New World Screwworm are blow fly larvae that infest the living tissue of warm-blooded animals causing a condition know as myiasis. Female flies lay eggs on or near open wounds, body orifices, or decaying tissue. The larvae hatch and burrow into the host's tissues, feeding on live flesh and causing inflammation and tissue damage.

New World Screwworm infestations can occur in all livestock, horses, wildlife, poultry, and other mammals. Humans can also be hosts to the New World Screwworm.

Female flies lay eggs on living animals at the edge of wounds or mucous membranes such as nostrils, ears, eyes, mouth, or genitalia. Eggs hatch and the larvae burrow into tissue and continue to feed and grow. As the larvae feed on the host, damage is caused by sharp mouth hooks, making the wound larger and deeper. Larvae fall from the wound, bury in the ground to pupate, then emerge as new flies to breed and infect new hosts.

Flies are able to travel long distances to find a suitable host, up to 10 - 12 miles. The movement of infested livestock or wildlife can lead to spread over even longer distances.

Adult Flies:

• Metallic blue-green appearance

• About 2-3 times larger than house flies

• Orange or yellow face

• Three dark stripes on back

Larvae (Maggots):

• Cream to pink colored

• Cylindrical, tapered shape

• Distinct bands of spines around body segments

• Found in wounds, feeding on living tissue

Signs in animals include:

• Foul-smelling wounds

• Maggots in wounds

• Enlarged wounds

• Animal distress

• Loss of appetite

• Fever

Any open wound or blood source can become an infestation site.

Common sites include:

• Umbilical Regions

• Castration Wounds

• Branding Locations

• Ear Tags

• Ear Notching or Marking

• Genitalia

• Dehorning Wounds

• Any Cuts, Scrapes, or Open Wounds

The Sterile Insect Technique (SIT) is the most effective way to prevent the New World screwworm. However, there is not enough sterile fly production at this time.

Livestock owners should take the following precautions with all livestock:

• Inspect animals daily for wounds

• Treat all wounds immediately

• Use approved insecticides around wounds

• Maintain proper sanitation

• Control fly populations

General Prevention:

• Use fly repellents

• Schedule procedures such as castration and calving during cooler months

• Monitor imported animals carefully

If you suspect your animals have a New World Screwworm infestation, please contact your local veterinarian. They can safely collect samples and coordinate with USDA and state animal health officials to verify the presence of New World Screwworm.

The Food and Drug Administration has approved treatments for various species. The list of approved treatments can be found here.

As of June 1, 2025, there are no animal movement restrictions for animals with New World Screwworm.