Lee: Unfortunately, it's definitely the latter. This is the early stages of reestablishment. When we look back at the Florida situation—I want to say happened around 2016—that cost millions of dollars to contain, and it came from a single dog. The unofficial report was that it was a dog smuggled in from Cuba, where screwworm is endemic. The problem now is that this has been a wave. These flies have broken across through Panama, hit every single country in Central America, then Mexico, and now the United States. This is not going to be an isolated incident. This is the first time we've documented it on US soil, but the likelihood is that we have potentially hundreds to thousands of these flies here already.
dvm360: What does this detection signal in terms of animal health surveillance and regional spread risk?
Lee: USDA has been doing a terrific job of monitoring, and I think many veterinary professionals have been working hard on submitting samples. There is a lot of active surveillance. The fact now is that this case is 50 miles in from the border, and while we have only a single [confirmed] case, there is a high concern that there are other cases within. How many other wildlife or other animals are being affected, we do not know yet.
dvm360: What are the earliest warning signs of infestation in an animal, and what should clinicians be looking for?
Lee: In a lot of ways, it's going to look very similar to myiasis. You have an animal, you have maggots, you have a lesion. One of the keys, if you're catching it in an early stage, is that you do not have a debilitated animal. The animal does not need to be debilitated first. You will actually have a healthy animal with what can be something as simple as a scratch, a tick bite, even a mucous membrane opening. All of a sudden, you will have maggots beginning to eat and destroy normal tissue. If you are catching it early, you may be sitting there with a fairly healthy animal that has these large, unexpected lesions. As time goes on, it will lead to death in many cases.
dvm360: Once symptoms appear, how urgent is treatment?
Lee: Treatment needs to be initiated right away. They’re eating live flesh, and the longer they're there, the more damage is done. They also attract more flies—the smell will draw additional females to lay eggs. These things are growing, molting, getting larger, eating more flesh. Time is definitely important.
dvm360: If reestablishment does occur, what would the real-world impact look like for livestock producers, the food system, and animal health broadly?
Lee: When we think about meat—any kind of frozen meat that is shipped—New World Screwworm really isn’t a health risk. However, it has the potential to change export markets. Certain countries may not want to import meat once we have screwworm within our borders. When we think about live animal export, that risk is higher, particularly for countries that do not already have New World Screwworm. Also, in places where this is endemic, there are beef producers losing 10 to even 30% of their livestock solely due to New World Screwworm. That is a margin that in the United States is not a sustainable loss long term. Luckily, USDA and our government have been working hard to increase our capacity to produce sterile flies and are looking at new novel therapies and prevention, and so hopefully we will be able to contain and re-eradicate this, but it is going to take time.
Screwworm eradication timeline (USDA)
1962: USDA-Agricultural Research Service launches the screwworm eradication program in the southwestern United States. Sterile flies are produced at Kerrville, Texas, with a mass-production facility established in Mission, Texas.
1966: The United States is declared free of indigenous screwworms, though residual infestations persist in Texas until 1982. The program expands cooperation with Mexico following feasibility studies for regional eradication.
1960s–1970s: The Mission, Texas facility reaches peak production capacity, producing up to ~200 million sterile flies per week. Between 1962 and 1975, more than 96 trillion sterile flies are released across the southern United States, Mexico, Puerto Rico, and targeted outbreak areas.
1974–1977: A new production facility is developed near Tuxtla Gutiérrez, Chiapas, Mexico, to support southward expansion of the eradication program. Operations are later transferred there from Mission, Texas.
1991: Mexico is declared screwworm-free.
1994: Screwworms are eradicated from Belize and Guatemala. Panama becomes the operational headquarters for the USDA screwworm program, and Panama–United States Commission for the Eradication and Prevention of the Screwworm (COPEG) is established to manage the sterile fly barrier system in Panama.
1995–2000: El Salvador (1995), Nicaragua (1996), Honduras (1999), and Costa Rica (2000) are declared screwworm-free, completing the regional eradication corridor.
Southwestern United States and Mexico Collection: Screwworm Eradication Program Records. US Department of Agriculture National Agricultural Library. Accessed June 5, 2026. https://www.nal.usda.gov/collections/special-collections/southwestern-united-states-and-mexico-collection-screwworm-eradication-program-records
dvm360: What containment tools are available today that weren't available when screwworm was originally eradicated from the US in the late 1960s?
Lee: Historically, [the primary tool was the Sterile Insect Technique]. [During the eradication program and subsequent barrier system], we had three production sites— Moore Air Force Base in Mission, Texas; Tuxtla in Mexico; and the COPEG facility in Panama—combined producing anywhere from 600 to 800 million sterile flies per week. After eradication was achieved, Mission and Tuxtla both converted over to fruit fly production, leaving [the COPEG facility] as a buffer producing 25 to 100 million flies a week. So now that we need to restart, it is not as easy as flipping a switch. It can take up to a year and a half, two years to get to full production. Both of those former sites are looking at reconverting or reconstruction to lead to getting back to those same numbers. That remains the large primary tool we have.
Luckily, as you know if you've been following dvm360,they've been very good about looking at all of the new emergency use authorizations, and so we do have some new drug tools that we did not have in the 1960s. Isoxazolines are excellent. There was also a coumaphos combination product that has been utilized. There are extra tools. I am hopeful we will be able to re-eradicate this more quickly, but this is probably not a problem that gets solved in the next month. This could realistically be a scenario where we're looking at a couple of years or more.
dvm360: What should veterinarians be communicating to their clients right now? Are dogs and cats at risk?
Lee: Yes—pets, and people, are at risk. A little lower risk, but they are definitely at risk. The nice thing is that if you look at CAPCvet.org, the American Heartworm Society, all pets in the United States should be on heartworm prevention. If you're using one of the many wonderful combination products—right now we have Credelio Quattro, Simparica Trio, NexGard Combo— and you have an isoxazoline on board, you are not going to get screwworm. I think one of the main things veterinarians can do right now is, 1) don’t panic, but 2) reinforce the use of these combination products on a monthly basis. This will not only be good for so many things, but also for screwworm.
The last thing I would say is, just because you see something crawling across the ground does not mean it is a screwworm. Some labs are being inundated with caterpillars right now. Let the butterflies be. If you see something weird inching across a tree limb, that is not a screwworm.
The adult flies themselves are completely harmless—they do not bite, they do not sting, but when they lay eggs on you, it’s going to look kind of like a typical maggot until you look closer.
dvm360: On that note, what are the key characteristics of a screwworm fly and maggot that would help veterinarians, livestock producers, and pet owners recognize them?
Lee: The flies are actually kind of pretty. They have bright red eyes, blue bodies, 3 stripes along their thorax. But again, the adult fly poses no direct problem other than the fact that the females can lay eggs. The problem is the maggots.
The problem is the maggots. From a distance, a lesion is going to look like a very typical myiasis—you have maggots sitting there. As we talked about, the big difference is that the animal or person does not need to be debilitated. New World screwworm prefers living, healthy flesh.
When you look at the maggot itself, there are a couple of things you can do fairly quickly to say, "I need to send these in." The short answer: if you have maggots and you're concerned, send as many as you can to your lab. Send them to the National Veterinary Services Laboratories (NVSL), preserved in alcohol. They would like at least 10, but give them everything you have.
If you want to have an idea yourself of whether this could be screwworm, these maggots are shaped a bit like a cigar, there's a pointy end and a blunt end. The pointy end is where the mouth is. All maggots have hooks there, but on an average secondary myiasis maggot they're fairly hard to see, because those maggots are feeding on dead tissue that's already oozing—they're essentially scooping it in.
New World Screwworm maggots have prominent hooks because they are actively cutting into live flesh.
On the other end of that cigar—the blunt end—look for 2 dark lines inside the maggot body. That’s the other key telltale sign. Those are pigmented tracheal trunks, the breathing tubes. Yes, maggots breathe from the posterior end, and in the New World screwworm those tracheal trunks are distinctly dark and pigmented. Other maggot species can share that feature, but in the context of animal myiasis, pigmented tracheal trunks are a strong indicator.
If you want to go further, you can section the posterior end and examine the spiracles under a microscope—they have a very distinctive appearance with three lines. But again, you don't need to go that far.
The average pet owner, technician, veterinarian, you can just take a look: Does the pointy end have hooks? Does the blunt end have two dark lines inside? If it does, it becomes a big concern. But regardless, for all of them, if you're worried, send them to the lab.
Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology), is an assistant professor of microbiology at Lincoln Memorial University's Orange Park College of Veterinary Medicine in Orange Park, Florida, and host of Vet Watch on dvm360.
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