Livestock Owners are Encouraged to be on the Lookout for New World Screwworm in the Wake of Outbreaks in Central America

by | Mar 21, 2025 | Beef Cattle, Dairy, Goats, Horse, Insects, Livestock, Pest Alert, Pest Management, Poultry, Rabbits, Sheep, Swine

New World Screwworm (NWS), Cochliomyia hominivorax is a type of fly found throughout the world. When Screwworm fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage to the animal. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people. The U.S. successfully eradicated NWS by 1966 using the sterile insect technique, which involved releasing millions of sterile male flies to prevent successful mating and reproduction.

While eradication eliminated this deadly pest from the U.S., recent outbreaks of NWS have been on the rise in Mexico and Central America, and the USDA is encouraging owners of horses and other livestock to monitor their animals carefully for any signs of screwworm infection. In 2023, NWS detections in Panama rose from an average of 25 cases per year to more than 6,500 cases in 1 year. Since then, screwworm has been detected in Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico – regions that are in close proximity to the United States. All warm-blooded animals are at risk for NWS infection, especially those who have recently suffered a wound or a surgical procedure.

Infections with NWS usually involve the larval (maggot) stage, and are characterized by non-healing wounds that contain maggots.  Unlike typical fly-larvae maggots, which only ingest dead or dying tissues, NWS maggots feed on living flesh and can cause serious and life-threatening injury to the host mammal.

New World Screwworm Fly Credit: Used with Permission of USDA APHIS

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Signs of a Screwworm infection

  • Irritated behavior, especially toward a wound
  • Head shaking
  • Biting at an open wound
  • The smell of  tissue decay
  • Fly strike
  • Presence of fly larvae (maggots) in wounds

Animal owners that suspect a NWS infection in their animals should contact their veterinarians immediately.

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More information on NWS from the USDA can be found by using the following links:
New World Screwworm
New World Screwworm -– What You Need to Know
Screwworm Poster
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