Updates June 26th 1:30pm: The number of cases has gone up to 25 as of writing. Any turnaround will be difficult. California has ramped up its response with sterile flies being dropped over San Diego County in attempt to mitigate any possible spread to the area.
June 22nd 6:30am: The USDA dashboard counts 15 cases as of today, with three new cases identified (two yesterday, one on the 20th). There is a huge lack of information surrounding this outbreak that makes it difficult to report on (at least from Orange County, CA without any farmer contacts). I’m hoping that turns around and regular updates become the norm.
June 14th 7:30am: The USDA’s dashboard now notes 12 cases, all but one of which are in Texas. The impacted counties are Sutton County, Tom Green County, Zavala County, Edwards County, La Salle County, and Gillespie County. These are non-contiguous counties, suggesting further spread is likely to be occurring.
June 11th 6:50pm: We are up to nine cases of NWS, eight of which are in Texas.
June 10th 2:30pm: There are now seven cases of NWS (five cattle, a goat, and the dog from NM). Despite the release of 100 million sterile flies weekly, Mexico has counted almost 30,000 cases this year. Significant work will need to be done to beat this back past the Darien Gap; an effort I expect to take multiple years even with significant further investment. This needs to be an international effort on behalf of the Western Hemisphere.
June 8th 11:35am PST: There have been two new cases of screwworm identified in Texas, one in a goat in Gillespie County, TX, and the first case outside of TX in a dog in Lea County, New Mexico. This brings the total to five cases and suggests the screwworm fly is likely more widespread than was previously thought and is a hugely negative update in terms of the first two having been anomalous cases. I expect more cases to be identified in the coming days via wildlife testing, more identification in cattle herds, and cases being detected in fly trips.
June 6th 1:00pm EST: Canada has made the call to halt all importation of cattle from Texas in the wake of the second screwworm case detection. The economic impacts of this outbreak are already starting to trickle in
On June 3rd, 2026, the USDA confirmed that Cochliomyia hominivorax, indigenous New World screwworm (NWS) fly, was detected on American soil for the first time in 60 years in a three-week-old calf in Zavala County, Texas (roughly 90 miles southwest of San Antonio and about 50 miles north of the border with Mexico). The National Veterinary Services Laboratory in Ames, Iowa confirmed the positive after being sent samples from the calf taken from the umbilical area. This was followed by a second case in a one-month old calk found some 5.6 miles from the first case being confirmed two days later on the 5th..
I won’t say this is definitive proof that the New World screwworm has reestablished itself in Texas, although I see it as more likely than when I started writing this given the second case was found after just two days. Two nearby detections in very young calves with umbilical lesions mean the state and federal officials working on this now have to treat the area as one with a local reproductive focus until surveillance tells us otherwise. In line with this, the USDA and the Texas Animal Health Commission established a 20-kilometer infested zone with imposed movement controls on warm-blooded animals leaving the zone, increased trapping, instituting wildlife surveillance, and started new targeted sterile-fly releases (more on those later).
First, a small clarifying point: USDA declared the United States free of indigenous screwworms in 1966, but the border is not some impermeable force. That’s what the “first time in 60 years” refers to. The Southwestern United States had serious post-eradication incursions in the 1970s, and Florida had a contained outbreak in 2016-2017. I’ll be the annoying guy to say technically the June 2026 cases are not the first screwworms ever found on American soil since 1966 even though they are the first confirmed U.S. animal detections in a while and likely come from the northward resurgence, with the first Texas detections being a new phase of the problem. Screwworm was never defeated completely, we had just pushed it south with labs, planes, quarantines, reporting by ranchers, wound treatment, fly factories, international agreements, and mountainous amounts of public health work.
What is New World Screwworm?
Many people in the US may not have ever heard of NWS with it having been eradicated in 1966, but it is a type of fly whose larvae eat living flesh (compared to other blowfly species whose larvae feed on dead tissue). To complete their lifecycle, they have to parasitize the living tissue of an animal, requiring a wound of some sort on a warm-blooded animal such as a cow, deer, dog, horse, human, etc. The female flies are attracted by the odor put off from the wound/bodily opening and will then deposit some 200-300 eggs at the edge of the wound/mucous membrane/orifice such as the nose, mouth, ears, eyes, umbilical area of newborns, or genitalia. Something as small as a tick bite could attract the female. Then, some 12-24 hours later the eggs hatch into larvae that burrow, quite literally screwing their way into the living tissue, tearing the flesh with their sharp mouth hooks as they feed. Over about seven days they cycle through three larval stages, dropping to the ground, burrowing into the soil, and pupating. Adult flies emerge anywhere from seven to 50+ days later depending on things like temperature and humidity.
The biological aspect most important to their eradication though is the fact that the female flies only mate once in their lifetime, with the ability to lay some 3,000 eggs across multiple clutches during the 10-30 day life as an adult. Untreated, the infestation can kill a full-grown steer in as little as a week or two, with wounds expanding and deepening as the hundreds of larvae feed. This can lead to secondary bacterial infections and septic shock. When humans get infected, its called myiasis, though this is rare in the United States. It’s a pretty simple biological cycle in the end though. Female finds a wound, lays some eggs, those become larvae, and those deepen the wound. That attracts even more females and the cycle continues until treatment or death stops it in its tracks.
Our War Against the Fly
Before the mid-twentieth century, screwworm was one of the American South and Southwest’s greatest agricultural pests, as the nuisance fly species was in effect a tax on animal births, a risk post-branding, castration, or dehorning, made barbed wire more of a liability, and turned ordinary scrapes into infestation risks. The USDA’s historical collections describe it as a parasite of warm-blooded animals and as obligate living flesh eaters. In the 1935 outbreak, an estimated 180,000 livestock were killed in Texas counties despite the constant work on the part of ranchers to treat wounds and segregate the infested from the healthy. The earliest efforts to combat screwworm were relatively simple, with ranchers being taught things like shifting calving, branding, castration, and dehorning away from fly season, treating fresh wounds immediately, isolating infested animals, the burning of carcasses, and attempting to get rid of the flies with things like pine tar oil, benzol, and later larvicides. These were taught in demonstrations and in state fair exhibits to get the word out.
Understanding the taxonomy was the first necessary discovery made. In 1933 Emory Cushing and Walter Patton showed that NWS was a different species from the other ordinary blowfly that ranchers knew. The second discovery was more practical when a method of breeding them with non-living tissue was discovered using a medium of ground meat, beef blood, water, and preservatives. This allowed for laboratory work to be done and led to the work of Edward Knipling who developed his theory of autocide: with the release of enough sterile males into the wild population, fertile females who mate with them would lay eggs that wouldn’t hatch. With each passing generation the reproductive rate would fall further and further and, with enough sterile pressure for long enough, completely crash. Knipling began looking for ways to create sterile, but otherwise healthy flies when Hermann Muller’s work on radiation in fruit flies suggested a route to him. Radiation would be tested and found to sterilize screwworms without ruining any ability to compete for mates and a new public health weapon was created.
The first tests were done at Sanibel Island off the western coast of Florida. In 1951 scientists started their program with sterilized males being dropped into the island. Wounded goats were placed around the island and researchers would track the infestation as fly traps helped to get an idea of the ratio of sterile to fertile flies. The population was pushed down enough to call the trial a success, but reinfestation kept the authors humble with the realization that screwworm could make its way back to where it once was even after an eradication event. Further proof came from Curacao when in 1953 veterinary officer B.A. Bitter wrote the USDA to ask advice on getting rid of livestock infestation. At 40 miles off the coast of Venezuela and with plenty of livestock, this was the first opportunity for a real-world test to see if his method would work. The USDA team started in 1954 with sterile flies being supplied from Orlando with round-the-clock operations producing millions of flies. Researchers worked to make the flies hardier, bigger, and more sexually competitive through the manipulation of their larval diets. From there the effort just became more industrial, working on infestations in Florida at the Sebring plant, facilities related to the screwworm effort popping up elsewhere in the Southeast and the Southwest, better rancher reporting and quarantining, and aerial releases finally leading to the 1966 declaration of the US being screwworm free.
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Why We Were Still Left Vulnerable
The 1970s made it clear that, despite our gains, we were still vulnerable to screwworm infestation with heavy screwworm activity in northern Mexico resulting in some of the worst US outbreaks since 1966’s eradication declaration. The year 1971 saw some 444 cases. Compare that to 1972 when there were over 125,000 cases across California, Arizona, New Mexico, Texas, Oklahoma, Arkansas, and northern Mexico, with 90k being in Texas alone and 30k in Mexico (likely an underestimate due to lesser testing capabilities compared to Texas at the time). That outbreak led to the 1972 agreement between the US and Mexico creating the Mexico-United States Screwworm Eradication Commission. As eradication moved south, they targeted the Isthmus of Tehuantepec in Mexico as a target to push beyond. Once that had been done, they moved their target to the Darien Gap between Panama and Colombia.
That much progress had required massive amounts of infrastructural investment and setup. It only worked because sterile flies were being produced in enormous numbers and were being released in the right places to be followed up with surveillance, reporting, quarantine, and further international cooperation. That would be tested in the 2016-2017 outbreak in the Florida Keys, largely among Key deer. While the origin couldn’t be definitively tracked down, it’s thought to have been a possible importation via cargo ship or accidental drift from the Caribbean or Central America. So, while the current outbreak is worrisome, it’s not unprecedented.
We had roughly two decades with the front line in eastern Panama, with the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG) maintaining the biological barrier near the Darien Gap, consistently releasing sterile flies to prevent the fertile ones from moving north into Central America. The USDA’s own budget documents describe it as a 102-mile jungle barrier along the Panama-Colombia border and a s a hub for field operations and sterile insect techniques. While the system held together, it looked almost miraculous in its effectiveness. But that all came crashing down under a mix of factors including migration of people and cattle through the Darien Gap, COVID’s impact on everything that took joint efforts between the involved countries and the economy, and funding cuts. In 2021 there were 78 positive cases in the barrier zone, a bit higher than normal after some field activity was suspended during COVID.
The situation just deteriorated further from there. The CDC noted that in 2023 Panama and Costa Rica had identified an outbreak, with all of the other countries in Central America and Mexico soon following suit, accumulating over 170,000 animal and over 2,000 human cases since. The Darien Gap itself was also changing as the corridor, long seen as incredibly difficult to trek through, ended up with an estimated 520,000 people crossing in 2023 alone. This likely complicated surveillance, animal inspection, land use, field access, and possibly introduced unregulated animal movement including cattle. Once screwworm had made it past that barrier, the geography played into their favor as the continent widened from the incredibly narrow straight, allowing for flies to spread out (they can travel some 180 miles). By November 2024 Mexico had reported a case in Chiapas. The next year the USDA was closing and reopening imports of cattle, bison, and horses from Mexico. You already know what happened this week with the two cases in Texas.
The Stakes are High
US beef markets were already strained before reports of NWS in Texas, with the national cattle herd having shrunk under multiple years of drought-driven liquidation, as well as the built-in lag that comes with breeding large animals which means a decision to expand the herd doesn’t impact the market until years later. The Dallas Fed reported in May of this year that beef prices had gone up by 57% since 2020, with another 3% rise just in the first quarter of 2026. So, screwworm is making itself known in an already brittle economic environment. The border closure to Mexican cattle removed a potential NWS crossover point, but it also removed some of the supply that normally comes to US feedlots. Authors from the Dallas Fed note damage could exceed $3 billion across the Southwest in an outbreak comparable to 1972, with that going up to $8 billion if it matched the size and timescale of the 1962-1980 episodes. Wildlife also produces its own risk for NWS to further spread. At least livestock can be inspected, treated, quarantined, and moved when needed. Good luck doing that to a population of white-tailed deer or feral pigs living out in the warm brush country this summer. Ramping up our sterile-fly capacity is going to be crucial. The COPEG facility in Panama produces about 100 million sterile flies per week and the USDA is planning a facility with similar output in Mexico. One effort in South Texas is expected to produce some 300 million files per week once complete at Moore Air Base.
I’m writing this the night of June 5th and will keep updates at the top of this post and on the Notes section of Substack. I and many others are hoping this does not become the reestablishment of a screwworm population in the US, no matter how short lived that may be. Our system worked for as long as it was in place without interruptions and it will work again under similar conditions.
My work will always be free to read at The Edge of Epidemiology. If you value historically grounded writing and want to help make it easier for me to keep doing, a paid subscription helps more than you know.
