Tweet
New World Screwworm
Last Modified: September 24, 2025
Traveler-Associated Case of New World Screwworm Reported
New World Screwworm (NWS, Cochliomyia hominivorax) is a devastating pest. When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.
NWS is endemic in Cuba, Haiti, the Dominican Republic, and countries in South America. For decades, the United States and Panama have collaborated through the Commission for the Eradication and Prevention of Screwworm (COPEG) to prevent the pest's northward movement. Using a biological control technique involving sterilized insects, this partnership eradicated NWS from the United States in 1966 and successfully eliminated a small outbreak in the Florida Keys in 2017.
USDA is implementing a sweeping, five-prong plan to protect the Nation’s livestock, wildlife, and public health from the growing threat of NWS. This includes proactively monitoring nearly 8,000 traps (734.96 KB) across Texas, Arizona, and New Mexico. To date, no NWS files have been detected.
Keeping NWS out of the United States is crucial to protect our livestock industry, economy (506.16 KB), and food supply chain.
Webinar for Animal Industry
This recorded event covers the NWS' life cycle and damage it causes; how to detect and report suspected cases; and USDA’s response plans, should there be a detection in U.S. animals.
Watch Video
What To Look For
The name screwworm refers to the maggots' feeding behavior as they burrow (screw) into the wound, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.
Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.
Report mammals and birds with the following signs:
How To Prevent This Disease
Protecting the United States from New World Screwworm (NWS)
The best way to keep screwworms out of the United States is to prevent their introduction. NWS can infest warm-blooded animals, including livestock, pets, wildlife, people, and even birds.
Some steps you can take to prevent infestation:
In the event of a NWS detection in the United States, APHIS will recommend additional measures to prevent spread, including:
If you think you have found a screwworm, report it immediately to your State animal health official and APHIS office. This will allow APHIS and partner agencies to respond quickly and remove the screwworms before a population becomes established.
Traveling with a dog? View U.S. entry requirements
How It Is Treated
Eradicating NWS is only possible through sterile insect technique (237.54 KB). With this method, sterile male flies are released into an area where a known population has become established. The sterile male flies mate with wild female NWS flies, which then lay unfertilized eggs. Because female NWS flies mate just once in their lifespan, the NWS population progressively reduces and ultimately dies out.
Animals infested with NWS should be treated according to their veterinarian's recommendations.
Currently, there are no animal drugs approved by FDA for the treatment or prevention of New World Screwworm (NWS) myiasis in animals. FDA provides information to support veterinarians with identifying FDA-approved animal drugs labeled for indications other than NWS myiasis that scientific literature indicates may be effective to prevent or treat NWS myiasis.
There are pesticides to potentially use against NWS. View list.
People who suspect they are infested with NWS should seek immediate medical treatment following the Centers for Disease Control and Prevention (CDC) guidelines. For information on how screwworms affect people or to report human disease, visit CDC at About New World Screwworm Myiasis and Clinical Overview of New World Screwworm Myiasis.
Learn More
Browse NWS Photo Gallery
View NWS Life Cycle
Download NWS Informational Products
Report Signs of Animal Disease
Producers or owners who suspect an animal disease should contact their veterinarian to evaluate the animal or herd. Find an accredited veterinarian.
Animal health professionals (veterinarians; diagnostic laboratories; public health, zoo, or wildlife personnel; and others) report diagnosed or suspected cases of nationally listed reportable animal diseases to APHIS Area Veterinarians in Charge and to the State animal health official as applicable under State reporting regulations.
Controlling New World Screwworm
NWS Outbreak in Central America and Mexico
APHIS is partnering with other USDA agencies, the U.S. Department of State, the Food and Agriculture Organization of the United Nations, and affected countries to respond to an outbreak of NWS in Central America and Mexico.
Learn More About the Outbreak in Central America and Mexico
Expand All Guidance for Veterinarians and Animal Health Officials
Emergency Response Information for Cooperators
History of NWS Eradication in the United States
https://www.aphis.usda.gov/livestock...icks/screwworm
Last Modified: September 24, 2025
Traveler-Associated Case of New World Screwworm Reported
On August 1, CDC notified USDA of a case of New World Screwworm (NWS) in a traveler returning from Central America. There have been no detections in U.S. livestock, and the risk to domestic animals and wildlife remains low. There is no evidence of further exposure beyond this individual case.
For public health questions, please contact CDC.
For public health questions, please contact CDC.
New World Screwworm (NWS, Cochliomyia hominivorax) is a devastating pest. When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.
NWS is endemic in Cuba, Haiti, the Dominican Republic, and countries in South America. For decades, the United States and Panama have collaborated through the Commission for the Eradication and Prevention of Screwworm (COPEG) to prevent the pest's northward movement. Using a biological control technique involving sterilized insects, this partnership eradicated NWS from the United States in 1966 and successfully eliminated a small outbreak in the Florida Keys in 2017.
USDA is implementing a sweeping, five-prong plan to protect the Nation’s livestock, wildlife, and public health from the growing threat of NWS. This includes proactively monitoring nearly 8,000 traps (734.96 KB) across Texas, Arizona, and New Mexico. To date, no NWS files have been detected.
Keeping NWS out of the United States is crucial to protect our livestock industry, economy (506.16 KB), and food supply chain.
Webinar for Animal Industry
This recorded event covers the NWS' life cycle and damage it causes; how to detect and report suspected cases; and USDA’s response plans, should there be a detection in U.S. animals.
Watch Video
What To Look For
The name screwworm refers to the maggots' feeding behavior as they burrow (screw) into the wound, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.
Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.
Report mammals and birds with the following signs:
- Irritated behavior
- Head shaking
- The smell of decay
- Presence of fly larvae (maggots) in wounds
How To Prevent This Disease
Protecting the United States from New World Screwworm (NWS)
The best way to keep screwworms out of the United States is to prevent their introduction. NWS can infest warm-blooded animals, including livestock, pets, wildlife, people, and even birds.
Some steps you can take to prevent infestation:
- Watch for signs of NWS in pets and livestock.
- Make sure pets traveling internationally are inspected for screwworm.
- Handle livestock carefully and inspect pens and equipment for sharp objects that can cause wounds.
- Treat the umbilical cords of newborn animals and all wounds immediately with an approved insecticide.
- Protect pets and livestock from other wound-causing parasites such as ticks.
In the event of a NWS detection in the United States, APHIS will recommend additional measures to prevent spread, including:
- Inspect your vehicle for screwworm flies when leaving a NWS-infested area.
- Postpone or avoid procedures that create wounds such as dehorning, branding, shearing, ear notching, tail docking, and castration if you are in an NWS-infested area.
If you think you have found a screwworm, report it immediately to your State animal health official and APHIS office. This will allow APHIS and partner agencies to respond quickly and remove the screwworms before a population becomes established.
Traveling with a dog? View U.S. entry requirements
How It Is Treated
Eradicating NWS is only possible through sterile insect technique (237.54 KB). With this method, sterile male flies are released into an area where a known population has become established. The sterile male flies mate with wild female NWS flies, which then lay unfertilized eggs. Because female NWS flies mate just once in their lifespan, the NWS population progressively reduces and ultimately dies out.
Animals infested with NWS should be treated according to their veterinarian's recommendations.
Currently, there are no animal drugs approved by FDA for the treatment or prevention of New World Screwworm (NWS) myiasis in animals. FDA provides information to support veterinarians with identifying FDA-approved animal drugs labeled for indications other than NWS myiasis that scientific literature indicates may be effective to prevent or treat NWS myiasis.
There are pesticides to potentially use against NWS. View list.
People who suspect they are infested with NWS should seek immediate medical treatment following the Centers for Disease Control and Prevention (CDC) guidelines. For information on how screwworms affect people or to report human disease, visit CDC at About New World Screwworm Myiasis and Clinical Overview of New World Screwworm Myiasis.
Learn More
Browse NWS Photo Gallery
View NWS Life Cycle
Download NWS Informational Products
Report Signs of Animal Disease
Producers or owners who suspect an animal disease should contact their veterinarian to evaluate the animal or herd. Find an accredited veterinarian.
Animal health professionals (veterinarians; diagnostic laboratories; public health, zoo, or wildlife personnel; and others) report diagnosed or suspected cases of nationally listed reportable animal diseases to APHIS Area Veterinarians in Charge and to the State animal health official as applicable under State reporting regulations.
Controlling New World Screwworm
NWS Outbreak in Central America and Mexico
APHIS is partnering with other USDA agencies, the U.S. Department of State, the Food and Agriculture Organization of the United Nations, and affected countries to respond to an outbreak of NWS in Central America and Mexico.
Learn More About the Outbreak in Central America and Mexico
Expand All Guidance for Veterinarians and Animal Health Officials
Emergency Response Information for Cooperators
History of NWS Eradication in the United States
https://www.aphis.usda.gov/livestock...icks/screwworm