Vigilance key to stopping New World screwworm

Progress is being made in the fight against the New World screwworm, according to Texas A&M’s Phil Kaufman.

The fight will be a battle that requires vigilance, technology, patience, cooperation, and continued awareness by farmers and ranchers. The threat of the New World screwworm led the U.S. Department of Agriculture 14 months ago to close the Southern Border because the pest had traveled northward from southern Mexico. Monitoring the situation remains a constant.

Mexico’s National Service of Agro-Alimentary Health, Safety and Quality, on Dec. 30, 2025, reported a new case in Llera, Tamaulipas, Mexico, which is 197 miles from the U.S.-Mexican border.

The detection was in a six-day-old bovine with an umbilical lesion. It was the first detection Tamaulipas and currently the northernmost active detection.

Livestock producers, especially those located near the border, should check their animals often and keep a close eye on clinical signs consistent with NWS.

No cases have been reported in the U.S., said Kaufman, a professor in the department of entomology at Texas A&M University and Texas A&M AgriLife Extension service entomologist.

When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious injury with the potential to kill an animal.  NWS is known to researchers as Cochliomyia hominivorax.

If an animal dies, Kaufman said, and if the NWS maggots are large enough to survive, the resultant adults can lay eggs on another host, continuing the cycle.

“I am cautiously optimistic still, because there are so many variables that no one knows what’s going to happen?” Kaufman said. “We’re a year out from when it crossed into southern Mexico. It rapidly moved up to where it is now.”

Since then, NWS hasn’t appreciably moved north since that rapid expansion occurred in Chiapas and Yucatan.

“I think that the efforts by USDA and Mexico are paying dividends, because we have not seen a big push of those flies coming up through Mexico,” Kaufman said. “We still have to be incredibly vigilant about this. I’m very pleased that we have kept it as far away from the U.S. as we have, and that these interceptions (in Mexico) have been in animals that once we’ve detected them they’ve been treated. They’re following the protocols and as long as we keep staying with that will be good.”

Resources

Kaufman encourages farmers and ranchers to familiarize themselves with USDA’s Animal and Plant Health Inspection Service at screwworm.gov. The information includes details about how Mexico is handling cases. SENASICA, an acronym for the National Service of Agro-Alimentary Health, Safety and Quality, is Mexico’s version of our USDA-APHIS.

It is encouraging that Mexico has been working very closely following the requirements and that has kept the NWS south of the border, he said. “Our situation has not really changed (in the past six months) other than our readiness is even more because we’ve had more time to prepare.” 

Edinburg, Texas, is ramping up to release sterile flies in about 18 months, and when they hit that benchmark it will mean about 300 million sterile flies a week will be produced in facilities in U.S., Mexico and Panama.

By mid-2027 the U.S. is expected to produce an additional 200 million sterile flies a week to reach the target of 500 million flies per week.

Treatment process

In the meantime, as of Jan. 1, southern ports of entry remain closed to livestock trade as a precaution.

Kaufman said if protocols continue to be followed and new cases are not identified, he has an expectation that the border could be loosened to allow some Mexican cattle into the Southern Plains and Southwest, but specific protocols will be needed.

“The Food and Drug Administration has conditionally approved, under veterinarian care, a couple of products that we did not have available back in August,” Kaufman said.

One is DECTOMAX-CA1 injectable. It has been an available product for beef cattle, but it did not have screwworm the label, so it could not be used on cattle for that purpose, but now it can under veterinarian’s care.

Among the latest news was that the FDA has conditionally approved a topical drug for cattle for the New World screwworm and cattle fever tick. Exzolt Cattle-CA1 (fluralaner) topical solution for the prevention and treatment of New World screwworm larval infestations, and the treatment and control of cattle fever tick in beef cattle 2 months of age and older and replacement dairy heifers less than 20 months of age.

“The producer pours it on the animal’s back then it gets absorbed,” Kaufman said. “It’s been approved not only for the screwworm, but for fever ticks because it is a very good tick control product, but it has a couple of restrictions. It has to be to be administered or authorized by a veterinarian with prescriptions and has a very long withdrawal period.”

To prevent unsafe drug residues in meat from treated cattle and ensure human food safety, the slaughter withdrawal period for Exzolt Cattle-CA1 is 98 days, he said. This product is not for use in lactating dairy cattle, dairy calves, veal calves, or bulls at least 1 year old that are intended for breeding.

“If you are going to send your animals to slaughter, the feedlot has to know you treated them with that product because most animals are not in a feedlot for 90 days,” Kaufman said.
“If you treat your animals today to ship it to a feedlot the feedlot operator must know that you treated them with Exzolt so they don’t send the animal to the slaughter plant until after those 98 days have expired.

The single-use, ready-to-use product is applied directly to the hair and skin in a narrow strip extending along the top of a bovine’s back from between the shoulder blades to the base of the tail (withers to tailhead along the dorsal midline).

Kaufman said when it comes to any preventative or treatment drugs, producers need to read the labels and follow them closely, so the industry does not lose access to those products.

Vigilance

Although the focus is rightfully on the border, Kaufman says all livestock producers in the U.S. need to watch for it. If the New World screwworm enters the U.S., then it can also move. Fifty years ago, it meant putting animal movement restrictions in place to help eradicate it, he said.

There are also concerns about wildlife being able to carry NWS infestations, and he said the Texas Parks and Wildlife Department is taking it seriously, but cases in wildlife are harder to track.

“We really need to work to make sure we keep the fly out of the U.S. as long as we can so that we can knock it out if it ever shows up here,” he said. “And I will say that if we do have an infestation here, and if people are willing to work collectively, there’s no reason we can’t eliminate it.”

He said if cases occur in a local area it does not mean New World screwworm will spread across the state or multiple states.

“If people are going to look out for the greater good and report that they have it, we have the capacity to keep this fly at minimal numbers while we work to eradicate it locally,” Kaufman said.

Dave Bergmeier can be reached at 620-227-1822 or [email protected].