The New World screwworm, a devastating parasitic fly that targets livestock, is spreading dangerously close to the U.S. Originally eradicated in 1966 using sterile flies, the pest has re-emerged in Central America, posing a serious threat to animal health, producer costs, and trade stability. 

The name "screwworm" comes from the maggots' feeding behavior—they burrow (or "screw") into wounds, feeding as they go, similar to a screw being driven into wood. Maggots cause extensive damage by tearing at the host’s tissue with sharp mouth hooks, enlarging and deepening wounds as more maggots hatch and feed. Left untreated, screwworm infestations can lead to serious, often fatal damage to the animal. 

Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs. 

Signs of screwworm infestation in mammals and birds: 

• Irritated behavior 
• Head shaking 
• The smell of decay 
• Evidence of fly strike 
• Presence of fly larvae (maggots) in wounds 

In response, the Livestock Marketing Association (LMA) supports the bipartisan STOP Screwworms Act, introduced by Rep. Tony Gonzales and Sen. John Cornyn. This legislation would authorize funding for a new sterile fly production facility in the U.S., addressing capacity limitations at the current site in Panama and ensuring swift action to prevent an outbreak. 

LMA urges members to contact their legislators in support of the STOP Screwworms Act. Increased funding for sterile fly production is critical to protecting the U.S. livestock industry from a potential outbreak. Reach out to your representatives and encourage swift passage of this bipartisan measure to safeguard animal health and producer viability. 

The best way to keep screwworm out of the U.S. is to prevent introductions. New World screwworm can infest mammals—including people—and birds. Take the following steps to avoid infestation: 

• Be alert for screwworm symptoms in pets and livestock. 
• Ensure that pets traveling internationally are inspected for screwworm. 
• Check vehicles for screwworm flies when returning from an infested area. 
• Report any suspected screwworm cases immediately to your State animal health official and APHIS office to allow for a rapid response before the pest becomes established. 

The only effective way to eradicate New World screwworm is through the sterile insect technique. This method involves releasing sterile male screwworm flies into infested areas. These males mate with fertile females, leading to the laying of nonviable eggs. Over time, the screwworm population declines and dies out. 

Animals infested with screwworm should receive veterinary treatment based on professional recommendations. 

Individuals who suspect they are infected should seek immediate medical attention following Centers for Disease Control and Prevention (CDC) guidelines. For more information on how screwworm affects people, visit the CDC’s New World Screwworm Myiasis resource. 

Producers are also encouraged to stay vigilant, monitor livestock for signs of screwworm infestation, and those who suspect an infestation should contact their veterinarian to evaluate the animal or herd immediately.  

U.S. Secretary of Agriculture Brooke L. Rollins announced on May 11, 2025 the immediate suspension of live cattle, horse, and bison imports through U.S. ports of entry along the southern border due to the continued and rapid northward spread of New World Screwworm (NWS) in Mexico.  NWS had been recently detected in remote farms with minimal cattle movement as far north as Oaxaca and Veracruz, about 700 miles away from the U.S. border. 

Additional Background: 

• The first case of NWS in Mexico was reported to the U.S. in November 2024. When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage to the animal. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people. 
• In November 2024, after a positive detection of NWS in southern Mexico, USDA shut down the border for live animal trade. 
• In February 2025, USDA resumed imports after APHIS and Mexico agreed to and implemented a comprehensive pre-clearance inspection and treatment protocol to ensure safe movement and steps to mitigate the threat of NWS. 
• Over the last two years, screwworm has spread north throughout Panama and into Costa Rica, Nicaragua, Honduras, Guatemala, El Salvador, Belize, and now Mexico. 
• APHIS is releasing sterile flies through aerial and ground release at strategic locations, focusing on Southern Mexico and other areas throughout Central America.