Cattle and bison imports from Mexico will resume following new screwworm inspection protocol

Cattle and bison imports from Mexico are scheduled to resume soon, according to a recent announcement by the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) after shipments of Mexican cattle and bison have been under pause since November 2024.¹ The lift comes after APHIS and Mexico implemented an extensive protocol for the pre-clearance inspection and treatment of ruminants following a New World screwworm (NWS), Cochliomyia hominivorax, detection in a cow in southern Mexico last fall.^1,2

New World screwworm larva. (Photo courtesy of United States Department of Agriculture, John Kucharski, and American Veterinary Medical Association)

The NWS fly is a parasitic fly species that lays eggs on living mammals’ open wounds. These eggs hatch into larvae that burrow into the host’s flesh, leading to severe damage, infections, and often death.^2,3

On November 22, 2024, the discovery of NWS in Chiapas, Mexico prompted an emergency response from the US to prevent the parasitic species—which was eradicated from the US in 1966—from entering the country. As part of the response, the USDA APHIS announced on December 2024, $165 million in emergency funding to safeguard US livestock, pets, and wildlife.³

“The question of whether it will arrive in the United States is not "if," but "when." If that happens, we will see myiasis cases on a regular basis, said Christopher Lee, DVM, MPH, DACVPM, in an interview with dvm360 on the growing concern of NWS in the US.⁴

“What makes the New World screwworm so concerning is that it causes primary myiasis,” he continued.⁴ “To clarify, myiasis occurs when a human or animal has live maggots feeding on them. Right now, in the US, we only see secondary myiasis, which means maggots are feeding on dead or decaying tissue.”

RELATED: USDA issues new import restrictions for certain animals from Honduras

Per the protocol for the importation of ruminants from Mexico to the US, cattle and bison will be inspected and treated for screwworm by trained and authorized veterinarians prior to entering the pre-export inspection pens, where they will be reinspected by Mexican officials. These pre-export inspection pens in San Jeronimo and Agua Prieta, Sonora; as well as Chihuaha, have been identified and prepared by Mexico and been visited, inspected and approved by APHIS.¹ Additionally, cattle and bison who have been approved for importation will be dipped in a solution to “ensure they are otherwise insect- and tick -free,” according to the USDA.¹

According to the announcement, the US and Mexico are closely collaborating to approve more pre-export inspection pens and resume trade through other ports of entry. “To support our efforts to keep NWS out of the United States, APHIS will continue working with partners in Mexico and Central America to eradicate NWS from the affected areas and to reestablish the biological barrier in Panama, which we have worked to maintain since 2006,” wrote the USDA in its announcement.¹

NWS continues to exist in Central and South American countries, with the parasite spreading north of the barrier in the last 2 years throughout Panama and into Costa Rica, Nicaragua, Honduras, Guatemala, El Salvador, Belize, and more recently, Mexico. The rise in NWS is being driven by new areas of farming in previous barrier regions for fly control, as well as an increase of cattle movement into the region, according to the USDA. To combat the growing issue, APHIS is releasing sterile flies through aerial and ground release at strategic locations, concentrating efforts in Southern Mexico and other regions across Central America.¹

“If screwworms establish themselves in the US again, we will be facing a severe and urgent problem in veterinary and public health,” said Lee.⁴

References

1. Cattle and bison imports from Mexico resume under new protocol. News release. United States Department of Agriculture. February 1, 2025. Accessed February 12, 2025. https://www.usda.gov/about-usda/news/press-releases/2025/02/01/cattle-and-bison-imports-mexico-resume-under-new-protocol
2. With screwworm protocols in place, US resuming livestock, equine imports from Mexico. American Veterinary Medical Association. February 11, 2025. Accessed February 12, 2025. https://www.avma.org/news/screwworm-protocols-place-us-resuming-livestock-equine-imports-mexico?utm_source=delivra&utm_medium=email&utm_campaign=todays-headlines-news
3. McCafferty C. Screwworm case in Mexico prompts emergency response in the United States. dvm360. January 16, 2025. Accessed February 12, 2025. https://www.dvm360.com/view/screwworm-case-in-mexico-prompts-emergency-response-in-the-united-states
4. Lee C. Screwworms are moving north—are we prepared? dvm360. February 4, 2025. Accessed February 12, 2025. https://www.dvm360.com/view/screwworms-are-moving-north-are-we-prepared-