The return of screwworm is testing US biosecurity and the cattle economy

New World screwworm has spread to southern Texas for the first time in about 50 years, with more than 30 confirmed infections in cattle, sheep and goats, Bloomberg reports.

Scientists remain uncertain how the flesh-eating parasite entered the U.S., making it difficult to predict or contain its spread. Although the issue has become politically contentious, experts caution there is no evidence linking the outbreak to immigration and say claims about its origin remain speculative.

Because adult screwworm flies travel only short distances, researchers believe the parasite was more likely introduced through infected animals than by flies crossing the border on their own. However, livestock imports from Mexico had already been largely suspended before the Texas cases emerged, leaving several possible routes, including infected wildlife, pets or illegal animal movements. 

No wild screwworm flies or infected wildlife have yet been detected in the U.S., adding to the uncertainty. An infected dog in New Mexico, which had never traveled to Mexico, also suggests local transmission may have occurred, though no additional cases have been identified there.

Authorities have imposed quarantines across parts of southern Texas, restricted livestock movement, expanded surveillance and are exploring the use of drones and artificial intelligence to monitor wildlife. The USDA has also increased staffing dedicated to the outbreak and, together with Mexico, opened a new sterile-fly production facility to help suppress the pest by interrupting its reproductive cycle.

Experts have greater confidence in explaining the parasite’s broader resurgence in Central America. After decades of successful control using sterile-fly releases, screwworm cases surged in Panama in 2023 before spreading north through Central America and Mexico. 

Researchers believe reduced animal disease surveillance during the COVID-19 pandemic and favorable environmental conditions likely contributed to its rapid expansion. They warn that sterile-fly releases alone will not eliminate the threat if infected animals continue to move through unauthorized trade routes, emphasizing that long-term control will require stronger surveillance, enforcement and international cooperation.

Bloomberg has the full story.