A flesh-eating parasitic fly, once eradicated from North and Central America, is making a troubling comeback. Known as the New World screwworm (Cochliomyia hominivorax), the fly poses a renewed threat to livestock, pets, and even humans, prompting coordinated efforts to prevent a wider crisis.
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The resurgence of this parasite spells potential disaster for the cattle industry. The New World screwworm is far from selective — it targets a broad range of warm-blooded animals, including domestic pets and people. Scientists and governments across the region are now scrambling to contain its spread. This article is based on an original piece by Hannah Rose Vineer, Senior Lecturer at the University of Liverpool’s Institute of Infection, Veterinary & Ecological Sciences, and Professor Livio Martins Costa Junior. Republished from The Conversation under a Creative Commons licence.
While flies are often considered beneficial for pollination and the decomposition of organic waste, some have evolved to feed on living tissue. The female New World screwworm is drawn to the smell of wounds, where she lays her eggs. The resulting larvae burrow into the flesh of the host, causing extreme pain, severe tissue damage, and potentially death if not swiftly treated.
In the 1960s, Texas cattle farmers estimated they were treating around one million screwworm cases annually. A massive eradication effort began during that decade, employing a strategy known as the sterile insect technique (SIT). Because female screwworms mate only once, the release of billions of sterilised male flies prevented them from reproducing. Combined with the application of chemical treatments and colder weather conditions, the strategy successfully eradicated the parasite from the United States by 1982. The campaign cost an estimated US$750 million (£555 million), yet it significantly boosted cattle productivity.
For decades, a facility in Panama has served as a biological barrier, routinely releasing sterilised flies to prevent re-infestation further north. However, since 2022, the parasite has begun advancing northward once again. Panama saw a surge in cases in 2023, and by November 2024, the parasite had been reported in Mexico.
Experts suggest several factors may be contributing to its resurgence. These include the transport of livestock carrying the parasite, rising temperatures improving screwworm survival and development, and possibly even changes in female mating behaviour that allow them to avoid sterile males.
An estimated 17 million cattle are now at risk across Central America. The danger intensifies as the parasite pushes further north into Mexico, which has double that number of cattle. If it crosses the US border once more, as many as 14 million cattle in Texas and Florida could be threatened. Humans are not immune. Since April, at least eight cases of human screwworm infestation have been reported in Mexico.
The US has responded by temporarily restricting the import of live animals from Mexico. Alongside governments from Central America and Mexico, it is intensifying surveillance and expanding the sterile fly programme in a bid to halt the parasite’s spread.
More than 100 million sterile screwworm pupae are currently being produced each week at a facility in Panama, a joint venture between the US Department of Agriculture (USDA) and Panama’s Ministry of Agriculture Development. Yet, this might fall short. Historical campaigns in Mexico required over 500 million sterile flies weekly. To close the gap, the USDA is investing US$21 million to retrofit a fruit fly production facility in Metapa, Mexico, to produce an additional 60 to 100 million sterile screwworm flies each week.
The production, sterilisation and release process is lengthy, meaning the effects won’t be immediate. Experts stress that integrated strategies — combining biological control with antiparasitic treatments — are essential. However, there are logistical challenges, particularly a lack of institutional memory. Many current veterinarians and farmers have never encountered screwworms, making education and training vital.
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Moreover, unlike in the 20th century, cooler weather may no longer assist eradication efforts due to climate change. Rising temperatures are creating more hospitable conditions for the parasite to thrive, making the task of eradication even more complex.
The re-emergence of the New World screwworm underscores the delicate balance between ecological control, veterinary intervention, and climatic stability. Without urgent and coordinated international action, this flesh-eating pest may soon pose a significant threat not only to livestock but to public health across the Americas.
