New World Screwworm

New World Screwworm (NWS) is a type of fly whose larvae infest and feed on the living tissue of warm-blooded animals, causing severe wounds and, if left untreated, death. Although NWS was eradicated from the United States in 1968, a rise in cases in Central America during 2023 and 2024 has led to its detection in Mexico.

NWS remains endemic in South America and the Caribbean. In 2023, containment efforts in Panama failed, and the parasite spread through every country in Central America and into Mexico, raising the risk of reintroduction to the U.S.

The USDA is working closely with Mexican authorities to contain the outbreak and prevent the fly from spreading to the U.S. If reintroduced, NWS could lead to billions of dollars in losses due to treatment costs, animal deaths, and control efforts.

Animal owners and veterinarians must stay alert for signs of NWS and report any suspected cases immediately to the State Veterinarian’s office.

What to do if you suspect NWS

• Contact WSDA animal health officials
  • ahealth@agr.wa.gov
  • (360) 902-1878
• Contact USDA-APHIS
  • Veterinarian in charge by state
  • After-hours line: (866) 536-7593
• Follow USDA-APHIS protocols in the "Guidance and response plans" below.

1. Collect larvae from multiple wound sites using gloves and forceps.
2. Place in 70% alcohol inside a tightly sealed vial.
3. Submit samples with a completed Parasite Submission Form to the National Veterinary Services Laboratories.
4. Quarantine the animal(s) immediately to prevent further spread.

• NWS is ruled out or confirmed.
• Appropriate treatment is completed.
• The supervising veterinarian verifies the animal is screwworm-free.

Animal treatment

• Remove all visible eggs and larvae.
• Clean and disinfect the wound.
• Provide analgesics as appropriate.
• Apply effective topical or systemic antiparasitics.
• Recheck the animal 24 hours later and retreat if any larvae remain.

All treatment must be performed under veterinary supervision. Use of gloves is advised for safety. There are no FDA-approved drugs for treating NWS. All antiparasitic use is extralabel and subject to FDA restrictions—especially for food-producing animals. Medicated feeds cannot be used extralabel. Refer to USDA protocols for specific antiparasitic recommendations for different species.

Environmental cleanup and control

To prevent reinfestation:

• Collect expelled larvae, place in sealed vials with alcohol, then bag and dispose in the trash.
• Apply approved larvicides to animal housing or quarantine areas.
• After spraying, wash surfaces with warm, soapy water to remove residues.
• Monitor premises weekly for one month post-quarantine.