06/16/2026
How far could they go?
USDA officially confirmed the first domestic case of New World screwworm in six decades, detected in a three-week-old calf in Zavala County, Texas (about 50 miles from the Mexican border).
The question of "how far north could they go" is no longer a hypothetical simulation, it is now an active biosecurity emergency. Leading veterinary parasitologists are warning that this single case likely signals the beginning of reestablishment, meaning hundreds or thousands of flies may already be across the border.
The threat is divided into two distinct biological zones…where the fly can live *permanently*, and how far north it can march during the warm months.
The “Summer Dispersal Zone” - NWS can reach Kansas, Missouri, Tennessee, and further north via wind/livestock. And the “Overwintering Zone” where NWS could become permanently established in South/Central Texas & Gulf Coast areas due to mild winters.
Winter is the ultimate limiting factor because screwworm pupae can’t survive hard, prolonged soil freezes. However, the winters of the 2020s are significantly milder than those of the 1950s when eradication began.
The new reality is that the permanent, year-round survival zone is no longer confined safely to Southern Mexico. South Texas, the Lower Rio Grande Valley, and the Gulf Coast could now be treated as year-round establishment zones.
Many entomologists believe that warmer winter trends will allow the permanent NWS overwintering line to push into Central Texas and across the deep Southeast (Louisiana, Mississippi, Alabama, Georgia, Florida).
*Bonilla, D. (n.d.). Cooperative Screwworm Eradication Program: Environmental Assessment. United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS).https://www.aphis.usda.gov/sites/default/files/screwworm-environmental-assessment.pdf
*Gutierrez, A. P., Ponti, L., & Arias, P. A. (2019). Deconstructing the eradication of new world screwworm in North America: Retrospective analysis and climate warming effects. Medical and Veterinary Entomology, 33(2), 282–295. https://doi.org/10.1111/mve.12362
*Timbie, S. (2026). Annotated bibliography of scientific research on new world screwworm (Cochliomyia hominivorax) myiasis in wildlife. U.S. Geological Survey Open-File Report 2026-1006. USGS Publications Warehouse. https://pubs.usgs.gov/publication/ofr20261006/full
Even if freezing temperatures wipe out northern populations every winter, the flies can travel massive distances during the spring and summer. A single fly can travel 10 to 30 miles in its lifespan. And the case that was discovered in Zavala County was 50 miles from the border, which leads one to believe there is already a population inside the US border. And of course, the primary driver of rapid expansion is the movement of infested livestock, vehicles, or wildlife.
Before eradication, summer outbreaks regularly pushed as far north as Kansas, Missouri, and Tennessee. If not aggressively contained in Texas, this summer range remains entirely possible today.
With live cases confirmed in South Texas, the immediate risk zones across the United States have grown.
CRITICAL / ACTIVE Regions include South Texas, Lower Rio Grande Valley. High humidity and thick brush provide ideal habitat. The parasite can easily establish a year-round lifecycle here if not eradicated quickly.
HIGH RISK regions include Central/East Texas, Gulf Coast, Coastal Louisiana, Southern Florida. These areas are at high vulnerability for permanent establishment. Mild modern winters mean soil temperatures rarely drop low enough for long enough periods of time to kill burrowed pupae.
MODERATE RISK regions include Northern Texas (Dallas/Panhandle), Oklahoma, Arkansas. These areas have a high vulnerability for summer infestation. While winter freezes will reliably clear out populations annually, unchecked spring/summer migrations could trigger devastating seasonal outbreaks.
From a wildlife management standpoint, an established population in South Texas is a nightmare for white-tailed deer. Biologists are deeply concerned about specific pressure points such as “Fawn Recruitment Collapse”. The Texas case was found in the navel of a 3-week-old calf. Newborn fawns are incredibly vulnerable because female flies target the raw umbilical stump. Historically, screwworm infestations caused 25% to 80% fawn mortality in heavy outbreak years.
Parasites like the Gulf Coast tick create bleeding bite wounds on deer hides and ears. These tiny lesions are exactly what female screwworms look for to deposit their 200–300 eggs.
In areas with dense deer populations or high feral hog numbers, the parasite has a virtually endless supply of warm-blooded hosts, allowing numbers to scale exponentially before experts can intervene.
The silver lining is that modern agricultural and wildlife authorities possess a tool that managers in the 1930s did not known as “Sterile Insect Technique (SIT)”.
The USDA and the Texas Animal Health Commission have already established a containment zone around Zavala County and are deploying millions of sterile male flies to help crash the wild fly population.
SIT is highly effective, but it relies entirely on containment. If the parasite goes undetected in wild deer or feral hog populations outside of the current quarantine zone, it can quickly expand. The primary concern right now is not whether a fly can survive a winter in Oklahoma, it is whether we can stop the current Texas situation from turning into a multi-state emergency.
