Mexico Reports Screwworm Cases

Mexico is facing a screwworm outbreak that has prompted heightened sanitary alerts, livestock controls and binational efforts to contain a pest long eradicated from much of North America. Mexican authorities confirmed a first case in the State of Mexico, with a second detection reported days later, drawing concern from agricultural and public health officials alike.

The New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae burrow into the living flesh of warm-blooded animals, causing painful infestations known as myiasis. Left untreated, these infestations can be fatal for livestock and difficult to eradicate. The pest was once considered eradicated from large parts of North America, but has re-emerged in Mexico following outbreaks in Central America.

In the first confirmed case in the central State of Mexico, officials with SENASICA and the Mexico-US Commission for the Prevention of Food and Mouth Disease and Other Exotic Animal Diseases (CPA) identified screwworm larvae in a goat from Tlatlaya, a region with active livestock production. The presence of the parasite was linked to a wound that had not received prompt veterinary care, illustrating how quickly the insect can exploit openings in animal health practices. Authorities declared a preventive alert and launched active surveillance in farms and ranches in the area to identify any further cases.

Just days later, Mexican authorities reported a second screwworm case in a goat in the same state, following a detection in a six-day-old calf in northern Tamaulipas at the end of December 2025. These cases represent part of a larger outbreak that began in late 2024 and has seen more than 13,000 reported infections in livestock across southern states like Chiapas, Oaxaca, Veracruz and Yucatan, regions with warmer climates conducive to the pest’s life cycle.

The resurgence of screwworm has had significant agricultural and trade implications. In May 2025, the United States halted imports of live Mexican livestock, including cattle, horses and bison, due to concerns that the pest could spread across the border and threaten domestic herds. The USDA estimated that an outbreak in Texas alone, the largest cattle-producing state in the U.S., could inflict up to US$1.8 billion in economic losses if the screwworm were to become established there.

To contain the outbreak, Mexico and US authorities have strengthened binational cooperation, deploying surveillance, traps and treatment protocols to manage infected animals and monitor fly populations. Mexico has also launched a pilot suppression plan in Yucatan, under which containment activities run over several weeks with the goal of reducing screwworm densities in key zones. This effort includes field surveillance and community engagement with ranchers to improve wound care and early detection.

Mexican agricultural authorities are also calling on research institutions and the scientific community to develop new tools and strategies for eradication, including biopesticides, attractants, genomic studies and natural enemy identification. These research collaborations aim to complement established containment measures and strengthen long-term responses to the pest.

The screwworm’s continued presence in Mexico comes after decades of eradication success using the sterile insect technique (SIT), a biological control method that releases sterilized male flies to suppress wild populations. Mexico and international partners are expanding sterile fly production facilities, with plans to bring operations online by mid-2026. These facilities are expected to produce millions of sterile flies to help suppress wild screwworm populations across key regions.

Beyond veterinary concerns, the screwworm outbreak has human health implications as well. While rare, cases of human myiasis linked to screwworm infestations have been confirmed in Mexico, highlighting the parasite’s broader risks when it spreads among livestock and companion animals.

Authorities are urging livestock producers to inspect animals daily for wounds, promptly clean and disinfect any lesions, and report suspicions of infestation. Given the pest’s potential to spread quickly via infested animals and its proximity to border regions, vigilance across both agricultural and public health sectors remains critical.