The New World screwworm fly (Cochliomyia hominivorax), a parasitic insect previously eradicated from the region, is spreading north through Central America and has reached Mexico, raising concerns for livestock industries and potentially human health across North America. This invasive fly’s larvae feed on living tissue, causing severe harm and potential death to infected hosts if untreated.
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What is the New World Screwworm Fly?
The New World screwworm is a type of fly with a unique and harmful life cycle for animals and humans. Female screwworms are drawn to the smell of wounds, where they lay their eggs. Once hatched, the larvae, or maggots, aggressively consume the living flesh of the host. This infestation causes immense suffering and can be fatal without intervention. Historically, this parasite caused significant losses in livestock. In the 1960s, cattle farmers in Texas reported treating around 1 million cases per year.
A History of Eradication
Between the 1960s and 1990s, a large-scale effort involving scientists and governments successfully eradicated the New World screwworm from the United States and Mexico. This was primarily achieved through the Sterile Insect Technique (SIT). SIT involves releasing vast numbers of male flies that have been sterilized by radiation. Since female screwworms mate only once, mating with a sterile male prevents them from producing viable offspring. Billions of sterile males were released from planes during the campaign. Combined with chemical treatments and cooler weather conditions, the screwworm population was eliminated in the US by 1982. The eradication campaign was costly, estimated at US$750 million, but it allowed cattle production to increase significantly. For decades, a facility in Panama has continued to release sterile flies to maintain a buffer zone against the fly spreading north from South America.
Head of New World screwworm larva showing mouthparts
Why is the Screwworm Returning?
Despite decades of successful containment, the New World screwworm has been spreading north again since 2022. Cases sharply increased in Panama in 2023, and the fly reached Mexico by November 2024. Scientists are exploring several potential reasons for this resurgence and spread. Hypotheses include flies possibly hitchhiking with cattle movements, warmer temperatures potentially improving fly development and survival rates, and the possibility that female flies are evolving their mating behavior to avoid sterile males.
The Impact and Risks
The return of the screwworm poses a significant threat to the livestock industry. Approximately 17 million cattle in Central America are currently at risk. The situation could worsen if the fly spreads further north. Mexico has twice as many cattle, and potential spread into US states like Texas and Florida could endanger around 14 million cattle in Texas and Florida alone. While livestock are the primary concern, humans are also susceptible. At least eight human cases of screwworm infestation have been reported in Mexico since April 2025.
Cattle grazing in a field, representing potential hosts for screwworm flies
Response and Challenges
In response to the outbreak, the US has implemented temporary restrictions on live animal imports from Mexico. The US, Mexico, and Central American governments are collaborating to enhance surveillance and increase sterile insect releases to work towards renewed eradication efforts in affected areas of Mexico and Central America. Currently, the facility in Panama produces and sterilizes over 100 million sterile screwworm pupae weekly, funded jointly by the US Department of Agriculture (USDA) and Panama’s Ministry of Agriculture Development. However, a successful large-scale eradication campaign may require a significantly higher production rate, potentially several times the current output, similar to the over 500 million flies per week used during the 1980s Mexico campaign. To address this shortfall, the USDA is focusing releases in critical areas and investing US$21 million to adapt a facility in Metapa, Mexico, to produce an additional 60 million to 100 million sterile screwworms weekly.
Implementing SIT is a lengthy process, and immediate reductions in wild fly populations are not expected. Past eradication efforts demonstrate that combining SIT with anti-parasitic veterinary medicines to treat infestations and repel flies is crucial. Effective surveillance by trained personnel is also vital but challenging due to a generation of veterinarians, technicians, and farmers who lack direct experience with screwworm infestations. Furthermore, climate change, particularly rising temperatures, may hinder current eradication plans, as past success benefited from cooler conditions.
