On November 22, The US Department of Agriculture temporarily halted the import of livestock from Mexico after a carnivorous parasite was detected in animals in the south of Mexico. Before the discovery of the 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 livestock and may not reopen until the new year.
The maggot 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 favorite feast of snails, but worms it can also feed on other livestock as well as fauna and companion animals. 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 throat.
The finding in Mexico follows the recent reappearance of the parasite in Costa Rica, Nicaragua, Honduras and Guatemala. Faced with the reappearance of the parasite, Mexico is stepping up health measures—requiring treatment of cattle wounds, larvicidal baths and deworming of cattle—and has introduced inspection stations like the one that discovered the case in Chiapas. But conservationists at the Wildlife Conservation Society and Mexican ranchers warn that the illegal cattle trade will be the disease’s real gateway to North America.
Before the closure of its border with the United States, the National Confederation of Livestock Organizations of Mexico had called on the government to curb cattle trafficking across Mexico’s southern border. The risk of the parasite is high, and if it re-establishes itself, the cost of eliminating it in Mexico would be high. The disruption of trade with the US was also very costly. In 2023 alone, live cattle and beef exports from Mexico to the US were worth $3 billion.
On the track of the Snail Worm
For nearly two decades, Cochliomyia hominivorax it had been removed from the United States as far as the Darien Gap in Panama. That was until the summer of 2023, when Panama detected an increase in infestations in animals 300 kilometers from its northern border with Costa Rica, which marked the beginning of the reappearance of the parasite in Central America.
Costa Rica, declared free of the aggressive parasite in 1999, documented outbreaks in July 2023. Nicaragua and Honduras, free of the snail worm since 1996, confirmed cases in April and September of this year respectively. Then, in October 2024, Guatemala reported the reappearance of the fly and its larvae, with a calf as the first death. The threat to the northernmost countries is clear. According to the Panama-United States Commission for the Eradication and Prevention of Snailworm, as of November 2, these four countries had accumulated 15,638 cases of snailworm in 2024, along with 20,890 documented in Panama.
In reports submitted to the World Organization for Animal Health, three of these countries—Costa Rica, Nicaragua and Honduras—pointed to the illegal traffic 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.
