The confirmation of a New World screwworm case in a South Texas calf on Wednesday has triggered an aggressive state and federal response aimed at preventing the flesh-eating parasite from becoming established in the United States once again.
The fly's larvae feed on the living tissue of warm-blooded animals, including livestock, wildlife and pets and, in rare cases, humans. Left untreated, infestations can cause severe injuries and death.
While federal officials say the U.S. food supply remains safe, they warn that screwworms pose a serious threat to livestock production.
The parasite was eradicated from the U.S. in the 1960s. Now, officials are deploying many of the same tools that successfully eliminated the pest more than half a century ago.
Releasing millions of sterile flies
One of the most important weapons against the parasite is an unusual one: more flies.
For decades, officials have used sterile insects to suppress and eventually eliminate screwworm populations (adult flies aren’t the problem; it’s the larvae). The strategy involves releasing sterilized male flies into the environment. When those flies mate with wild females, no larvae are produced, gradually causing the population to collapse.
Federal officials say roughly 4 million sterile screwworm flies are already being released each week by air in areas near the U.S.-Mexico border. Following the Texas detection, authorities are accelerating those efforts.
U.S. Department of Agriculture Secretary Brooke Rollins on Thursday said the agency aims to increase production to 30 million sterile flies per week by the end of the summer. That figure is expected to rise to 100 million flies per week by the end of the year, Rollins added.
“The release of the sterile flies, that's how we beat this back,” Rollins said.
In April, officials broke ground on a new $750 million sterile fly production facility at Moore Air Base near Edinburg. Rollins said the facility is expected to produce up to 300 million sterile screwworm flies per week when it becomes operational in fall 2027.
Quarantines and movement restrictions
State and federal officials have also established a roughly 12-mile “infested” zone around the location where the infected calf was found. Within that zone, authorities are implementing quarantines and restricting movement of animals to prevent the parasite from spreading to other livestock or wildlife populations.
“This fly typically moves great distances because humans move animals,” Rollins said. “These flies do not fly to new areas on their own.”
Increasing surveillance
Officials are also expanding trapping operations along the border and around the detection site.
Over the last year, Rollins says more than 8,000 traps have been deployed along the U.S.-Mexico border. Those traps allow scientists to monitor for adult screwworm flies and determine whether the parasite is spreading beyond the initial detection area.
Veterinarians, ranchers and animal health officials are being asked to closely inspect animals for wounds that could become breeding sites for the parasite.
