On November 22, the United States Department of Agriculture (USDA) enacted a temporary ban on cattle imports from Mexico due to the detection of the flesh-eating parasite, the cattle screwworm (Cochliomyia hominivorax), in animals in the state of Chiapas. This larvae, which comes from a metallic blue-green fly, had been completely eradicated in North America since the late 19th century. The discovery of the parasite at an inspection point has raised significant concerns regarding its potential spread, resulting in the closure of the US-Mexico border to cattle, which may remain in effect until the new year. The screwworm’s larvae inflict severe damage by feeding on the living flesh of various mammals, particularly cattle, which can lead to fatal infestations if not addressed.
The alarming findings in Mexico are part of a broader pattern of the parasite’s resurgence in Central America, with recent outbreaks reported in Costa Rica, Nicaragua, Honduras, and Guatemala. In response, Mexican authorities have ramped up sanitary measures to combat further spread, which include treating livestock wounds, administering larvicide baths, and deworming cattle. Additionally, new inspection stations have been established to monitor and control the situation. However, there are considerable concerns that illegal cattle trading may provide a significant avenue for the reintroduction of the disease into North America, as conservationists and ranchers alike recognize smuggling as a potential threat to the eradication efforts.
Before the closure of the border, Mexico’s National Confederation of Livestock Organizations had advocated for stringent measures to prevent cattle smuggling across its southern borders to mitigate the risks associated with the parasite. The implications of Cochliomyia hominivorax becoming established in North America are dire. If this occurs, the financial burden associated with its eradication in Mexico could be staggering, not to mention the potential economic fallout from disrupted trade routes with the United States, which were valued at around $3 billion in live cattle and beef exports alone in 2023.
The cattle screwworm, often referred to by the name of its larvae, was previously under control in the United States and parts of Central America, demonstrating a successful eradication effort that lasted for nearly two decades. However, the summer of 2023 marked a troubling shift, as Panama reported increasing infestation rates within 300 kilometers of its border with Costa Rica. This uptick signaled the beginning of the parasite’s return to Central American countries, with Costa Rica, Nicaragua, and Honduras all confirming outbreaks this year, reestablishing the risk level across the region.
Costa Rica had been free of the cattle screwworm since 1999 but faced new challenges when outbreaks were confirmed in July 2023. Nicaragua and Honduras also encountered their first cases since 1996, with the situation escalating rapidly. By October 2024, Guatemala documented its first fatality attributed to the parasite, underscoring the severity of the reemergence. Together, these countries faced a dramatic increase in screwworm infections, with the Panama–United States Commission for the Eradication and Prevention of Screwworm documenting thousands of cases across the region by November 2, 2024.
Reports submitted to the World Organization for Animal Health reveal a troubling trend, as three of the affected countries—Costa Rica, Nicaragua, and Honduras—identified illegal animal transit as a primary source of the infections in their regions. For instance, Honduras noted a significant outbreak following the inspection of 68 illegally imported horses just a short distance from its border with Nicaragua. With the increased incidence of the cattle screwworm, controlling illegal trade and enforcing biosecurity measures will be critical as authorities work to prevent a broader impact and protect livestock health in North America and beyond.
