By Patty Wilber
Screwworm has recently been detected in Texas and New Mexico. It is a nasty little fly that lays eggs on open wounds (even super tiny wounds and in noses and ears…eww.). The eggs hatch, and the maggots eat the living flesh. It infects all mammals, but has been especially devastating to the cattle industry in the past.
The major control method has been the release of sterile males. Females breed only once, and dropping millions of sterile males from airplanes prevented viable eggs from being produced. This really worked! By 1981, the fly-rearing program in Texas was shut down. The last reported case in the U.S. was 1982…But maybe that was last reported case in livestock? because…..
There was an outbreak in deer in 2016 in Florida, that was quashed by 2017. “Dr. Jack Shere, the U.S. Department of Agriculture’s chief veterinarian, said in a March 23 announcement from the USDA Animal and Plant Health Inspection Service that the eradication effort involved the release of 154 million sterile flies, 17,000 animal inspections, and about 700 hours of surveillance.” The flies were raised in Panama.
“USDA continues to disperse 100 million sterile insects per week in Mexico and along the U.S.-Mexico border and closely evaluates the location and circumstances of each new case. The sterile insect release area, or polygon, is adjusted as needed to maintain broad suppression and help prevent the pest from moving into the United States.” Even though that didn’t quite work. Also, those flies are coming from Panama…because…
There is only one active fly breeding facility, and it is in Panama. It is there because that piece of land between North America and South America is very narrow and a great spot to create a virtual wall. The screwworm was considered eradicated all the way through Panama in 2006.
Clearly, the wall has been breached. A Mexican facility sponsored by the U.S. was handed over to the Mexican government, who shut it down in 2012. It used to produce 500 million flies per week and was closed because that stuff is expensive (which of course is why we gave it to them in the first place). Two new fly breeding facilities are being constructed (one in Texas and one in Mexico), and Panama is upping its production from 10-20 million to 100 million flies per week.
Change in climate that has allowed flies to survive year-round in more places has contributed, and possibly lax inspections of transported animals.
As of 6.11.2026, there have been nine reported cases in the U.S. You can check out the USDA dashboard here.
For us with small numbers of animals, infections would be pretty obvious, and the USDA has released some drugs to help. Also, physical removal of eggs and maggots–ALL– of them is necessary. If you were to suspect a screwworm infection in one of your animals, CALL YOUR VET. The USDA has a protocol to follow.
Fortunately, the U.S. has successfully beat screwworm before, and it seems likely we can do it again. In addition, new genetic modification techniques might give new tools in the fight.
Below is a link to a RadioLab story about screwworm that I found interesting. My son told me about it. It turns out he may be collaborating on an epidemiological model to help better understand factors driving the northward spread. (Proud mom.) Spending money on science has real-world benefits.
Was screwworm eradicated https://www.agdaily.com/livestock/was-screwworm-eradicated-from-the-u-s-in-1966-ranchers-say-no/
Screwworm in deer in Florida https://www.avma.org/javma-news/2017-05-15/screwworm-again-eradicated-florida
USDA https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/current-status?page=1
Fly breeding https://www.aphis.usda.gov/livestock-poultry-disease/stop-screwworm/sterile-fly-production-dispersal-facilities
What to know about screwworm https://cals.ncsu.edu/news/what-to-know-about-the-new-world-screwworm/
Mexican and Florida plant shut down https://wildlife.org/another-screwworm-incursion-inevitable-experts-warn/
Climate change and fly survival https://yaleclimateconnections.org/2026/06/what-to-know-about-the-new-world-screwworm-fly-and-its-reappearance-in-the-us/
Meds to help livestock https://www.fda.gov/animal-veterinary/cvm-updates/fda-issues-emergency-use-authorization-over-counter-injectable-drug-new-world-screwworm-dairy-cattle
USDA screwworm protocol https://www.aphis.usda.gov/sites/default/files/aphis-sop-detection-nws-in-animals.pdf
RadioLab Return of the Flesh Eater; https://radiolab.org/podcast/return-of-the-flesheaters
Well, DOGE shut down our program and the USAID that was helping in SA, so…yeah. I mean, NO ONE could have predicted the predictable… This is great info, though, gotta share it!