Protect your herd from New World Screwworm

The New World Screwworm (NWS) is emerging as a significant and growing concern for cattle producers, posing potential threats to their operations and animal welfare. Its documented northward spread, moving from Panama and establishing a presence in southern Mexico, highlights an escalating challenge.

The confirmation of the first human case in the United States in August 2025 serves as a stark indicator that this pest is no longer a remote issue, but a tangible reality. This means its implications extend far beyond just the U.S. cattle industry, impacting a broader range of domestic livestock, various wildlife populations, household pets, and crucially, human health. This expanding reach demands increased vigilance and strategic responses from all affected sectors.

What is New World Screwworm and how does it harm animals?

NWS is a parasitic fly whose larvae infest open wounds on warm-blooded animals, feeding on live tissue. This infestation causes severe pain, tissue damage and even death if untreated. Cattle, especially newborns like young beef or dairy calves, are particularly vulnerable. The parasite is zoonotic, meaning it can be transmitted across species, like between cattle and humans.

The U.S. Department of Health and Human Services (HHS) confirmed the first human case of NWS in the United States in August 2025. The individual had recently traveled to El Salvador, and the case was confirmed by the Centers for Disease Control and Prevention (CDC) and Maryland Department of Health. The parasite has not been reported in U.S. livestock this year.

Why ranchers and dairy farmers should be concerned with New World Screwworm

• Economic impact. The U.S. Department of Agriculture estimates the New World Screwworm threatens over $100 billion in U.S. livestock-related economic activity. Further economic impact in the form of higher consumer beef prices is possible if cattle trade remains disrupted for an extended period.
• Livestock mortality. Infestations can lead to high mortality rates, especially in young or injured animals.
• Wildlife spread. Migratory animals can carry NWS across borders, making containment difficult.

What ranchers and dairy farmers can do to prevent NWS damage and losses

Effectively preventing New World Screwworm in a cattle herd is an effort on multiple fronts. Watch for a few key symptoms, including irritated behavior, head shaking or biting or licking wounds. If you see large numbers of larvae or egg masses in and around open wounds (including where they've been branded, castrated or dehorned, or around umbilical cords or genital areas), that's a sign animals are experiencing negative health effects. As with other illnesses, infected animals may stop eating or segregate themselves from the rest of a herd.

"Surveillance and early detection are so important to preventing economic losses from NWS," said Nationwide Senior Dairy Consultant Dan Rice. "Even if you're not 100% certain but see some of the telltale symptoms, report cases to your veterinarian immediately. This will help contain its spread."

There isn't currently a veterinary drug approved for NWS. But veterinarians can administer products labeled for other fly infections to prevent myiasis, or when a fly strikes an animal to begin feeding on host tissue. The U.S. Food and Drug Administration (FDA) has a list of approved treatments that, though considered "extra-label," can help prevent NWS damage. 

What’s being done to stop NWS around the country

After the first U.S. case was confirmed, U.S. Department of Agriculture (USDA) officials announced an enhanced strategy to prevent the spread of NWS. The plan includes:

• A $100 million investment in new technologies
• Construction of a sterile fly production facility in Edinburg, Texas, capable of producing 300 million sterile flies each week
• Strengthened border protections with expanded patrols or “Tick Riders” as well as newly trained detector dogs

“This strategy focuses on surveillance and aims to strengthen protocols to keep NWS from crossing into the U.S.,” Rice said. “Until there’s adequate supply of sterile flies to help eradicate the pest, it’s best for ranchers to stay watchful for NWS and report any potential infection to their veterinarian or other local animal health experts.”

See more about New World Screwworm. Talk to your veterinarian, local Extension livestock specialist or Nationwide Farm Certified agent to learn more about how you can help keep NWS away from your herd and take the right action if it does infect your animals.