New World screwworm (NWS) is a parasitic fly that infests warm-blooded animals and can cause significant financial losses for cattle producers. The female lays eggs in wounds, and the hatching larvae eat living tissue and can lead to death of the animal. NWS was eradicated from the United States in 1966 and periodically returns. The flies re-established in North America in 2023 and now threaten the U.S. southern border.
How to recognize New World Screwworm
Wounds as small as a tick bite can be infested with NWS, and they are more likely to be around the face and genitals. Animals will be off-feed, painful, and often exhibit scratching and head shaking. Infested wounds may smell like dead carcass, have dripping bloody discharge or pus, and may suddenly get bigger. The ridged larvae can get up to 17 mm (2/3 in) long and generally burrow too deep to see but they may be visible. White eggs may be seen along the edges of the wound. The flies are the size of a typical house fly with orange eyes and green iridescent bodies.
View USDA's New World screwworm photo gallery for images of flies, maggots, and infested wounds.
How to prevent New World Screwworm
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Wound prevention: Delay dehorning, branding, castration, shearing, and vaccination until after fly season
- Inspect pens for sharp objects
- Treat for ticks
- Treat wounds promptly, including the umbilical stump of young animals; Use fly spray and bandage when possible
- Closely monitor the herd for wounds around face and genitals
- Check your vehicle for screwworm flies if you are in an NWS-infested area
If you suspect New World Screwworm
Contact your veterinarian, and report to the ODA State Veterinarian at 503.986.4711.
What to expect if infestation is confirmed
State officials will collect samples and assist with medical care of affected animals. Treatment involves cleaning the wound and removing the larvae, use of dewormers and insecticides, and, if needed, pain control and antibiotics.
If a case of NWS is confirmed, a quarantine will be initiated until the larvae and any flies have been eliminated. ODA and USDA will investigate NWS cases and determine if additional control measures are warranted, such as insecticidal treatment of the environment or sterile fly release.
Treatment does not include destruction of livestock. The animal(s) may be released from quarantine when it is confirmed that no screwworm larvae remain.
Resources
ODA Resources
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New World Screwworm Livestock Producer Handout -
English,
Español
Oregon Department of Agriculture (ODA) New World Screwworm Informational Handout for Livestock Producers.
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New World Screwworm - What to Expect -
English,
Español
Oregon Department of Agriculture (ODA) New World Screwworm - What to Expect Informational Handout.
Other Resources