A Parasite That Eats Cattle Alive Is Creeping North Toward the US
On November 22, the United States Department of Agriculture temporarily halted the import of cattle from Mexico after a flesh-eating parasite was detected in animals in southern Mexico. Before the discovery of cattle screwworm (Cochliomyia hominivorax) at an inspection point in the state of Chiapas, the species had previously been eliminated in North America since the end of the 19th century. The US–Mexico border remains closed to cattle and may not reopen until the new year.
The worm is the larva of a metallic blue-green fly that spends the early part of its life cycle devouring the living flesh of mammals. Infestations can be fatal. Cows are the screwworm’s favorite feast, but the maggots can also feed on other livestock as well as wildlife and pets. Flies often lay their eggs near open wounds, and if the larvae can find a hole in the skin to deploy their sharp mouth hooks, they will then bury themselves in the animal’s flesh and gorge.
The finding in Mexico follows the recent reappearance of the parasite in Costa Rica, Nicaragua, Honduras, and Guatemala. In the face of the reemergence of the parasite, Mexico is intensifying sanitary measures—calling for the treatment of wounds in livestock, larvicide baths, and deworming of cattle—and has introduced inspection stations like the one that discovered the case in Chiapas. But conservationists from the Wildlife Conservation Society and Mexican ranchers warn that the illegal cattle trade will be the real gateway for the disease to enter North America.
Prior to the closure of its border with the US, Mexico’s National Confederation of Livestock Organizations had called on the government to clamp down on cattle smuggling across Mexico’s southern border. The risk from the parasite is great, and if it becomes established again, the cost of eliminating it in Mexico would be high. Disruption of trade with the US was also be highly costly. In 2023 alone, Mexico’s exports of live cattle and beef to the US were worth $3 billion.
On the Trail of the Screwworm
For nearly two decades, Cochliomyia hominivorax had been eliminated from the United States down to the Darien Gap in Panama. That was until the summer of 2023, when Panama detected a spike in infestations in animals within 300 kilometers of its northern border with Costa Rica, marking the beginning of the parasite’s reappearance in Central America.
Costa Rica, declared free of the aggressive parasite in 1999, then documented outbreaks in July 2023. Nicaragua and Honduras, free of the screwworm since 1996, confirmed cases in April and September of this year respectively. Then in October 2024, Guatemala reported the reemergence of the fly and its larvae, with a calf as its first fatality. The threat to countries further north is clear. According to the Panama–United States Commission for the Eradication and Prevention of Screwworm, as of November 2, these four countries had accumulated 15,638 screwworm cases in 2024, along with 20,890 documented in Panama.
In reports submitted to the World Organization for Animal Health, three of those countries—Costa Rica, Nicaragua, and Honduras—pointed to the illegal transit of animals as the origin of infections in their territories. Honduras detected an outbreak after inspecting 68 horses that entered the country illegally, for example, just 8 kilometers from its border with Nicaragua.
