New World Screwworm in the U.S: What Livestock Producers Should Know

New World screwworm (NWS, Cochliomyia hominivorax) is a serious pest recently detected in the Southwestern United States. It can lead to significant economic losses for livestock producers, highlighting the importance of awareness. This fact sheet answers common questions about NWS and how to protect your livestock.

What is NWS?

NWS is a parasitic fly widespread in the Caribbean and South America that mainly affects animal health. Their larvae (maggots) feed on the living tissue of warm-blooded animals by burrowing into the flesh. This infestation, called myiasis, causes painful lesions that can be lethal when left untreated.

Why is NWS a concern for livestock producers?

NWS infestations can cause severe lesions and predispose animals to secondary infections, resulting in economic losses due to reduced productivity, treatment costs, and mortality. Additionally, NWS not only affects livestock but can also affect wildlife, pets, birds, and occasionally humans.

How do animals get infested with NWS?

Female NWS flies lay eggs in wounds and sometimes in body openings such as the mouth, nose, vulva, and anus. The eggs hatch into larvae that look like screws. Animals are most at risk after routine procedures that create wounds, such as castration, dehorning, tail docking, and shearing. Other risk factors include untreated newborn navels, placenta retention, uterine prolapses, tick bites, and injuries caused by sharp objects, equipment, or fencing.

Are all maggots in wounds New World screwworm?

No. Several fly species can cause myiasis, and their larvae can be difficult to differentiate. Infested wounds should be promptly examined and treated by a veterinarian, and samples should be submitted for diagnostics.

What do NWS infestations look like?

• Maggots present in the wound
• Wounds that fail to heal normally and are enlarging or deepening
• Bloody wounds with foul-smelling discharge
• Excessive licking, biting, or irritation around the wound
• Isolation from other animals or signs of discomfort

How can I protect my animals?

• Regularly inspect animals for wounds, especially after management procedures
• Perform procedures that create wounds in cooler months when possible.
• Clean and treat wounds promptly
• Treat newborn navels properly
• Ensure that pens and handling areas are free of objects that can cause injuries
• Reinforce tick control protocols

Are there any treatments available?

Yes, several medications have been authorized for use for NWS. Early detection allows animals to be treated according to veterinary recommendations. It is important to maintain a valid veterinarian-client-patient relationship so producers can receive immediate support in the event of an outbreak.

What should I do if I suspect NWS?

NWS is a reportable disease. The risk of NWS introduction in Pennsylvania remains low, and larvae would be unlikely to survive winter temperatures. However, stay vigilant and immediately contact your local veterinarian or a state animal health official if you suspect NWS.

Is consumption of animal products safe?

Yes. NWS is not transmitted through the consumption of animal products. Once animals are fully recovered and medication withdrawal periods have been met, they can safely enter the food supply.

For suspected cases, contact:

Bureau of Animal Health and Diagnostic Services (BAHDS, 717-772-2852).

USDA-APHIS Area Veterinarian (717-540-2763).