A nasty, nasty pest that poses a severe threat to the livestock industry has been working its way northward from South America. Eradicated from Central America, Mexico, and the U.S. over 30 years ago, the pest has made a rapid comeback in just the last couple of years. Known as “New World Screwworm” (NWS), the pests infest living tissue of warm-blooded animals, causing serious, often deadly damage. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.
NWS most often enter an animal through an open wound and feed on the animal’s living flesh. The name screwworm refers to the maggots’ feeding behavior as they burrow (screw) into the wound, 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.
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. While they can fly much farther under ideal conditions, adult flies generally do not travel more than a couple of miles if there are suitable host animals in the area.
Screwworm infestations are difficult to detect at first. Check your pets and livestock for draining or enlarging wounds, and signs of discomfort. Also look for screwworm larvae or eggs. Screwworm eggs are creamy and white and are deposited on near the edges of superficial wounds.
If you suspect your animal is infected with screwworm, USDA advises you contact your veterinarian. Once an infestation is uncovered, it can be treated. According to APHIS, euthanasia of the animal is not usually required. “Although some wounds may be surgically excised, most are treated with a suitable larvicide and allowed to heal without closure. Treatment is usually repeated at intervals until the wound has healed. Removal of necrotic tissue may be necessary, and antibiotics may be given when secondary bacterial contamination is present.”
While not common in humans, if you notice a suspicious lesion on your body or suspect you may have contracted screwworms, seek immediate medical attention.
USDA eradicated NWS from the US, Central America, and Mexico back in the 1960s using sterile insect technique to the tune of around $800 million. Since 2006, USDA’s Animal and Plant Health Inspection Service (APHIS) has worked with Panama to maintain a barrier zone to prevent the northward spread of NWS.
However, over the last two years, NWS has moved north into Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico. According to the USDA, cases of screwworm leaped in Panama to more than 6,500 cases in 2023 from a usual average of just 25 cases per year. APHIS says the spread is likely due to several factors, “including new areas of farming in previous barrier regions for fly control and increased cattle movements into the region.”
According to the Wildlife Conservation Society and other experts, the resurgence of NWS is directly tied to illegal cattle trafficking. Major screwworm outbreak hotspots closely mirror cattle smuggling routes identified in InSight Crime’s 2022 report, “Cash Cows – The Inner Workings of Cattle Trafficking from Central America to Mexico,” according to WCS Regional Director Jeremy Radachowsky.
Mexico’s cattle producers have been calling for a crackdown on the illegal cattle crossings along the country’s border with Guatemala. The industry is especially alarmed after a positive detection of NWS in Mexico in November, which led the U.S. to halt the importation of live cattle originating from or transiting Mexico.
APHIS last week announced $165 million in emergency funding from the Commodity Credit Corporation to protect U.S. livestock and other animals from NWS and to increase USDA’s ongoing efforts to control its spread in Mexico and Central America. APHIS and Mexican authorities have taken additional measures to prevent further spread through surveillance, animal health checkpoints, and domestic preparedness. In addition, USDA is working with partners in Mexico and Central America to establish a barrier on the Isthmus of Tehuantepec, eradicate NWS from the affected areas, and reestablish the biological barrier in Panama. The APHIS announcement is HERE.
