New World Screwworm (NWS) myiasis occurs when larvae of the NWS fly (Cochliomyia hominivorax) burrow into the flesh of a living animal. NWS is a serious, immediately reportable disease that can be fatal if untreated; it affects livestock, pets, wildlife, occasionally birds, and in rare cases, people. The disease is most often detected in cattle, swine, horses, small ruminants, and dogs.
Female flies lay eggs in open wounds or other body orifices, such as the nose, eyes, ears, mouth, or genitalia. Hatching larvae eat living tissue and cause very painful wounds. Animals that have recently given birth, suffered an injury, or had a surgical procedure, such as dehorning or branding, are most vulnerable. Even tick bites or vaccination sites may attract flies and become infested.
NWS is a foreign animal disease (FAD) in the United States since the pest was eradicated in 1966. However, NWS is endemic in many countries of South America and some Caribbean islands. Recently NWS has been moving northward through Central America and was found in Mexico in 2024.
Potential Hosts
Any warm-blooded animal.
What to Look For
The best way to keep screwworms out of the United States is to prevent their introduction. Veterinarians and animal owners should watch for signs of NWS in pets and livestock, especially in those with a travel history to a country currently experiencing an NWS outbreak. Signs to look for include:
- General signs of illness, including pain, inappetence, lethargy, and possible self-isolation.
- Irritated behavior, scratching, or head shaking.
- Wounds that have bloody discharge and smell of decaying flesh. Wounds become deeper and larger as larvae grow and feed.
- White egg masses that may be present along the edges of a wound.
- Presence of larvae (maggots) in a wound.
- Visualizing NWS larvae may be difficult as they generally burrow deep into flesh, but if seen they appear as cream/tan colored, cylindrical, ridged maggots with a tapered shape blunt on one end rather than tapered on both as in other species of maggots. Secondary myiasis with blowflies or botflies on the surface may occur.
- Adult NWS flies are the size of a typical house fly (or slightly larger) with orange eyes, blue or green iridescent bodies, and three dark stripes along their backs.
Report Signs of NWS
Immediately report any suspicious wounds, maggots, or infestations to the ODA State Veterinarian at 503-986-4711.
- ODA will assist in getting samples properly submitted to USDA's National Veterinary Services Laboratories. See
ODA NWS Submission Guide below
- Wearing disposable gloves, remove maggots (various shapes and sizes) from several sites within the open wound using forceps.
- Place the specimens in a tight-closing vial or test tube and preserve them in 70 percent alcohol (ethyl or isopropyl) at room temperature.
- Sample collection kits are available from ODA at no charge to Oregon veterinarians. See resources below for ordering information.
Treatment
Animals infested with NWS, myiasis, or secondary infection should be immediately treated by a veterinarian.
- If NWS is suspected, contact the ODA State Veterinarian at 503-986-4711 for treatment guidance.
- Treatment of suspected or confirmed NWS infestations generally includes:
- Thoroughly clean the affected area and remove all visible larvae. Use antiseptics to prevent secondary infections.
- Administer approved topical or systemic antiparasitic treatments to eliminate remaining larvae.
- Left untreated, animals may die within one week of infestation with NWS.
Prevention
- Watch for signs of NWS in pets and livestock, especially in those with international travel history, and immediately report suspicions.
- Educate owners on monitoring their animals for wounds, especially around mucous membranes and umbilicus.
- Advise animal owners when travelling, especially to NWS-infested areas, to ensure pets and vehicles are inspected for NWS flies and larvae.
What to Expect
- If you report a suspect NWS case, ODA staff will collect case history and owner information.
- The ODA State Veterinarian or an ODA District Veterinarian will discuss the suspect case with you and provide instructions on next steps for sample collection, submission and treatment recommendations.
- Specimen testing is provided for free through the National Veterinary Services Laboratory (NVSL). ODA will provide necessary submission forms, case approval, and a prepaid shipping label, as well as coordinate with NVSL staff to ensure rapid processing of samples.
- In the event of a NWS detection in the United States, additional measures will be taken by state and federal authorities to prevent spread and eradicate NWS which may include:
- Sterile fly releases
- Regional quarantines
- Regional and interstate movement restrictions
- Regional inspection/preventative treatment
- Veterinarians are encouraged to contact ODA with any questions by email: StateVet@oda.oregon.gov, or by phone: 503-986-4711.
Resources
ODA Resources
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New World Screwworm - Information for Veterinarians
Oregon Department of Agriculture (ODA) one-page handout with information on New World Screwworm for livestock and companion animal veterinarians.
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New World Screwworm Veterinary Submission Guide
Oregon Department of Agriculture (ODA) New World Screwworm Veterinary Submission Guide for Suspection, Sample Collection, and Shipping Instructions.
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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.
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Veterinary NWS Sample Kit Request
ODA will provide sampling kits veterinarians to have supplies on hand for collecting samples of suspected NWS cases. Kits may be requested online through this link.
Suspected cases of NWS must be reported to ODA prior to sample collection and laboratory submission.
Other Resources