sterile Insect Technique, or SIT. However, given the presence of tens of millions of screwworms in the Americas, the number of sterile males that SIT required was daunting.
That was where Dr. Raymond C. Bushland came in. Working from Texas, Bushland developed a technique to breed huge stocks of flies raised on a diet of meat, blood, water, and formalin. This gave labs sufficient supplies of screwworms to test treatments, but could also be scaled up to produce sterile males for SIT programs.
Work on SIT was briefly interrupted by the advent of WW2, with scientists assigned to researching other parasitic infections affecting soldiers on the frontlines in Europe and the Pacific. By 1950, however, the final breakthrough needed to implement SIT had been made: radiation sterilization. By exposing male flies to radioactive gamma rays from cobalt-60, scientists realized that screwworms could be sterilized by the ton.
In 1954, the island of Curaçao was chosen to test the effectiveness of SIT on an isolated real-world population. Massive stocks of sterile male screwworms were released by USDA scientists to resounding success. The same rapid generation of screwworms that allowed them to grow rapidly also allowed for their rapid decline, and after ten weeks, Curaçao was parasite-free.
So began a decades-long eradication program that scaled up the release of sterile males across the Southeastern US and eventually all of the Americas. Facilities produced tens to hundreds of millions of sterile males a year, flooding US screwworm populations starting in Florida. Combined with timely treatment of affected animals and regular livestock inspections and quarantines, the mass implementation of SIT meant that the US was declared screwworm-free by 1966. However, screwworm populations that plagued farmers in the Southwestern US spent winters across the border in Mexico, and outbreaks continued into the 1970s.
This led to the signing of the Screwworm Eradication Program Agreement in 1972, establishing a joint US-Mexico effort to eliminate the parasite in Central America. A new facility in Mexico produced 500 million sterile male flies a week, aiming to drive screwworms further south. By 1994, the infestation was completely gone from Mexico as SIT programs expanded into more countries. Nicaragua, Costa Rica, Belize, and El Salvador quickly followed. By 2006, with the eradication of screwworms in Panama, a new line of eradication was established at the Darien Gap. The Gap was a perfect natural barrier: 60 miles of dense swamp lands and forests that made crossing from South to Central America difficult even for humans. Officials hoped that the screwworm would stay to the south of the gap, never threatening US livestock again. Unfortunately, that hope was short-lived.
Return
From 2006 to 2022, constant efforts largely maintained eradication of the screwworm in Central America and the US, with the exception of a large outbreak among deer in the Florida Keys in 2016. Fortunately, after releasing 188 million sterile flies and establishing 20 medication sites, US officials were able to re-establish the country’s eradication status.
Then, in August of 2022, the screwworm crossed the Darien Gap, infecting animals in Panama for the first time since 2006. Cases didn’t stop there. By the start of 2024, Costa Rica had reported its first human screwworm infection and declared a state of emergency over growing outbreaks in animals. Cases quickly spread across Nicaragua and Honduras, with tens of thousands of animals affected.
The number of human cases in the current outbreak is uncertain, but over 100 had been reported as of October 2024, including three deaths in Costa Rica. Additional human infections have since been reported as far north as Southern Mexico, where dozens of animal cases have also occurred.
With the screwworm now within 700 miles of the US border, US Agriculture Secretary Brooke Rollins suspended all live animal imports across the southern border on May 11. Citing the parasite’s “unacceptable northward advancement,” a US press release called for measures to remain in place until a “significant window of containment” was made. Livestock producers were also advised to watch for signs of infestation among their animals, and border surveillance efforts were stepped up.
In the US Congress, a bipartisan effort is being implemented to fund a new facility to produce sterile screwworms, hoping to stop or mitigate the parasite directly from the US border. The Strengthening Tactics to Obstruct the Population of Screwworms (STOP Screwworms) Act allocates a total of 300 million dollars to the effort, though any facility would still take time to construct. By then, screwworms could already be in the US.
What’s Next?
The economic impact of a full-scale resurgence of New World screwworms in the US could be enormous. Its eradication is currently estimated to save 900 million dollars each year, and any outbreak would devastate farms and affect everyday consumers. Since the COVID-19 pandemic, beef prices have already been increasing, partially due to a combination of reduced supply and increasing demand. Tariffs announced by the Trump Administration are now expected to further add to the strain. Without a let-up in existing factors, any screwworm outbreak would drive prices to record highs.
Doctors and veterinarians must now also be on watch for screwworm infections in both humans and animals. The US has already reported one human case linked to international travel in 2024, and domestically-acquired cases and even deaths may soon occur if the parasite spreads as it did in Central America. Since very few American vets and doctors have seen the parasite since its eradication in the US decades ago, diagnosis alone could be difficult. A variety of insects endemic to the US can cause parasitic myiasis with similar symptoms, meaning early outbreaks caused by the New World screwworm could be misdiagnosed.
As American farmers and health officials brace for the return of one of the country’s most hideous parasites, the race is on. A new sterile screwworm facility is under construction in Mexico, hoping to augment SIT to stop the infection in its tracks in Mexico. Preventing further spread also depends on cooperation with Central American governments and increased awareness among medical and agricultural professionals. History tells us that screwworms can be contained, but the cost of containment all depends on the speed of the human response.
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