Screwworm Cases Surge Past 40 in November as Mild Weather Aids Parasite

By Daniel Okafor

More than 40 screwworm cases were confirmed in the US in the first stretch of November, making it the worst month since the parasite arrived in Texas in June, according to the US Department of Agriculture. Mild temperatures and humidity are driving the surge. Nearly 130 cases have been recorded nationwide, and more than 60 are currently active.

The acceleration started in October, when the pest spread to Arizona for the first time. That jump reversed a quieter summer that had surprised entomologists.

Why did the summer outbreak stall?

When the New World screwworm reached the US in June, experts braced for heavy losses. "A lot of us were like, 'Okay, yep, we're in for a rough summer,'" said Caleb Hubbard, an assistant professor of urban entomology at New Mexico State University. The feared wave never came.

The explanation was weather, not luck with the parasite itself. Screwworm flies like warmth, but not the extreme heat above 100F that Texas and New Mexico endured in July and August. Above that threshold, the flies move less and reproduce less. Below 59F, they turn sluggish. The Southwest's summer sat in the wrong zone for the pest.

"It was something that I think we all were like, 'Whew, we got through it, we're all good,'" Hubbard said of the hottest days. "But as temperatures have started to decline, at least in the Southwest, we're kind of in this Goldilocks zone right now."

Texas A&M University puts the parasite's ideal life cycle at two to three weeks, in conditions above 80F, below 100F, with high humidity. Screwworms mate only once, but in that temperature band they move more and the population multiplies faster.

Cooler air has now arrived with more rain. "We finally have started getting some cooler weather — not cold, but cooler weather here, and there has been an uptick in rain events, particularly over Arizona, New Mexico, and West Texas," said Phillip Kaufman, an entomology professor at Texas A&M — exactly where the fly has been detected.

The spread into Arizona is "part of a natural progression" as the pest pushes west from Texas and north from Mexico, said Mykaela Martin, public information officer for the Arizona Department of Agriculture, in an email.

How does this compare with 1972?

The outbreak remains small next to history. The worst US outbreak, in 1972, racked up roughly 95,000 cases, stretching across all of Texas, into Oklahoma and as far west as California.

The USDA contained this year's outbreak through the summer with three tools:

• Livestock movement controls
• A monitoring and surveillance program
• Releases of sterile flies to curb reproduction

The agency said in an emailed statement that current weather patterns are "certainly conditions that could cause an increase in fly survivability, though it is difficult to determine a sole reason" for the jump. Animal movement is another factor. The fly travels poorly on its own and depends on livestock or wildlife to spread.

Why are ranchers worried about Mexican cattle?

Mexico currently has about 1,500 active screwworm cases. The US halted cattle imports from Mexico to stop the spread, then resumed them in late August. More than 65,000 cattle have crossed the border since ports reopened, according to the USDA.

Some ranchers argue the agency acted prematurely. The USDA says rigorous inspection protocols make it unlikely that infected animals will enter the country, and it plans to keep the ports open despite rising case counts.

The main eradication weapon is still months away. A Texas facility that would sharply expand sterile fly production is not slated to open until next spring. The USDA said in a posting on X that it has shifted sterile fly releases to "the areas of greatest need" in Arizona and New Mexico, while continuing some releases in Texas.

Can winter stop the fly?

That leaves the US dependent on a cold, dry winter. The Southwest is unlikely to stay cold long enough, and dry weather would hurt ranchers who need rain for pastures.

A fast-building "super" El Niño will likely break the drought across much of Texas and parts of the Southwest. The pattern typically drenches the southern third of the country through winter, and wetter soil helps flies grow. El Niño usually cools the southern US, but long-term climate change has made the region notably hotter over three decades, said John Nielsen-Gammon, who directs the Southern Regional Climate Center and serves as Texas state climatologist.

In some counties fighting screwworm, the coldest winter nights have recently held in the mid-50s. "When Arctic air makes it down this far, it's not as cold," Nielsen-Gammon said. "The weather tends to be more cloudy and dreary rather than having sharp cold during an El Niño."

The practical stakes for the cattle industry now rest on whether USDA surveillance, sterile fly drops and border inspections can hold the line until the new Texas facility opens next spring.