On August 27, the US Department of Agriculture (USDA) announced the detection of New World screwworm larvae — a flesh-eating parasite — in a three-week-old calf in La Pryor, Texas, about 50 kilometers from the Mexico border. This is the first animal infection in the US since the species was declared eradicated in 1966.
Experts say the screwworm likely migrated from Central America through Mexico before entering Texas, overcoming biological barriers that had contained the disease for decades. The USDA has established a 20-kilometer (12.4-mile) quarantine zone around the affected farm, banning the movement of any animals, including pets. The infected calf is receiving treatment, and the larvae will be destroyed.
Screwworm is caused by the female Cochliomyia hominivorax fly, which lays eggs in scratches or open wounds on warm-blooded animals, including livestock, wildlife, and humans. The eggs hatch into hundreds of larvae, which use sharp mouthparts to burrow into living tissue for about a week, then drop to the ground to pupate and develop into adult flies within one to eight weeks. Adult flies can fly several kilometers to find new hosts, enabling the fast spread of the disease through livestock herds, wildlife, and between animals and humans.
Symptoms include foul-smelling, rapidly expanding open wounds with visible maggots inside. Infected animals often become weak, lethargic, lose weight, and exhibit abnormal behavior. In humans, symptoms include severe pain, swelling, fever, and secondary infections. Without treatment, the infection can lead to tissue destruction and death. Treatment requires manual removal of hundreds of larvae from the wound, disinfection, antibiotics to fight bacterial infection, and pain management. In animals, insecticides are used to kill remaining larvae and prevent re-infection.
Experts say the return of screwworm is alarming for several reasons. First, it is the first animal case in the US since eradication, following a large-scale sterile male fly release program. Second, the outbreak poses a serious threat to the livestock industry — already weakened by prolonged drought — and could further push already record-high beef prices. The USDA estimates screwworm could cause $1.8 billion in losses for the Texas economy alone.
The US has stopped importing livestock from Mexico for a year due to fears of screwworm spread, typically importing over one million Mexican cattle annually. This suspension has reduced beef supply and driven up prices. Experts warn most US livestock ranchers lack the experience and tools to diagnose and treat screwworm, since the disease was eliminated long ago. Meanwhile, between mid-July and mid-August 2025, Mexico saw a 53% rise in animal screwworm cases. Rare as it is, humans can still be infected, and homeless people are at higher risk due to sleeping outdoors and lacking hygiene and medical care.
