The US has suspended cattle imports from Mexico following evidence of the northward spread of the new world screwworm (NWS).

The latest case of NWS was identified in Veracruz, around 370 miles south of the US-Mexico border.

The US had previously closed its borders to the import of cattle, bison and horses from Mexico due to fears around NWS but was in the process of a phased relaxation of these restrictions.

The resumption of livestock imports from Mexico has now been halted, the US Department of Agriculture (USDA) stated.

“While USDA announced a risk-based phased port re-opening strategy for cattle, bison, and equine from Mexico beginning as early as 7 July, 2025, this newly reported NWS case raises significant concern about the previously reported information shared by Mexican officials and severely compromises the outlined port reopening schedule of five ports from 7 July to 15 September,” the USDA stated.

The US secretary of agriculture, Brooke Rollins, said she had to take “quick and decisive action” to respond to the spread of what she described as “this deadly pest”.

“The United States has promised to be vigilant — and after detecting this new NWS case, we are pausing the planned port reopenings to further quarantine and target this deadly pest in Mexico. We must see additional progress combatting NWS in Veracruz and other nearby Mexican states in order to reopen livestock ports along the Southern border,” said Rollins.

NWS is a species of parasitic fly that is well known for the way in which its larvae – which are like maggots - eat living tissue.

Indeed, while the maggots of many fly species eat dead flesh, and may occasionally infest an old and putrid wound, screwworm maggots are unusual because they attack healthy tissue.

NWS is found in the tropics of Central America, South America and the Caribbean.

Mexico is a major supplier of live cattle to the US. Close to one million Mexican cattle were exported to the US in 2024.