A flesh-eating parasite that was recently discovered in southern Mexico has officials in the United States concerned that the previously eradicated pest could make its way back into the country for the first time since the 1960s.
The parasite known as the New World screwworm (NWS) was found in a cow at an inspection checkpoint near the border of Guatemala, according to Texas Parks and Wildlife. Officials say its detection in Mexico follows the progressively northward movement of NWS through both South and Central Americas -- and the U.S. could be next.
Officials are asking residents along the southern Texas border who hunt or spend time outdoors to keep an eye out for clinical signs of NWS and immediately report potential cases. Though NWS primarily affects livestock, officials say humans, wildlife and pets can also be infected.
The name screwworm refers to the maggots' feeding behavior as they burrow into a wound, feeding as they go like a screw being driven into wood, according to the U.S. Department of Agriculture. The maggots cause extensive damage by tearing at the hosts' 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 have orange eyes, a metallic blue or green body, and three dark stripes along their backs.
An infection begins after a fly lay eggs in a wound or areas of exposed tissue — such as nostrils, eyes or mouth. The eggs later hatch into dangerous parasitic maggots that burrow into an opening to feed. Larvae feed in the host for about a week and may migrate to other tissues in the body, often causing extreme damage in the process.
If a wound is infested, it can become larger and cause damage or death to an animal.
Screwworm larvae only feed on living flesh and never on dead tissue.
Symptoms of NWS myiasis include irritated behavior, loss of appetite, head shaking, smell of decaying flesh, and the presence of fly larvae in wounds.
Prevention measures include covering wounds and using insect repellent to help avoid contact with NWS and other harmful insects. Treatment includes surgical removal of the larvae.
The NWS was last eradicated from the U.S. in 1966, with costly efforts by federal and state animal health officials, livestock producers and veterinary practitioners. Eradication efforts have continued in Central America, but the pest is considered widespread in Cuba, Haiti, Dominican Republic and South American countries.
Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama that prevents NWS from moving north from South America to screwworm-free areas in Central and North America.
In 2023, NWS detections in Panama exploded from an average of 25 cases per year to more than 6,500 cases in 1 year, according to the USDA. Since then, screwworm has been detected in Costa Rica, Nicaragua, Honduras, Guatemala and Mexico, north of the biological barrier that's successfully contained this pest to South America for decades. Officials are now working to curb the outbreak.