New World Screwworm Update

Livestock | February 2026

Written by:

Jennifer Ryan has been writing about veterinary topics for nearly 20 years. She has a master’s degree in journalism, a bachelor’s degree in agricultural communications and decades of experience caring for both livestock and pets.

Key points
  • New World screwworm (NWS) is a species of parasitic fly that feeds on live tissue.3
  • It primarily affects livestock, but it can also infest people.3
  • NWS is endemic in Cuba, Haiti, the Dominican Republic and countries in South America, with cases spreading north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador and Mexico.4
  • NWS pupae are killed if the soil is consistently below 8° C/46° F.5
  • Dectomax-CA1 (doramectin injection) Injectable Solution and Exzolt Cattle-CA1 (fluralaner) topical solution are conditionally approved by the FDA to prevent and treat NWS larval infestations.

In May 2025, a case of New World screwworm (NWS) was identified about 700 miles south of the U.S. border. This was too close for comfort for the U.S. Department of Agriculture, which suspended live cattle, horse and bison imports from Mexico into the United States.1 In turn, the closed border contributed to a rise in beef prices for American consumers. The move emphasized the importance of keeping NWS out of U.S. cattle herds and the potential devastation the pest could cause.

Cattlemen everywhere held their breath until colder temperatures arrived, effectively slowing the northward encroachment of the fly.

“We are feeling much better today than we were six months ago,” said Sigrid Johannes, senior director, Government Affairs for the National Cattlemen’s Beef Association (NCBA). “The weather is playing in our favor. We made it to the time of year when there’s less pressure and risk. We’re not out of the woods yet, but we’re making meaningful progress on prevention and mitigation strategies.”

Wintertime will allow progress on two fronts: control efforts designed to keep the fly south of the U.S. border and preparedness for detection on American soil.

Keeping the fly outside the United States

The United States eradicated the NWS in the 1960s, largely using the sterile insect technique. This involves releasing sterile flies into the environment to gradually reduce the population.

A new sterile insect production facility is being built in Mexico, and an existing facility is being renovated. Both are funded using a $21 million investment from the U.S. Department of Agriculture.2 Johannes said the surveillance network in Mexico for the pest is being reinforced to help ensure better detection.

“With screwworm, prevention is worth everything because treatment can be so difficult. It requires constant monitoring of animals, checking once or twice daily,” Johannes explained. “Screwworm took decades to eradicate from the U.S. last time. Our goal — and we share this with the USDA — is to eradicate it from Mexico entirely, ideally push it back south of the Darién Gap in Panama. Our short-term goal is to give our producers that buffer zone and normalize our supply chain. We are down 1.25 million cattle that would have been imported in a normal year. That closed border definitely had an impact.”

Get the latest updates from the USDA APHIS

On Dec. 26, 2025, in Tamaulipas, Mexico, a NWS case was detected in the umbilical lesion of a 6-day-old calf. This indicates a fly population just 197 miles from the U.S.-Mexico border, according to NCBA.

Generational gap in NWS knowledge

The NCBA is currently working with the USDA, American Veterinary Medical Association (AVMA) and other groups to increase awareness of NWS.

“There are a huge number of producers who have not lived with screwworm in the United States,” Johannes said. “It’s important that veterinarians and producers take this time to get educated on what it looks like, understand how they report it, take a sample and treatment options. Also, we don’t have the manpower in rural America that we did a few decades ago. NWS was a concern in the best of times.”

The USDA is primarily responsible for screwworm testing, protocols and action plans. NCBA and other groups are supporting USDA in that work. This will help ensure identification. Testing can be performed rapidly, and cattle can continue to move throughout the country.

Prevention and treatment options for U.S. cattle

Headed into spring and summer, cattlemen now have two conditionally licensed pharmaceutical products approved for prevention and treatment of NWS.

Cattle producers regularly treat and prevent damage from flies. Yet, the NWS is a different pest altogether. It is a type of blowfly that lays its eggs on living tissue, causing painful wounds that can leave the infested animal vulnerable to secondary infections.

“Most commonly, you’ll see it at the site of some wound or disruption of the skin, even a tick bite or injection site could do it — it doesn’t have to be a huge, gaping wound,” explained J.P. Pollreisz, DVM, Cattle and Equine Technical Services, Zoetis. “In fact, the New World screwworm can utilize mucus membranes; it doesn’t have to be a wound.”

After the female NWS lays eggs, the cycle continues within the living tissue of animals while the eggs hatch and develop into larvae and maggots. There can be hundreds of larvae within an infected site, causing a distinct rotten smell. Untreated animals can die from the infestation, especially combined with the secondary bacterial infection that inevitably occurs, Pollreisz explained.

Advice for southern cattle herds

Until NWS is identified in the United States, Pollreisz recommends the ongoing diligent surveillance of cattle already in place at most operations today, using your nose and eyes to look for this pest. As warm weather consistently returns, cattle should be observed daily, even if utilizing drones.

“Just the good animal stewardship cattlemen already employ is enough,” he advised. “Be aware and watching. The key areas are wounds. The screwworm loves the newborn calf navels. As we get into calving season, that’s going to be one of the No. 1 areas to scrutinize. Also, anytime you’re castrating, dehorning or using needles. That’s the second most likely point of a screwworm strike.”

Veterinarians and producers should review the USDA Animal and Plant Health Inspection Service (APHIS New World screwworm playbook). It outlines reporting, treatment and prevention procedures, should New World screwworm myiasis be detected. USDA/APHIS will employ fly traps close to affected premises, and the results will be publicly available online. This can help producers and veterinarians decide if, and when, to implement herd prevention measures.

“Remember that this is a reportable condition, and it is in the producer’s best interest to do so, hastening the control and eradication efforts to be deployed locally, such as the sterile pupae dissemination,” Pollreisz added.

Dectomax-CA1 (doramectin injection) Injectable Solution is conditionally approved for NWS in the 250 mL and 500 mL sizes. The 100 mL size of Dectomax Injectable does not have the NWS indication because a conditionally approved product cannot be used off label on the recommendation of a veterinarian. The 100 ml size with the original labeling allows veterinarians to utilize it off-label, for example in sheep and goats. The product is effective against NWS for 21 days in cattle, and Pollreisz recommends producers take that into consideration when building a treatment and prevention program.

“It’s the same Dectomax formulation we’ve had since 1995,” he said. “This allows producers to kill the larvae in the wound. Don’t count on traditional fly control measures. The adult fly doesn’t use manure or vegetation to lay eggs.”

In addition to a conditionally approved larvicide, the myiasis should also be treated with physical excavation and destruction of larvae, application of a topical insecticide approved by the EPA to treat New World screwworm myiasis, administration of a systemic antibiotic to treat the secondary infection and something for pain management.

“As I talk to producers, we’re cautiously optimistic that we have some breathing room for the next couple of months, given the climate and life cycle of the New World screwworm,” Pollreisz said. “It’s important that we use this time to further prepare.”

References:

  1. Secretary Rollins Suspends Live Animal Imports Through Ports of Entry Along Southern Border, Effective Immediately | USDA. (2025, May 11). usda.gov/about-usda/news/press-releases/2025/05/11/secretary-rollins-suspends-live-animal-imports-through-ports-entry-along-southern-border-effective
  2. Update on USDA Efforts to Fight New World Screwworm in Mexico | USDA. (2025, May 27). usda.gov/about-usda/news/press-releases/2025/05/27/update-usda-efforts-fight-new-world-screwworm-mexico
  3. CDC. (2025, September 8). About New World Screwworm. New World Screwworm. cdc.gov/new-world-screwworm/about/index.html
  4. New World Screwworm Prevention for Animals. (n.d.). Retrieved December 15, 2025, fromaphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm
  5. Disease Response Strategy: New World Screwworm Myiasis. (June 2025) USDA. Retrieved December 15, 2025, fromaphis.usda.gov/sites/default/files/nws-myiasis-disease-strategy.pdf

Photo Credits:

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