Skip to main content
Handout
verifiedPeer Reviewed

Identify and Prevent New World Screwworm

Authors
Shujuan (Lucy) Li
Dawn H. Gouge
Publication Date: July 2026 | Publication Number: az2216 | View PDF
Get the latest information on New World screwworm
Access animal inspection guides, fact sheets, and additional resources from Extension experts on our Screwworm Resources Page.

 

New World screwworm can infest wounds and natural body openings of livestock, pets, wildlife, birds, and people. Newborn animals are especially vulnerable due to umbilical cord exposure, and infestation of newborns is almost always fatal. Learn more by visiting our New World screwworm information page and screwworm.gov. Download the handout for a detailed photos of New World screwworm flies and larvae with important features that will help you identify them.

  • New World screwworm infestations are serious. Contact your vet or doctor if you suspect an infestation of New World screwworm.
  • Recognize signs of NWS in pets and livestock.
  • Never attempt to treat an active infestation, contact a vet or doctor immediately.
  • Protect pets and livestock from wound-causing parasites and sharp objects.

 Adult flies are about the size of a common house fly. They have red eyes, a yellow-orange face, shorter center stripe, and a metallic blue-green body as shown in the graphic below.

New World screwworm fly with it's red eyes, yellow-orange face, shorter center stripe, and metallic blue-green body labeled on example flies.

USDA-APHIS screwworm.gov

Larvae burrow into and feed on the living tissue of warm-blooded animals. Fully grown larvae vary in size from 6.5 to 17 mm long. Larvae can cause extension damage by tearing host tissue with sharp mouth hooks.

New World screwworm larvae are small, rice-sized pale larvae with dark spine bands, dark rear tracheae, and are tapered at both ends. Fully grown larvae vary in size from 6.5-17mm long.

USDA-APHIS screwworm.gov