New World Screwworm: Information for Veterinarians

As frontline defenders of animal health, veterinarians play a critical role in early detection, treatment, and reporting of new or emerging diseases such as New World Screwworm (NWS). This update includes guidance on identifying signs of NWS, reporting procedures, and next steps. For more information about New World Screwworm, visit the ODA NWS webpage.

What is New World Screwworm?

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. On September 21, 2025 the USDA announced the detection of NWS in the Mexican state of Nuevo León, less than 70 miles from the U.S.-Mexico border. A second case of screwworm was reported in early October in a cow in Nuevo Leon, Mexico, near the Texas border.

Potential NWS Hosts

New World Screwworm may infest any warm-blooded animal including livestock, pets, birds, and humans.

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 Disease Reporting Hotline at 503-986-4711.
• ODA will assist in getting samples properly submitted to USDA’s National Veterinary Services Laboratories for identification. See ODA NWS Submission Guide.
• 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.

Treatment

• Animals infested with NWS, myiasis, or secondary infection should be immediately treated by a veterinarian.
• If NWS is suspected, contact the ODA Disease Reporting Hotline at 503-986-4711 for sampling and 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.
• The U.S. Food and Drug Administration conditionally approved Dectomax-CA1 (doramectin injection) injectable solution for the prevention and treatment of New World screwworm larval infestations, and prevention of NWS reinfestation for 21 days. Dectomax-CA1 is conditionally approved for use only in cattle. Dectomax is already fully approved under a New Animal Drug Application for treatment and control of certain nematode and arthropod parasites in cattle and swine. Dectomax and Dectomax-CA1 contain the same active ingredient (doramectin injection) at the same dose. The withdrawal period for cattle is 35 days for Dectomax-CA1, as it is for Dectomax. Warnings to prevent drug residues in meat from treated cattle are the same for Dectomax-CA1 as for Dectomax: Not for use in female dairy cattle 20 months of age or older. A withdrawal period has not been established for this product in preruminating calves. Do not use in calves to be processed for veal. ectomax-CA1 injectable solution is available in 250 mL and 500 mL bottles. Its label will contain both Dectomax and Dectomax-CA1 indications while each drug has a unique application number. To reduce the risk of antiparasitic resistance and preserve drug effectiveness against other parasites, producers and veterinarians are encouraged to use antiparasitic drugs like Dectomax-CA1 only when medically necessary, in accordance with the product labeling, and as part of a comprehensive parasite management strategy.
• The U.S. Food and Drug Administration has issued an Emergency Use Authorization (EUA) for Credelio (lotilaner) chewable tablets, concluding that, based on the scientific evidence available, it is reasonable to believe that Credelio may be effective in treating New World screwworm (NWS) infestations in dogs and puppies and the known and potential benefits of the product outweigh its known and potential risks. While the majority of dogs in America are at low risk of NWS due to their geographic location, dogs near the U.S.-Mexico border and dogs that have traveled to countries with active NWS cases are more likely to be exposed to NWS. The FDA originally approved Credelio in 2018 for flea and tick-related indications in dogs and puppies.
• The U.S. Food and Drug Administration conditionally approved Exzolt Cattle-CA1 (fluralaner) topical solution for the prevention and treatment of New World screwworm (NWS) larval infestations, and the treatment and control of cattle fever tick in beef cattle 2 months of age and older and replacement dairy heifers less than 20 months of age.

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 or by phone: 503-986-4680.

Source: Oregon Department of Agriculture, FDA