Mexico’s New World Screwworm outbreak fast approaching border, threatening U.S. livestock

One of the contributing factors to beef cattle shortages in the United States is that live cattle imports from Mexico have been shut down due to the discovery of New World Screwworm cases in some Mexican cattle. Mexico has had a reportedly 53% increase in cases of the dangerous pest since July.

The news got worse this week as the USDA announced Mexico’s National Service of Agro-Alimentary Health, Safety and Quality had confirmed a new case in the state of Nuevo Leon, a mere 70 miles from the U.S. border. This is now the northernmost discovery of the pest and is located near a major highway to Laredo, Texas—one of the most heavily traveled commercial highways in the world.

U.S. Agriculture Secretary Brooke Rollins said that protecting the United States from New World Screwworm is a top priority and she considers it a national security priority.

“We have given Mexico every opportunity and every resource necessary to counter NWS. Nevertheless, American ranchers should know that we will not rely on Mexico to defend our industry, our food supply or our way of life,” she said.

The affected animal, an 8-month-old cow, had recently been moved to a feedlot in Nuevo Leon from a region in southern Mexico known to have active cases of NWS.

Even before this discovery, U.S. ports have been closed to imports of cattle, bison and horses from Mexico—all of which can be affected by screwworm.

Cattle groups are concerned over another Reuters report noting that cattle smuggling has resulted in as many as 800,000 illegal cattle being brought into Mexico each year from Guatemala, Honduras, Nicaragua and perhaps other countries.

Apparently, organized crime doesn’t just stick to drugs and human trafficking but has now added illegal cattle smuggling too, according to the report. The Reuters story alleges that these illegal cattle enter Mexico with falsified papers and bogus ear tags. Mexican veterinary and animal health experts believe the cattle smuggling may have been a factor in bringing New World screwworm into Mexico.

The pest has been eradicated in the United States since the 1970s and has been controlled by a robust national program that agriculture officials believe has saved the U.S. cattle industry $2.3 billion per year. But with the smuggling of cattle from Central America and the pest’s re-emergence in Mexico, there are concerns for the U.S. cattle industry, for wildlife and even for humans that can be affected by the flesh-eating screwworm. The pest had been held at bay until 2023 by a “biological barrier” and other measures.

U.S.-Mexico border must remain closed until NWS is eradicated in Mexico

Through late August, USDA information showed that there have now been 5,542 screwworm cases in Mexico with 777 of them considered “active.” Those numbers tell Bill Bullard, CEO of the cattle group R-CALF, that the outbreak is still escalating at a rapid pace in Mexico. Archival reports from the USDA show that screwworm flies can travel distances of up to 180 miles.

“This means that none of our southern states can be free of screwworms until the pests are eradicated in Mexico,” Bullard said. “New and historical evidence support maintaining every possible defense, including keeping the border closed until that is accomplished.”

Since discovering the pest’s re-emergence earlier this year, the United States and Mexico have begun to invest in increased biological countermeasures, surveillance and detection measures. The USDA has said that it may take several years to eradicate the current outbreak of the pest.

The screwworm has been detected among cattle in Mexico and Central America since 2022.

International animal health professionals believe that one of their best options to control the pest is to ramp up the production of sterile flies to be released into the endemic areas. There is also discussion of using gene editing as part of the solution.

According to the New York Times, the USDA has dedicated more than $270 million in the past two years to increase weekly production of these sterile flies in Panama where production went from 20 million to 100 million flies per week. They also invested in establishing additional animal checkpoints and increasing awareness in Central America.

In August, a coalition of nearly 200 groups representing cattle ranchers, dairy producers, hog and sheep farmers, horse breeders and wildlife managers appealed to the USDA to fund the construction of a domestic facility within the United States to produce the sterile flies.

The agency is using 9,000 sticky fly traps and hundreds of additional screwworm traps along the Texas-Mexico border and will spend more on efforts to monitor the movement of livestock.

Rollins said that USDA is analyzing all new information related to the recent case in Nuevo Leon and will “pursue all options” to release sterile flies in this region as necessary. The screwworm’s maggots can infest livestock and other warm-blooded animals, including people. They most often enter an animal through an open wound and feed on that animal’s living flesh.

The agency is urging residents on the southern border to check their pets and livestock for signs of NWS and look for maggots and eggs in or around body openings–nose, ears, genitalia and navels of newborn animals.

Mexico recently confirmed the discovery of its first case of screwworm in a human.