Last November, the U.S. temporarily suspended some livestock imports through the southern border over concerns about “New World screwworm” moving northward. That was lifted in late January after Mexico met U.S. demands for “additional mitigations to guard against introduction” of NWS. On May 10, USDA once again halted livestock imports, again citing the northward spread of NWS. This time, it’s been spotted just 700 miles from the U.S. border. Below are more details about NWS and what USDA is doing to prevent it from being reintroduced into the U.S..
What is New World Screwworm? New World screwworm (NWS, Cochliomyia hominivorax) is a devastating pest. The name screwworm refers to the maggots’ feeding behavior as they burrow (screw) into the the flesh of living animals, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.
Eradication from the US: Historically, the original distribution area was the southern states of the USA, Mexico, Central America, and the northern states of South America up to Uruguay and northern Argentina. The United States officially eradicated the screwworm in 1982 using the “sterile insect technique.” That campaign also allowed Mexico to be declared NWS-free in 1991. Today, NWS is considered endemic in Cuba, Haiti, the Dominican Republic, and countries in South America.
What is Done to Prevent NWS in the US: To prevent the northward movement of this pest from South America to NWS-free areas in Central and North America, USDA’s Animal and Plant Health Inspection Service (APHIS) has worked with Panama since 2016 to maintain a barrier zone in eastern Panama. The cornerstone of this collaboration is the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG). Through COPEG, APHIS releases sterile male screwworm flies in the Darien Province of eastern Panama to create a biological barrier.
Recent Spread: Over the past two years, NWS has spread rapidly through Panama and into Costa Rica, Nicaragua, Honduras, and Guatemala. In November 2024, Mexico notified the U.S. that NWS was confirmed near Mexico’s border with Guatemala. On May 11, Secretary Rollins announced that NWS had been confirmed as far north as Veracruz and Oaxaca—roughly 700 miles from the U.S. border. Looking at the USDA maps below, you can see that the number of cases in Mexico as of the end of April has exploded versus the detection map from December.
Cost of NWS: Prior to NWS eradication in the U.S., producers in the southeastern U.S. experienced losses up to $10-$20 million per year. Although NWS has been considered extinct throughout the U.S. since 1966, recurrent outbreaks plagued the Southwest due to the continued presence of NWS in Mexico up until the 1990s. . An analysis of NWS economic impacts in Texas in 1976 indicated that livestock producers spent $132.1 million per year to manage NWS outbreaks, with a total impact to the Texan economy of $283–375 million. An outbreak of NWS in Texas in 1976 affected more than 1.4 million cattle and hundreds of thousands of sheep and goats. USDA says such an outbreak today would cost the Texas economy some $1.8 billion.
How Can Producers Protect Against NWS? The Texas Animal Health Commission (TAHC) is urging people to monitor their animals for screwworms. Clinical signs include: irritated or depressed behavior, loss of appetite, head shaking, smell of decaying flesh, evidence of fly strike, presence of fly larvae (maggots) in wounds and isolation from other animals or people. To avoid introduction of NWS, TAHC encourages keeping open wounds clean and covered. People should also treat clothing, gear and people with proper repellents. Those traveling, especially in NWS-infested areas, should ensure pests and vehicles are inspected for NWS flies and larvae. Suspected cases of NWS should be reported to your veterinarian or local animal health commission. You can also learn more about NWS and its prevention at APHIS HERE.
