Screwworm Outbreak in Mexico Prompts U.S. Livestock Import Ban
May 11, 2025 – A growing outbreak of the parasitic New World screwworm in Mexico has triggered a swift response from U.S. authorities, including a temporary ban on live cattle, horse, and bison imports from its southern neighbor.
What Is Screwworm?
The New World screwworm (Cochliomyia hominivorax) is a dangerous parasitic fly whose larvae infest and consume healthy flesh in warm-blooded animals. Female flies lay eggs in open wounds or natural body openings; after hatching, the larvae burrow into the tissue, leading to severe infections and often death if left untreated.
Unlike other maggots, screwworms require live tissue to complete their life cycle, making them particularly destructive to livestock and wildlife.
Extent of the Outbreak
The outbreak began in late 2024 with confirmed cases in Chiapas, Mexico, near the Guatemala border. Since then, it has spread to several states including Oaxaca and Veracruz — hundreds of miles north. Though exact figures are limited, the geographic spread suggests a substantial threat to Mexico’s livestock industry.
The U.S. Department of Agriculture (USDA) confirmed the parasite’s northward advance is approaching a level that risks reinfestation of previously eradicated zones in the U.S.
U.S. Response: Import Ban and Biosecurity Measures
On May 11, the USDA issued a 15-day suspension on live cattle, horse, and bison imports from Mexico via land border crossings. The decision is part of a broader effort to prevent the reintroduction of screwworm into the U.S., where it was eradicated in 1966.
According to Agriculture Secretary Brooke Rollins, the suspension is a precautionary biosecurity measure and not a political statement. “We must act decisively to protect our domestic herd and food supply,” she stated.
Conditions to Lift the Ban
The USDA will review the restriction on a monthly basis. In order for it to be lifted, Mexico must meet key criteria:
- Containment: Clear evidence that the outbreak has been halted or is receding.
- Surveillance: Comprehensive monitoring systems to detect and report new infections.
- Collaboration: Ongoing eradication efforts including release of sterile flies and cross-border coordination.
Officials say the earliest lifting of the ban could occur in June, depending on Mexico’s progress.
Historical Context: Past Outbreaks and Lessons
The U.S. eliminated screwworm by 1966 using a sterile insect technique (SIT), releasing millions of sterilized male flies to disrupt reproduction. Mexico successfully followed suit in the 1990s. However, the pest remains endemic in parts of Central and South America, and reintroductions remain a threat.
In 2016, an outbreak in the Florida Keys killed dozens of endangered Key deer before being contained. The success of past SIT programs provides hope for similar efforts now being relaunched in southern Mexico.
What’s Next?
Both nations are intensifying eradication efforts and surveillance. The USDA has pledged resources for sterile fly programs and is working with Mexican officials to mitigate risks. If left unchecked, the outbreak could cause economic damage exceeding $1 billion annually in livestock losses and trade disruptions.
This biosecurity scare underscores the importance of coordinated international action in preventing agricultural disease spread across borders.
Sources: USDA, U.S. Customs & Border Protection, SENASICA
