Current Status of New World Screwworm (NWS) Infestations in the U.S.

– Gustavo M. Schuenemann, Department of Veterinary Preventive Medicine, The Ohio State University

New World Screwworm, a parasitic fly

The NWS has been largely eradicated in the United States for over 50 years, but a current outbreak in Central America and Mexico is moving closer to the U.S. On August 18, 2025, the Secretary of Health and Human Services issued the emergency declaration after determining that NWS “has a significant potential to affect national security or the health and security of United States citizens living abroad and that involves New World Screwworm.” This emergency declaration allows veterinarians to use certain treatments under extra-label use provisions, to help control potential infestations.

What is NWS?
It is a parasitic fly (Cochliomyia hominivorax) that lays eggs in and on open wounds and mucous membranes of warm-blooded animals. When larvae hatch (maggot), they burrow into the flesh of these animals and eat healthy living tissue causing the myiasis. This infestation is seasonal in livestock, with outbreaks linked to warm conditions that favor the fly’s life cycle and host availability, often peaking in the warmer spring and summer months, and being suppressed by cold winters. NWS can infest all warm-blooded animals, including livestock, pets, wildlife, humans, and even birds.

What to Look For in my Animals?

Be alert for the following clinical signs that may indicate NWS infestation:

• Unusual or irritated behavior (restlessness, excessive movement)
• Isolation from the herd or flock (reduced interaction)
• Frequent head shaking or rubbing against objects
• Foul odor with strong smell of tissue decay
• Visible fly larvae (maggots) in wounds or natural body openings
• Bloody or pus-like discharge from wounds

Even small wounds (e.g., castration, dehorning, navel in newborns) can become infested. EARLY DETECTION AND TREATMENT ARE CRITICAL.

Fly Cochliomyia hominivorax. Source: New World Screwworm by USDA-APHIS

Larvae or maggots. Source: New World Screwworm by USDA-APHIS

Here is the Photo Gallery. Warning: Graphic Images!
NWS Life Cycle, click here.
NWS Information in English and Spanish.

What are the Treatment Options?
Treatment for NWS involves killing and removing larvae from wounds using topical parasiticides, wound dressings, and insecticides, with potential for systemic treatments. Currently there are no animal drugs approved by the FDA to treat or prevent of NWS myiasis in animals. However, the emergency declaration by the Secretary of HHS allowed the FDA to authorize the emergency use of certain animal drugs to treat NWS infestation. This requires a prescription from a licensed Veterinarian due to the emergency use authorization and the need for extra-label use of these drugs under a valid Veterinary-Client-Patient Relationship. Farmers and livestock owners should work closely with their regular herd veterinarian to access and use these emergency treatments.

How to Prevent NWS?

The most effective way to protect the United States from NWS is to prevent its introduction. To help prevent NWS infestation in your animals, follow these key steps:

• Monitor animals closely for early signs of infestation in pets and livestock.
• Promptly treat all wounds, including umbilical cords of newborn animals, using an approved insecticide or wound dressing to prevent egg-laying by adult flies.
• Ensure returning pets from international travel are properly inspected for NWS before entering the U.S.
• Requirements for pet dogs returning from FMD or Screwworm-affected countries.

When it comes to protecting your animals from preventable diseases like New World Screwworm, early detection and timely treatment are critical to protect your animals. After all, an ounce of prevention is worth a pound of cure!

This article has been reposted from the Ohio BEEF Cattle Newsletter. To view the original article, click HERE.