Screwworm precautions urged as parasite returns to U.S.
Flesh-eating parasite detected in Texas won't survive Canadian winter, but experts say farmers should strengthen biosecurity.
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The screwworm—a flesh-eating parasite that devastated Canadian cattle ranchers for decades—has returned to the U.S. for the first time since 1966, but experts say Canadian farmers don't need to panic about a major outbreak north of the border.
The parasite cannot survive Canadian winter, so the risk of a sustained infestation in Canada is minimal. However, a single infested animal crossing the border could still disrupt export markets and herd health, said Leigh Rosengren, chief veterinary officer with the Canadian Cattle Association.
"As we're seeing in Texas, even a single incursion can cause some hiccups in export markets," Rosengren said. "Canada's extremely dependent on our export market, so we would want to make sure to prevent that."
Screwworm flies were an annual summer scourge from at least the 1930s through the 1960s, when the U.S. eradicated them by breeding sterile male flies and dropping swarms from planes. The deadly flies remained contained to southern Panama and South America until a 2023 outbreak in Panama spread to Costa Rica and Nicaragua, and by late 2024 reached Mexico.
Last week, an infestation was discovered in a three-week-old calf in La Pryor, Texas, about 80 kilometres from the U.S.-Mexico border.
Rosengren recommends Canadian producers update their biosecurity plans, increase monitoring of animals—especially vulnerable neonatal cows—and reconnect with their veterinarians. The New World screwworm fly is unusual because its larvae eat live flesh instead of dead material, burrowing into open wounds and mucous membranes. An untreated infestation can cause death.
In past decades, ranchers suffered tens of millions of dollars in losses—potentially billions in today's dollars. Though Canadian winter will protect the herd, the situation underscores how important it is to maintain national biosecurity and stay vigilant about disease.
Infectious disease specialist Isaac Bogoch at the University of Toronto recently treated a Canadian man who became infested after falling on a hike in Costa Rica—a reminder that even travel can bring the parasite home.