New World Screwworm enters the U.S.

NWS infestations were discovered just days apart on calves that grazed
roughly 5.6 miles from each another. (Photo via Hoosier Ag News) 

Despite ongoing efforts from the U.S. Department of Agriculture, the Mexican government and on-the-ground teams along the U.S.-Mexico border, last week the USDA's Animal and Plant Health Inspection Service (APHIS) confirmed two cases of New World screwworm in Texas livestock, reports Jennifer Carrico of Progressive Farmer. In both cases, the flesh-eating blowfly larvae were discovered in young calves about 6 miles apart in Zavala County, Texas.

After the second case was confirmed, Texas Governor Greg Abbott "issued an updated statewide disaster declaration for the NWS infestation," Carrico writes. At a news conference, Abbott encouraged the use of "all state resources to combat the NWS." Additionally, the executive director of the Texas Animal Health Commission, Budd Dinges, reminded the public that NWS isn't a threat to consumer food safety but rather poses a threat to livestock production. 

In response to the infestations, APHIS has "deployed mobile response trailers, and sterile fly releases are underway with 2 million aerially and 4 million released on the ground per week," Farm Journal reports. "Movement control zones have been activated at a 20-kilometer (12.4-mile) radius around each site."

Both NWS cases were found in Zavala, Texas, 
in red above. (Wikipedia map)

The discovery prompted the Canadian Food Inspection Agency to place "temporary restrictions on livestock imports from Texas," reports C.J. Miller of Hoosier Ag Today. "Under the order, cattle, horses and other livestock that originated in Texas or were present in the state within 21 days before entering Canada will be denied entry."

Although the USDA intensified its battle to prevent the NWS from reentering the U.S. including banning Mexican livestock from entering the U.S. beginning in July 2025, some ranchers don't think the government's response was enough.

"On Friday, about 100 ranchers in mud-splattered boots and cowboy hats packed a small high school cafeteria for a Texas Animal Health Commission briefing on screwworm, peppering officials with questions and venting frustration over what they saw ​as a slow federal response,” reports Heather Schlitz of Reuters. 

Even as the USDA works to contain NWS infestations, they are likely to spread throughout the summer months, Miller reports. Prior to last week, the NWS hasn't been found in the U.S. for at least 60 years.

New World screwworm detected roughly 70 miles from the Texas border as USDA efforts intensify

New World screwworm was found roughly 70 
miles from Laredo, Texas. (USDA photo)

 

The New World screwworm and its flesh-eating larvae have made their way through Mexico to hover fewer than 70 miles from the U.S.-Mexico border. "The case was reported near Sabinas Hidalgo in Nuevo Leon," reports Joshua Baethge of Farm Progress. "That city is close to a major highway connecting Monterrey, Mexico’s second-largest metro area, and Laredo, Texas."

Screwworm larvae were discovered in "an 8-month-old cow that was moved to the area from southern Mexico, which has active NWS cases," Baethge writes. Concerned lawmakers and livestock owners would like Agriculture Secretary Brooke Rollins to persuade the Mexican government to restrict cattle movement to contain the pest.

While NWS larvae will eat the flesh of any warm-blooded animal, female screwworm flies most often lay their eggs in the open wounds or scratches of sheep or cattle. The larvae are considered a threat to U.S. livestock and rancher economies as well as the U.S. food supply.

Rollins said "protecting the U.S. from New World screwworm is a 'national security priority," Baethge reports. Rollins added that "U.S. farmers and ranchers should know that the U.S. 'will not rely on Mexico' to defend the cattle industry, the food industry or 'our way of life.'"

State of Nuevo León within Mexico
(Wikipedia map)

The USDA and Mexican authorities have been working together to push the NWS away from both countries' livestock herds through sterile fly releases, animal surveillance and fly trapping and testing work. 

Since July, the USDA and Mexico "have been monitoring almost 8,000 traps across Texas, Arizona, and New Mexico," reports Jennifer Carrico of Progressive Farmer. "Currently, all the 13,000-plus screening samples have detected no NWS flies."

While the latest detection in Nuevo Leon, Mexico, appears to be limited to one cow, the USDA is still investigating. "Rollins said the U.S will be 'taking no chances,'" Baethge reports.