Department of Agriculture Secretary Brooke Rollins dispatched a sharp letter to Mexican authorities on April 26, threatening to halt some imports of cows, horses, and bison as early as April 30 unless Mexico cooperates in stopping a potential infestation of New World screwworm (NWS). Critics who object to tariffs on Mexico and other countries do not appear to oppose the possible ban. Perhaps this is due to the destructive potential of a screwworm infestation in the United States, which could decimate domestic beef herds.
The Threat of Flystrike
The US cattle herd is reportedly at a 74-year low. Limitations on imports of cows would be a double-edged sword: They could drive up record-high beef prices even further, but inaction could expose the nation’s cattle population to a scourge of flesh-eating New World screwworm maggots that enter mammals (including humans) and can cause serious sickness and even death. Mammalian infection with these larvae is called myiasis, more commonly known as flystrike. The CDC advises, "Larvae under the skin may move, but usually remain under the skin and do not travel throughout the body. If you have myiasis, a healthcare provider will remove the larvae through surgery.”
Human infection with these insects is rare, but the gruesome impacts will elicit sympathy for cows and an understanding of Rollins’ urgency. Other species of flybots can inflict myiasis: As a sheep and cow farmer for 26 years, I have experienced them only once, and we lost several young lambs even after treating them. The condition is rare in humans in the Northern Hemisphere and usually impacts tourists returning from trips abroad, but it is a horrible experience for ranchers and humans alike.
The United States successfully eradicated New World screwworm in 1966 using sterile insect technique (SIT), which introduces sterile male flies into a population of fertile females. This continues to be the method used. The Daily Caller reported:
“The United States has established biological barriers in Central America at the Darien Gap between Panama and Colombia to try and prevent northern spread. As part of the sterile insect technique, the U.S. and Panama produce 20 million sterile screwworms every week which they then release on the Darien Gap, according to APHIS.”
Rollins Forceful on Threat to Cows in US
Rollins alleges that Mexican authorities have not kept up with their side of the fly-sterilizing bargain. Her letter stated, “We do not understand how our official efforts to stop a common pest can be subject to such burdensome customs duties.” Rollins charges that Mexican officials have imposed costly import duties on sterile flies, aviation parts, and dispersal equipment and have limited crucial aviation operations required to bombard female flies with eunuch males hatched in Panama. Mexican intransigence is particularly galling because the United States is supplying these resources for free but is still being taxed.
President Joe Biden opened the southern border up to illegal immigrants, gang members, sex traffickers, and convicted murderers, but even he blocked New World screwworms by halting imports of Mexican livestock in November 2024 after authorities detected an infected cow near the Guatemalan border. The Biden administration released $165 million from the Commodity Credit Corporation to increase efforts to eradicate screwworm spread, including temporarily increasing production at a Panama-based facility to about 95 million sterile flies a week from 20 million over the previous year. The United States resumed imports on Feb. 1, but detection in a cow from Tabasco state later that month prompted heightened concern and the ultimatum by Rollins.
Humans resist flystrike better than many livestock. New World screwworm presents a severe and continuing problem in many livestock industries worldwide. An infestation in US cows could prove catastrophic to farmers and ranchers. The USDA states on its website:
“The USDA continues to take all necessary action to stop the spread of New World Screwworm in southern Mexico to protect America’s livestock industry, food supply, and wildlife populations before it reaches the U.S. border.
“An NWS infestation in the U.S. will result in significant losses to livestock industries. The U.S. successfully led the eradication of NWS in the U.S. and Mexico, but recent detections in Mexico show that this dangerous pest is back and remains a serious threat. Ongoing vigilance and control efforts are critical to prevent its spread northward and protect animal health and agriculture.”
Rollins is exhibiting strong leadership and sharp demands for good reason, warning:
“I must inform you that if these issues are not resolved by Wednesday, April 30, USDA will restrict the importation of animal commodities, which consist of live cattle, bison, and equine originating from or transporting to Mexico to protect the interest of the agriculture industry in the United States.”
It is hoped Mexico will quickly capitulate for the benefit of both nations. If not, beef futures and grocery store hamburgers could be in for another price rise.
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