The latest on New world screwworm

On August 26, 2025, The HHS and the USDA confirmed the first human case of travel-associated New World screwworm (NWS). The case was first identified by the CDC on August 4, 2024, when the infected person from Maryland returned to the US after traveling to El Salvador. Currently this case is being investigated by the CDC and the Maryland Department of Health.

“As this is a human case, CDC is the lead response agency and is conducting an epidemiological assessment in coordination with local health authorities. Currently, the risk to public health in the United States from this case is very low. In support of CDC’s activities and out of an abundance of caution, USDA initiated targeted surveillance for NWS within a 20-mile radius of the affected area, encompassing portions of the District of Columbia, Maryland, and Virginia,” the USDA stated in its release.

“To date, all trap results have been negativing for NWS. There have been no detections of NWS in the U.S. in livestock or other animals since the last outbreak of NWS in the Florida Keys was resolved in 2017,” it continued.1

The USDA also shared in its announcement that there have been previous cases of travel-associated cases in the US in past years, but all were isolated and designated as closed after precautionary targeted surveillance in the vicinity was negative. The USDA stated that there may be more cases of traveler-associated NWS but it will work with the HHS and CDC to conduct targeted surveillance to help make sure there is no active spread of it in the US. At this time, there is no cause for alarm because human risk remains low, and the several isolated cases the organizations have seen in recent years has not resulted in livestock transmission.

Eradication efforts

In April 2025, Brooke Rollins, US Secretary of Agriculture, announced that Mexico committed to eliminate restrictions on USDA aircraft as well as waive customs duties on eradication equipment to help the response to the spread of NWS. This agreement meant the ports will remain open to livestock imports, but if the terms are not upheld, port closures could be revisited.

In a letter Rollins addressed to Mexico Secretary of Agriculture Julio Antonio Berdegué Sacristan letter outlined the negative impact on efforts to curtail the spread of NWS from restrictions for aircraft operated by the USDA are “uniquely equipped to conduct the high-volume, precision aerial releases required to suppress and eliminate the NWS population” as well as “substantial import duties on critical aviation parts, dispersal equipment, and sterile fly shipments.2

As the detection of NWS continued to increase in Mexico, the USDA issues a 5-part eradication plan, which included a $8.5 million sterile fly dispersal facility. The facility will be built at the Moore Air Base in Mission, Texas due to its proximity to the boarder, and is expected to be completed by the end of this year.3

Buck Wehrbein, president of the National Cattlemen’s Beef Association, stated that having a sufficient supply of sterile flies is the only way the US can push NWS away from the border, meaning this facility is crucial to eradication efforts.4 In the 1960s, Moore Air Base was part of the eradication efforts, and will now be the cornerstone of the fight against NWS.

Besides the Moore Air Base, The USDA is reviewing other options for another facility in the US that can generate 300 million sterile flies a week, with a possibility of the agency building a modular sterile fly production facility along with different technologies to help provide faster construction and surge capacities in the future.

Besides inside the US, the USDA is investing $21 million to assist in the renovation of a sterile fly production facility located in Metapa, Mexico, is expected to be completed in in a little over a year from now. The facility is estimated to generate 60-100 million sterile flies weekly.

On a treatment level, HHS announced that the FDA can issue emergency use authorization for animal drugs to prevent or treat infestations caused by NWS. This authorization at this time only applies to animal drugs.5

“We have been seeing [NWS] moving up from South America, through every single country in Central America, and has touched down into Mexico,” Christopher Lee, DVM, MPH, DACVPM, host of dvm360’s Vet Watch, said in a recent clip from the show.6 “Currently, we are looking at the numbers nearly doubling every couple of weeks.”

NWS presentation

For veterinary professionals, NWSare similar in size to common house flies, or could be slightly larger according to the American Veterinary Medical Association.7 The flies will also have orange eyes, metallic blue or green bodies, and 3 dark stripes along their back. If a patient is infected with NWS, they will present with:

• Larvae visible in wounds or orifices
• Egg masses in or around wounds
• Wounds with bloody or purulent discharge and an odor of decay
• Wounds that enlarge or deepen over time
• Behavioral signs of irritation
• Signs of pain like depression, irritability, inappetence, or isolating from other animals or people

After 3 days of the initial infestation, hundreds or thousands of larvae could be present. The larvae are normally positioned with their heads down, with their posterior ends will be visible at the surface of the wound. For wounds that are deep or more narrow, the only visible sign they are there may be subtle movement.

dvm360’s Vet Watch continues to cover NWS and its updates, which you can find on dvm360.com or wherever you listen to podcasts. dvm360 will also continue to monitor progress or advancements made for the eradication of NWS, or if more cases continue to pop up in the US.

References

1. HHS and USDA Confirm Singular Traveler-Associated New World Screwworm Case; Precautionary and Proactive Surveillance Ongoing. News release. US Department of Agriculture. August 26, 2025. Accessed September 3, 2025.
2. Coppock Crossley K. US-Mexico agreement allows screwworm eradication efforts to resume. D360. Published May 2025. Accessed September 3, 2025. https://www.dvm360.com/view/us-mexico-agreement-allows-screwworm-eradication-efforts-to-resume
3. Bautista-Alejandre A. Clinic center: USDA announces sterile fly facility at Texas base for screwworm eradication, and more. dvm360. Published July 9, 2025. Accessed September 3, 2025. https://www.dvm360.com/view/clinic-center-usda-announces-sterile-fly-facility-at-texas-base-for-screwworm-eradication-and-more
4. USDA unveils Texas screwworm facility, eradication strategy amid reopening of southern ports. American Veterinary Medical Association. July 2, 2025. Accessed July 9, 2025. https://www.avma.org/news/usda-unveils-texas-screwworm-facility-eradication-strategy-amid-reopening-southern-ports?utm_source=delivra&utm_medium=email&utm_campaign=todays-headlines-news
5. Bautista-Alejandre A. US Department of Health and Human Services authorizes emergency use of animal drugs for New World screwworm. dvm360. Published August 19, 2025. Accessed September 3, 2025. https://www.dvm360.com/view/us-department-of-health-and-human-services-authorizes-emergency-use-of-animal-drugs-for-new-world-screwworm
6. Lee C, Alaburda R. Vet Watch update: New World screwworm. dvm360. May 16, 2025. Accessed August 19, 2025. https://www.dvm360.com/view/vet-watch-update-new-world-screwworm
7. American Veterinary Medical Association. New World screwworm (NWS). American Veterinary Medical Association. Published 2025. Accessed September 3, 2025. https://www.avma.org/resources-tools/one-health/veterinarians-and-public-health/new-world-screwworm