Global AgriTrends
Back06.02.25
The Basics of New World Screwworm (NWS) in Mexico
Background on New World Screwworm: The worm is hatched from fly larva (maggots) laid by a blowfly. These blow flies are unique in that they lay eggs in LIVING flesh as opposed to DEAD flesh. Any mammal with an open wound is at risk if NWS flies are in the vicinity. And there have been human cases in Central America.
NWS was eradicated from the U.S. in 1966 and from Mexico in 1991. An ambitious program of hatching sterile male flies into affected areas (creating sterile offspring) has held the outbreaks at bay in Central America until 2023 when cases surged higher; no clear answer on why this system has failed. The risk of flies follows the cattle to higher prices as they move North from Central America into Mexico and potentially into the U.S.
Background on U.S. Feeder Cattle Imports from Mexico: The U.S. imports around 1.2 million feeder cattle from Mexico annually. These are brought for grazing and feeding, ultimately contributing to the U.S. annual fed cattle slaughter (25.2 million in 2024). Note the recent Drover’s article on the link between cattle imports and Mexican drug trade here .
Findings of New World Screwworm in Mexico led to a U.S. border closure to Mexican feeder cattle from November 14th 2024 to January 21, 2025. During that time Mexican feeder cattle imports stopped backlogging an estimated 450k or more. This tightened U.S. cattle supplies, supporting higher prices. Another NWS finding in May led USDA to close the border again on May 2, 2025. The border remains closed today.
The Risk: Findings of NWS in U.S. cattle could lead to some temporary export restrictions. This should not happen as the infected cattle pose no risk. The affected wounds would be excluded from the food supply and flies would not transit into boxed beef. But there could be some temporary export restrictions. It is unlikely to expect any serious extended export disruptions should a U.S. case be found. Cattle health is the key concern as findings would necessitate an aggressive anti-fly animal health campaign, but without universal efforts the flies would continue to find vulnerable animals.
Going forward: The U.S. pledged $165 million back in December with an additional $25 million committed last week, to assist in Mexican efforts to stop the flies. In the meantime, the U.S. feeder cattle market tightens, supportive to prices. And Mexican feeders continue to backlog as prices decline on the lost market outlet. One USDA employee reported that the border could remain closed for the remainder of 2025.
