Colorado State Veterinarian Maggie Baldwin said there are direct costs associated with the individual animals and herds affected by new world screwworm, as far as surveillance, detection, quarantine and treatment. The bigger picture though, she said, is the bigger problem.

“One of the bigger things we have to think about as far as economic consequences is animal movement restrictions that will be put in place and international trade implications that we will have with our beef and other livestock commodities depending what commodity it’s detected in in the United States,” Baldwin said. “Because it affects all warm-blooded animals, it could really impact a lot of international trade if we were to have a detection. There are direct and indirect consequences we need to be aware of, and our producers need to be aware of.”

The flesh-eating maggot is a zoonotic pest, meaning it can infect any warm-blooded mammal, including humans. While other maggots eat only dead or decaying flesh, the new world screwworm feeds on living tissue resulting in extensive tissue damage and can be life-threatening.

STERILE INSECT TECHNIQUE

Baldwin said the sterile insect technique is one of the most unique ways the U.S. has been able to control a pest threat. The NWS was eradicated from the U.S. in 1966 through the use of sterile insect technique, though there were outbreaks into the 1970s due to the pest’s persistence in Mexico. McGrath said collaboration between the U.S. and Mexico was borne out of the eradication efforts and efforts to push screwworm populations back into southern Mexico by 1986.

Animals at risk are those with open wounds, including tick bites; recent surgeries; newborn animals via umbilicus; and via body orifices.

She said the first outbreak was controlled by the sterile insect technique in which irradiated sterile flies are released. When the sterile males mate with fertile female flies (females mate once in life) it results in unviable eggs. The fertile fly population dies over a few lifecycles, which can be as short as weeks. It is due, McGrath said, to the life cycle and mating habits of the flies that make the technique effective.

She said this technique has created a biological barrier between the U.S. and more southern countries, which has been maintained by the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG).

Historically, outbreaks in the U.S. have cost livestock producers millions of dollars. The largest outbreak in 1972 affected Arizona, Arkansas, California, New Mexico and Texas. In Texas, where the previous year documented 500 cases, experienced 90,000 cases and shouldered the majority of the $172 million in losses. McGrath said a 1976 outbreak in Texas cost economic losses of about $330 million, which adjusted for inflation is about $1.8 billion in 2024.

The biological barrier, she said, has been breached and since 2024, there has been a significant uptick in cases as far north as Mexico.

“Last year was when we saw a big surge in cases that breached the Panama Canal,” Baldwin said. “There have been thousands of cases at this point that have been north of the Panama Canal. Historically, because the new world screwworm was endemic south of that, and because that’s where they built the facility producing the sterile flies, that is where they’ve been successful at maintaining that barrier.”

The point for producers in this area to understand, Baldwin said, is why the pest is moving north. The illegal cattle movement in that area has begun encroaching into southern Mexico and that is likely why the pest has continued its movement northward.