What do you need to know?
In a press release dated September 21, 2025 from the USDA, Mexico has confirmed a case of New World Screwworm in Nuevo Leon, which is less than 70 miles from the U.S.-Mexico border. Texas Agriculture Commissioner Sid Miller announced that the state is taking immediate and aggressive steps to prevent the re-establishment of the NWS. He is calling for the immediate use of pesticide baits to suppress the pest.
“The screwworm is dangerously close,” Miller warned.
New World Screwworm (NWS) is a serious pest of livestock and other mammals. Screwworms are a fly larvae, or maggots, that burrow into the flesh of living animals causing serious and even deadly damage to the animal. The maggots cause extensive damage by burrowing and tearing at the tissue with sharp mouth hooks. The wound becomes deeper and larger as more maggots’ hatch and feed on living tissue. Adult screwworm flies are about the size of a common housefly or slightly larger. They are identified by the red eyes, a yellow-orange face and a metallic blue-green body.
NWS is common in Cuba, Haiti, the Dominican Republic and countries in South America. Cases of NWS have spread north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador and Mexico. The USDA eradicated NWS from the United States using a sterile insect technique but there is a constant risk of re-introduction.
Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama to prevent the NWS from moving North from South America. In 2023 APHIS confirmed an unusual higher number of NWS cases in Panama. Since then, cases have been detected in every Central American country and Mexico.
NWS infestations are often hard to detect in the first 1-3 days as the larvae burrow beneath the skin. They are normally not seen crawling on the surface but as many as 200 larvae can be found below the surface of what appears to be just a small opening in the skin. As more larvae hatch and continue to feed, the wounds can expand and deepen. Left untreated, myiasis caused by NWS can be fatal within 7-10 days.
Preventing an infestation of NWS is the key to protecting your herd. NWS are attracted to open wounds and mucus membranes where they lay their eggs, and they can be carried by both domestic and wild animals. Pay close attention to tick bites, cuts and dehorning/branding sites. Immediately treat any open wounds, scratches, or scabby areas. Screwworms in wounds can be killed by direct application of a wound dressing. Closely monitor newborn calves and cows as NWS often lay eggs on the navel area of calves and the vulva and perineum of cows. On bulls and steers, inspect the sheath/prepuce as well.
Animals that are suffering from NWS infestation can show the following signs:
• Bloody or light-colored drainage from a cut or wound
• A wound that rapidly enlarges
• White or cream-colored runny substance (the eggs) in and around a wound
• Presence of fly larvae/maggots in wounds
• Irritated behavior such as head shaking
• The smell of decay
Animals may become depressed, go off feed and separate themselves due to pain in the wound.
The Texas Department of Agriculture has outlined a series of actions to be taken against the NWS. Pesticide baits are the fastest line of defense when paired with attractants such as ‘TDA SWORMLURE’, a technology designed to lure the NWS by smelling like rotting flesh. The sterile fly program to follow the bait. Vaccine research for a development of a non-mRNA vaccine to prevent larvae from maturing in livestock. Ivermectin, available as injection or pour on for both treatment and prevention. Expanded testing and deployment of specimen kits for ranchers, veterinarians, and TDA staff to rapidly collect and submit sample for USDA confirmation.
The USDA is currently dispersing 100 million sterile flies per week in Mexico, sourced from the COPEG facility in Panama and is also providing support to Mexico to renovate a production facility in Metapa. Construction on a domestic sterile fly facility at Moore Air Force Base in Edinburg, Tx. is expected to be close to completion near the end of 2025. Planning is also underway with the U.S. Army Corps of Engineers for construction of a facility in Southern Texas.
Currently, U.S. ports remain closed to imports of live cattle, bison and horses from Mexico.
Although rare in people, screwworms can infest humans through open wounds. On August 4, 2025, the CDC confirmed the first human case in the U.S. in a patient who had returned from travel to El Salvador.
On September 30th, the U.S. Food and Drug Administration conditionally approved Dectomax-CA1 (doramectin injection) injectable solution for the prevention and treatment of NWS larval infestations, and prevention of NWS reinfestation for 21 days. Dectomax-CA1 is conditionally approved for use in cattle only.
If you think you have found a screwworm, contact your veterinarian who can help identify and report it immediately to your state animal health official and APHIS office. It will be important to respond quickly and remove the screwworms before a population becomes established. Hopefully we will not see the screwworm make its way any farther north, but it is important to stay vigilant and aware of the risks and signs.
