On November 22, The United States Department of Agriculture temporarily halted the importation of livestock from Mexico after a carnivorous parasite was detected in the animals. in southern Mexico. Before the discovery of the cattle screwworm (Cochliomyia hominivorax) at an inspection point in the state of Chiapas, the species had been eliminated in North America since the late 19th century. The US-Mexico border remains closed to livestock and may not reopen until the new year.
The worm is the larva of a metallic blue-green fly that spends the first part of its life cycle devouring the living flesh of mammals. Infestations can be fatal. Cows are the screwworm’s favorite feast, but worms It can also feed on other animals. 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 burrow into the animal’s flesh and become stuck.
The discovery in Mexico follows the recent reappearance of the parasite in Costa Rica, Nicaragua, Honduras and Guatemala. Faced with the resurgence of the parasite, Mexico is intensifying sanitary measures – including treatment of wounds in livestock, baths with larvicides and deworming of livestock – 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 North America.
Before closing its border with the United States, Mexico’s National Confederation of Livestock Organizations had called on the government to clamp down on livestock smuggling across Mexico’s southern border. The risk of the parasite is great, and if it re-establishes itself, the cost of eliminating it in Mexico would be high. The interruption of trade with the United States was also very costly. In 2023 alone, Mexico’s exports of live cattle and beef to the United States were worth $3 billion.
On the trail of the screwworm
For almost two decades, Cochliomyia hominivorax They had been eliminated from the United States to the Darien Gap in Panama. That was until the summer of 2023, when Panama detected an increase in animal infestations within a 300-kilometer radius 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, 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 resurgence of the fly and its larvae, with a hatchling being its first fatality. The threat to countries further north is clear. According to the Panama-United States Commission for Screwworm Eradication and Prevention, as of November 2, these four countries had accumulated 15,638 cases of screwworm in 2024, along with 20,890 documented in Panama.
In reports presented to the World Organization for Animal Health, three of these countries – Costa Rica, Nicaragua and Honduras – pointed out the illegal transit of animals as the source 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.