New World Screwworm isn’t just another headline. It’s a flesh-eating parasitic fly (scientific name Cochliomyia hominivorax) whose larvae burrow into living tissue of warm-blooded animals—and, rarely, people. After decades of peace following a historic eradication campaign, the pest has resurged across Central America and into Mexico since 2023.
On August 25, U.S. officials confirmed the nation’s first travel-associated human case in years: a patient in Maryland who had recently returned from El Salvador (initial reports pointed to Guatemala), with the CDC confirming diagnosis on August 4. Risk to the general U.S. public remains low, but the livestock sector is on alert.
Quick primer: what makes New World screwworm dangerous
Female flies lay eggs along the edges of a wound or in mucus membranes; within hours, larvae hatch and feed on living tissue. Untreated infestations can be fatal. While primarily an animal disease, human myiasis does occur in areas where the fly is present. The CDC notes there’s no curative drug; treatment is rapid removal of larvae plus wound care and prevention.
Where it is now, and why the map has Americans nervous
USDA’s APHIS reports that since the 2023 explosion of cases in Panama, detections have moved north through Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador—and into Mexico, north of the long-maintained biological barrier. That’s why U.S. agencies tightened border controls and stood up new response plans in 2025.
Social media has amplified alarming mileage timelines (e.g., “1,100 miles to 350 in 7 months; Texas in under 4”). The official picture is simpler and more sober: detections are undeniably moving north, but spread is patchy and driven by climate, animal movements, and surveillance—not a straight-line march. Still, proximity to the border has spurred aggressive prevention steps.
First U.S. human case this year: what we know
- Location: Maryland (patient had recently traveled internationally).
- Source country: U.S. Health and Human Services says the traveler returned from El Salvador; earlier accounts referenced Guatemala. CDC confirmed the species on Aug. 4 via telediagnosis.
- Public-health risk: Officials describe risk to the general U.S. population as very low. No U.S. animal cases have been confirmed in 2025 as of the latest reporting.
Important clarification: Screwworm is not passed person-to-person. People or animals acquire it when flies lay eggs on an open wound. Claims that “humans transmit it” conflate “humans can be infested” with “humans spread it.” The CDC’s clinical overview is explicit on exposure and transmission.
Why Texas is taking this personally
Texas still remembers the misery before eradication: widespread livestock losses and catastrophic impacts to wildlife, particularly white-tailed deer. Historical analyses estimate economic damage in the 1950s on the order of $200 million/year (roughly $1.8 billion today). Modern estimates from industry and news reports suggest a new Texas outbreak could again run to the $1.8 billion range.
State leaders are mobilizing. The Texas Department of Agriculture has launched an outreach and tools push—everything from Swormlure traps to feed additives and future vaccines—while urging vigilance among landowners who remember the 1950s–60s devastation.
USDA’s 2025 playbook: build capacity before it arrives
The United States and Panama jointly operate COPEG, which for decades produced sterile screwworm flies to maintain a “biological barrier” far south of the U.S. border. That program succeeded in eradicating NWS from the U.S. (1960s) and in stamping out a Florida Keys re-introduction in 2016–17.
The method—called the sterile insect technique (SIT)—releases vast numbers of sterile males; when they mate with wild females, no viable offspring are produced and the population crashes.
In August, USDA announced a major expansion: a domestic sterile fly production facility at Moore Air Base in Edinburg, Texas, sized to produce up to 300 million sterile flies per week, alongside a dispersal center already progressing in South Texas. That triples current North American capacity and reduces reliance on Panama alone—crucial insurance if releases must surge.
Why ramp so high? During earlier eradication campaigns, releases peaked at 400–500 million sterile flies weekly to overwhelm wild populations. Today’s COPEG facility can scale but doesn’t meet that level alone, hence the Texas build.
What about Minnesota headlines?
Viral posts claim “it’s in Minnesota.” As of mid-July, the Minnesota Board of Animal Health urged producers and pet owners to stay alert and learn the signs—but did not report a detection in the state. Upper Midwest officials say summer conditions could allow temporary survival if introduced, which is why surveillance and rapid reporting matter.
Symptoms, what to watch for, and how to report
In animals, watch for wounds that inexplicably worsen—foul odor, agitation, visible larvae, tissue breakdown, head-shaking, or isolation from the herd. The adult fly is slightly larger than a housefly with orange eyes and a metallic blue-green body with three dark stripes on the back. For suspected cases in animals, contact your state animal health official or a USDA veterinarian immediately; APHIS provides a national brochure with contacts and visuals.
In people, the CDC lists slowly healing wounds, pain or odor, and (sometimes) sensation of movement in a wound, nose, mouth, or eyes. Treatment is mechanical removal of larvae and wound management—seek medical care immediately if you suspect myiasis.
Practical prevention for ranches, rescues, and pet owners
- Harden your perimeter: Fix fencing to limit animal mixing; quarantine new arrivals; scrutinize imported livestock (USDA suspended certain live-animal imports at southern ports this spring as a precaution).
- Wound management: Treat, cover, and monitor cuts (branding, dehorning, birthing navels). Screwworms target fresh wounds first.
- Set traps and schedule checks: State programs are distributing lure traps where appropriate; daily checks during calving/fawning seasons are non-negotiable.
- Document & report fast: Photos, GPS location, animal ID, and veterinary contacts save time if regulators need to investigate.
What the Maryland human case does and doesn’t mean
The Maryland case confirms the U.S. can see travel-associated human myiasis while outbreaks rage to the south. It doesn’t prove the fly is established in the U.S. this year. Officials emphasize no U.S. animal cases have been confirmed in 2025 to date, and public-health risk is low; the economic risk, however, is real if the fly crosses and breeds. That’s why sterile fly capacity, inspections, and public reporting lines are being scaled now.
Myths vs. facts
- Myth: “Humans spread screwworm.”
Fact: Only the fly spreads it by laying eggs on wounds; humans can be hosts but are not a transmission vector. - Myth: “Cold states are safe.”
Fact: Summer conditions in northern states can support survival long enough to cause local cases if introduced. That’s why Minnesota and other states ask for vigilance. - Myth: “It’s already rampant in U.S. livestock.”
Fact: As of the latest reports, 2025 has no confirmed U.S. animal cases. Prevention aims to keep it that way.
Why sterile insect technique (SIT) is the only proven exit ramp
Insecticides can help protect individual animals, but eradication at landscape scale has one track record: releasing massive numbers of sterile males to collapse the population. COPEG perfected this over decades; the Florida Keys re-introduction in 2016–17 was eliminated using the same playbook. With cases now in every Central American country and Mexico, scale is the name of the game—which is why the Edinburg facility is pivotal.
What producers and pet owners should do today
- Bookmark official guidance. CDC’s overview of New World screwworm myiasis is concise and practical—start there for human health questions (CDC NWS myiasis explainer).
- Set a wound-check routine. Daily checks during birthing and hot months; train hands to spot early signs.
- Designate a reporter. One person on your team should own the call tree (vet → state → APHIS) so minutes aren’t lost when something looks wrong.
- Follow state updates. Minnesota and other Upper Midwest states post practical advisories even without detections; Texas agencies are sharing trap and treatment info.
Bottom line
Yes, the map is moving north. Yes, the U.S. has a confirmed travel-associated human case this month. But the path to preventing a domestic livestock disaster is clear and already underway: scale up sterile fly production and dispersal, keep import and surveillance defenses tight, and empower producers and pet owners to report and respond fast.
If Texas and the federal partners hold the line—and Edinburg’s new facility hits its stride—the United States can repeat the success of the 1960s rather than relive the pain.
