Key Takeaways
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The New World screwworm, a parasitic fly whose larvae feed on living tissue, has re-emerged in the US for the first time in decades, with 31 confirmed domestic cases as of July 2, 2026, primarily in Texas.
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The outbreak has triggered strict livestock trade restrictions, including US border closures to live animal imports from Mexico and Canadian bans on livestock from affected US areas, impacting North American livestock markets.
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Authorities are responding with expanded surveillance, quarantines, and the release of sterile flies—a proven eradication method—to contain and eliminate the pest.
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While the screwworm poses little direct risk to human health or food safety, it threatens livestock health, production, and could cause significant economic losses if not controlled.
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The US previously eradicated the screwworm in 1966, but recent outbreaks in Central America and Mexico have allowed the pest to spread northward, highlighting the need for ongoing vigilance and rapid response.
The New World screwworm (NWS), a flesh-eating parasitic fly whose larvae feed on living tissue, has re-emerged as a major animal-health threat in the Americas after spreading northward through Central America and Mexico.
Federal and state authorities have expanded surveillance, quarantines, and sterile-fly releases in an effort to prevent the pest from becoming re-established in the country.
Expana is closely tracking the situation—see below for related coverage and an updated factsheet with the latest developments.
July 2, 2026
- Total confirmed US domestic cases: 31
- Latest confirmed detections: Sheep, Crockett County, Texas (June 30)
July 1, 2026
- Total confirmed US domestic cases: 29
- Latest confirmed detection: Dog, Pecos County, Texas (June 29)
- June 29 update: One additional cattle detection in Uvalde County, Texas
June 29, 2026
- Total confirmed US domestic cases: 27
- Latest confirmed detection: Cattle, Crockett County, Texas (June 27)
- June 26 update: One additional cattle detection in Jim Hogg County, Texas
June 25, 2026
- Total confirmed US domestic cases: 25
- Latest confirmed detections: Sheep, Crockett and Edwards County, Texas (June 24); Cattle, Medina County, Texas (June 24)
June 24, 2026
- Total confirmed US domestic cases: 19
- Latest confirmed detections: Cattle, Terrell County, Texas (June 23)
June 22, 2026
- Total confirmed US domestic cases: 16
- Latest confirmed detection: Goat, Terrell County, Texas (June 22)
June 15, 2026
- Total confirmed US domestic cases: 12
- Latest confirmed detection: Sheep, Sutton County, Texas (June 12)
- June 11 update: Four additional detections in Texas (three cattle, one goat)
June 9, 2026
- USDA confirmed a new case in a calf in La Salle County, Texas
- Total confirmed US cases increased to five
June 8, 2026
- USDA confirmed three additional New World screwworm cases, including two in Texas and one in New Mexico
- APHIS later determined that the Andrews County dog case originated in New Mexico, making it the state’s first confirmed case
June 3, 2026
- US authorities confirmed the first US case in decades in a calf in Texas
What is the New World screwworm?
The New World screwworm is not a worm, but the larval stage of the fly species Cochliomyia hominivorax. Unlike most maggots, which feed on dead tissue, screwworm larvae consume the living flesh of warm-blooded animals.
Adult female flies lay eggs in open wounds or body openings. After hatching, the larvae burrow deeper into tissue, enlarging wounds and potentially causing severe injury or death if untreated.
The parasite can infest cattle, sheep and goat, swine, wildlife, pets, and, in rare cases, humans. Livestock are the primary concern because infestations can spread rapidly and cause significant economic losses.
Trade restrictions
The spread of NWS has prompted a series of livestock trade restrictions across North America as authorities seek to prevent the parasite from becoming established in new areas.
The United States currently has its southern border closed to imports of live cattle, bison, and horses from Mexico because of the screwworm outbreak. The restrictions were first imposed after detections in Mexico in late 2024 and were tightened in May 2025 as the pest continued moving northward.
Following the June 2026 confirmation of screwworm cases in Texas, the Canadian Food Inspection Agency announced temporary restrictions on livestock imports from the affected areas in the US. Under the measures, Canada will not allow the entry of livestock originating from Texas or animals that have been present in Texas within 21 days before crossing the border.
The restrictions are expected to remain in place until animal-health authorities determine the risk of further spread has been reduced.
Was it previously eradicated?
Yes.
The United States eradicated New World screwworm in 1966 using the sterile insect technique, which involves releasing large numbers of sterile male flies that mate with wild females, preventing reproduction. The program later pushed the pest south through Mexico and most of Central America.
A sterile-fly barrier has been maintained in eastern Panama for years to prevent northward spread from endemic regions in South America.
Why is it back in the headlines?
Since 2023, outbreaks have expanded across several Central American countries and into Mexico, raising concerns that the parasite could re-establish itself in North America. Authorities reported increasing detections as the pest moved northward beyond the traditional containment zone.
In June 2026, US officials confirmed a screwworm infestation in a calf in Texas, the first US detection in decades. Additional cases were subsequently reported.
Does it threaten humans?
Human infestations are rare but possible. Most cases occur in livestock and wildlife.
People with open wounds or certain medical conditions may be vulnerable if exposed. Health authorities say the parasite is primarily an animal-health threat rather than a broad public-health emergency.
Is it a food safety risk?
US officials say NWS does not pose a food-safety risk. Instead, concerns center on its impact on animal health, livestock production, interstate commerce, and international trade in live animals.
How is it controlled?
The principal control measure remains the sterile insect technique.
Authorities release sterile male flies into affected areas. When wild females mate with sterile males, no viable offspring are produced, causing populations to collapse over time. The strategy was central to the original eradication campaign and remains the cornerstone of current control efforts.
Other measures include:
- Animal movement restrictions
- Surveillance and inspections
- Rapid treatment of infestations
No animal drugs are specifically approved for New World screwworm (NWS) myiasis, but some FDA-approved products labeled for other uses may help prevent or treat infestations. Certain pesticides may also be effective against the parasite.
Current status
As of June 2026, federal and state authorities are conducting intensive surveillance and containment operations in Texas, alongside continued sterile fly releases and movement controls.
In a June 8 press conference, Texas Governor Greg Abbott said the state aims to stay aligned with Texas ranchers in addressing the NWS threat. Governor Abbott directed the Texas Division of Emergency Management (TDEM) to activate the Texas State Emergency Operations Center (SOC) to Level II (Escalated Response) to support the state’s NWS response.
In April, the USDA issued a New World screwworm (NWS) Response Playbook outlining federal plans for detecting, containing, and eradicating the parasite in the US.
Written by Courtney Shum
