What do I need to know about New World screwworm?
The New World screwworm (NWS) is a fly that produces larvae that can cause a parasitic infestation in animals, including in livestock, pets and wildlife. Infestation in people is rare. The NWS does not contaminate food.
The New World screwworm (NWS) has re-emerged as a major threat in North America. After being eradicated from the U.S. in 1966, the flesh-eating parasite broke through a permanent biological barrier in Central America and moved rapidly northward through Mexico. The outbreak crossed the U.S. border into Texas in early June 2026.
The timeline of the parasite's northward migration highlights its spread and subsequent arrival in the United States:
2022–2023: The New World screwworm breaches the sterile-fly barrier at the Darién Gap in Panama, beginning its northward movement through Central America and Mexico.
Late May 2026: The screwworm is confirmed in Coahuila, Mexico, just 25 miles from the Texas-Mexico border.
June 3, 2026: The USDA officially confirms the first U.S. case of the current outbreak in a 1-month-old calf in Zavala County, Texas.
June 8, 2026: The infestation spreads further into Texas and crosses into New Mexico, with a confirmed case in a dog in Lea County.
June 9, 2026: Additional cases are confirmed in La Salle County, Texas, triggering widespread movement restrictions and state of emergency declarations across the region.
What is the New World screwworm?
The New World screwworm (NWS) is a species of fly, known by the scientific name Cochliomyia hominivorax.
The fly lays its eggs in open wounds or body cavities (for example, nasal passages) of warm-blooded animals such as farm animals, wildlife, pets, or humans. After the eggs hatch, the larvae (maggots) feed on living tissue, which is called NWS myiasis.
How does New World screwworm affect animals' health?
New World screwworm is found most commonly in animals, not humans. Pets, livestock, and wild animals are vulnerable to infestation. Animals affected may lose weight, have wounds that do not heal, or even die if the larvae are not removed.
How can you protect yourself and your animals from New World screwworm?
Keep wounds clean and covered.
Regularly check pets and livestock for wounds or maggots if you live in or travel to areas where New World screwworm (NWS) is present.
Seek medical or veterinary care if wounds do not heal or show signs of maggot infestation.
Report any suspicious cases to local health or animal authorities, particularly after traveling to regions where NWS infestations are occurring currently or have occurred in the past.
How is a New World screwworm outbreak controlled?
Sterile insect technique (SIT): This method involves releasing sterile male flies to reduce the New World screwworm (NWS) population. It has been very effective in preventing the return of NWS to North America.
Quarantine and inspections: Animals transported from areas where NWS exists are carefully checked for signs of infestation.
Prompt treatment: Removing NWS larvae from wounds and treating infections quickly are essential for both humans and animals.
COLORADO DEPARTMENT OF AG. REGULATIONS FOR ALL WARMBLOODED ANIMALS MOVING ACROSS STATE LINES.
New NWS Import requirements.
If you plan to bring your pets or livestock into the state, it's crucial to understand the regulations that apply to each species.
CDA adopted an Emergency Rule on June 10, 2026 outlining new requirements for animals entering Colorado from an NWS infested state.
The New World screwworm (NWS) is a fly that produces larvae that can cause a parasitic infestation in animals, including in livestock, pets and wildlife. Infestation in people is rare. The NWS does not contaminate food.
The New World screwworm (NWS) has re-emerged as a major threat in North America. After being eradicated from the U.S. in 1966, the flesh-eating parasite broke through a permanent biological barrier in Central America and moved rapidly northward through Mexico. The outbreak crossed the U.S. border into Texas in early June 2026.
The timeline of the parasite's northward migration highlights its spread and subsequent arrival in the United States:
2022–2023: The New World screwworm breaches the sterile-fly barrier at the Darién Gap in Panama, beginning its northward movement through Central America and Mexico.
Late May 2026: The screwworm is confirmed in Coahuila, Mexico, just 25 miles from the Texas-Mexico border.
June 3, 2026: The USDA officially confirms the first U.S. case of the current outbreak in a 1-month-old calf in Zavala County, Texas.
June 8, 2026: The infestation spreads further into Texas and crosses into New Mexico, with a confirmed case in a dog in Lea County.
June 9, 2026: Additional cases are confirmed in La Salle County, Texas, triggering widespread movement restrictions and state of emergency declarations across the region.
What is the New World screwworm?
The New World screwworm (NWS) is a species of fly, known by the scientific name Cochliomyia hominivorax.
The fly lays its eggs in open wounds or body cavities (for example, nasal passages) of warm-blooded animals such as farm animals, wildlife, pets, or humans. After the eggs hatch, the larvae (maggots) feed on living tissue, which is called NWS myiasis.
How does New World screwworm affect animals' health?
New World screwworm is found most commonly in animals, not humans. Pets, livestock, and wild animals are vulnerable to infestation. Animals affected may lose weight, have wounds that do not heal, or even die if the larvae are not removed.
How can you protect yourself and your animals from New World screwworm?
Keep wounds clean and covered.
Regularly check pets and livestock for wounds or maggots if you live in or travel to areas where New World screwworm (NWS) is present.
Seek medical or veterinary care if wounds do not heal or show signs of maggot infestation.
Report any suspicious cases to local health or animal authorities, particularly after traveling to regions where NWS infestations are occurring currently or have occurred in the past.
How is a New World screwworm outbreak controlled?
Sterile insect technique (SIT): This method involves releasing sterile male flies to reduce the New World screwworm (NWS) population. It has been very effective in preventing the return of NWS to North America.
Quarantine and inspections: Animals transported from areas where NWS exists are carefully checked for signs of infestation.
Prompt treatment: Removing NWS larvae from wounds and treating infections quickly are essential for both humans and animals.
COLORADO DEPARTMENT OF AG. REGULATIONS FOR ALL WARMBLOODED ANIMALS MOVING ACROSS STATE LINES.
New NWS Import requirements.
If you plan to bring your pets or livestock into the state, it's crucial to understand the regulations that apply to each species.
CDA adopted an Emergency Rule on June 10, 2026 outlining new requirements for animals entering Colorado from an NWS infested state.
- No animal with NWS myiasis may enter Colorado.
- Animals from an NWS Infested Zone must meet all enhanced requirements, as outlined in the Emergency Rule.
- All dogs and cats entering from states with an Active NWS Infested Zones must have a CVI (health certificate) within 5 days prior to entry into Colorado.
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