The fly that came north
In 18 months a flesh-eating fly crossed Mexico and reached Texas. Winter slowed it down. It did not stop it.
From Chiapas to the Rio Grande
New World screwworm is a fly whose maggots feed on the living flesh of animals, entering through wounds and body openings. Mexico was declared free of it in 1991. On November 21, 2024, it confirmed a new case at Catazajá in Chiapas, near the Guatemalan border.
From there the fly worked its way up the country. By the government bulletin of August 29, 2026, Mexico had counted 40,468 cases in 30 of its 32 states, and by October 15 the national total had reached 48,071. Press play to watch the front move north month by month.
Confirmed cases in each state so far
Cases in Mexico, bulletin by bulletin
Cold slowed it down. It did not stop it
The fly needs warmth. Below an average of about 58°F it stops developing, and cold soil kills its pupae. Nuevo León, the first border state to report a case, logged it in September 2025 at Sabinas Hidalgo. Then the count froze: 3 cases at the end of November, still 3 at the end of February.
In January 2026, 76% of Texas and 81% of Chihuahua were too cold for the fly to breed, according to ERA5-Land weather data. On the Gulf coast, Tamaulipas stayed warm and went from no cases to 38 over the winter. When March warmed the whole border, the front moved again: Coahuila in April, Texas on June 3, New Mexico four days later.
Warm days each month in northern Mexico and the US border states
55 cases in Texas and New Mexico
USDA has confirmed 55 animal cases since June 3: 24 cattle, 16 sheep, 7 goats, 5 dogs and 3 horses. Four are still under treatment. The latest, a horse and a calf, came on October 15 in Grant County, New Mexico. A wild fly was also caught in a surveillance trap in Brewster County on September 25.
Weather explains when the fly spread, not where. From mid-June to mid-September every county in both states had breeding weather, so where cases turned up depended on animals, wounds and movement. Switch the shading to compare the season, the recent weather and the cattle.
Weather for the fly in the two weeks before the date shown
Cattle and calves per county
Months a year warm enough for screwworm flies to breed
Warm nights by the river
Brewster and Presidio counties, on the Big Bend of the Rio Grande, have had 6 animal cases and the trapped fly. NASA's ECOSTRESS thermal camera on the International Space Station measured the ground there at night in 70-meter detail, about 15 clear passes per spot from June through August.
Even on the coldest summer nights, only 6% of the area dropped below the point where the fly stops developing. About a quarter of it runs at least 2°C (3.6°F) warmer at night than the area as a whole, much of it in the river lowlands. Those warm pockets are where the fly has the best chance of holding on through winter.
Night surface temperature, Brewster and Presidio counties
Night surface temperature compared with the area as a whole
Coldest summer nights, June to August: the coldest tenth of clear nights
Where the cattle are
Texas has 12,543,300 cattle and calves, according to the 2022 Census of Agriculture, more than any other state. So far the cases sit in dry ranch country with modest herds. Further east, long-season counties with big herds, such as Gonzales, Wilson and Lavaca, have had none yet. Each bubble is a county; tap one for details.
Where the cattle are, and where screwworm can breed
Counties with cases as a table
| County | Cases | Cattle | Per sq mi | Breeding months |
|---|
Sterile flies and closed gates
The proven weapon is the sterile insect technique: flies bred by the millions, sterilized and released by air, so that wild females mate with sterile males and lay eggs that never hatch. It is how the screwworm was pushed out of the United States and Mexico last century. US and Mexican officials are releasing sterile flies again, and a new sterile fly facility opened in Edinburg, Texas, in February 2026.
Movement is the other lever. In Texas, animals cannot leave an infested zone without permission from the Texas Animal Health Commission, and a zone is released only after at least three months with no new case. Mexican cattle can enter the United States at a single crossing, Douglas, Arizona, which reopened on August 24, 2026.
Check your animals
- Look at every wound, including fresh brands, tags, castration and dehorning sites, and newborn navels.
- Watch the nose, ears and genitals for swelling, drainage or a foul smell.
- Maggots that burrow into living flesh and a wound that keeps growing are the warning signs.
- Keep wounds clean and covered, and ask your veterinarian about preventive treatment.
- Check pets and vehicles for flies and larvae after travel through an infested area.
- Untreated animals can die within a week, so treat and report the same day.
Report a suspected case
Why the season matters
The fly breeds only in warm weather, so the cold months slow it down but do not end the threat. Where winters stay mild, the fly can survive until spring and spread again. The season shading on the Texas map counts the months whose normal average temperature is warm enough for the fly to develop.
This time the front starts closer
Last winter the cold held the front in Nuevo León for four months. This winter starts at the border. In Chihuahua, across from New Mexico and West Texas, cases rose from 213 at the end of August to 612 by October 15, and the state now has more active cases than any other in Mexico: 182 of 2,025.
If this winter is like the last, most of Texas and New Mexico will be too cold for the fly by January. The places to watch are the warm pockets: the Rio Grande lowlands, the Gulf coast, and anywhere a mild winter lets pupae survive until spring.