Livestock producers work tirelessly to protect herd health, but even small, unnoticed wounds can open the door to dangerous parasites. One of the most concerning threats now resurfacing near the U.S. border is the New World screwworm — a parasite capable of devastating livestock herds in a matter of days.
A Fast-Moving Parasite
The New World screwworm is a fly whose larvae feed on living tissue, not dead flesh like typical maggots. Once eggs are laid in an open wound, the larvae burrow into the animal, causing severe pain, infection, and potentially death if not treated quickly.
Dr. Jennifer Schleining, head of Large Animal Clinical Sciences at Texas A&M, explained,
“The larvae basically eat the animal from the outside in. It can happen very fast if you don’t catch it early.”
While the parasite was eradicated from the United States decades ago, recent detections just south of the border have renewed concerns. The screwworm can affect any warm-blooded animal — including cattle, horses, goats, sheep, and even feral hogs — and can occasionally infect humans.
Increased Risk in Warm Regions
Livestock in warmer climates face greater risk, as flies remain active year-round. Calving season is particularly vulnerable since newborns can attract flies to the umbilical area. Without a winter freeze, these parasites can persist, reproduce, and spread rapidly if unchecked.
Feral hogs also serve as carriers, making it crucial for producers to monitor domestic herds closely. “Because we can’t treat wild populations, watching our own animals becomes even more important,” Schleining noted.
Signs to Watch For
Early detection is critical. Producers should look for:
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Small white dots or larvae inside open wounds
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Restlessness or discomfort in animals
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Foul odors or visible tissue damage
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Weight loss or general weakness
The timeline from infestation to severe damage can be as short as one to two weeks, underscoring the need for daily monitoring and quick response.
Prevention and Control
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Apply fly control methods such as pour-ons, pyrethrins, and fly tags.
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Clean and cover all wounds to prevent egg-laying.
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Keep injured or newborn animals in monitored areas near the barn.
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Maintain clean pastures and remove carcasses or waste that attract flies.
Schleining reminds producers that prevention remains the best defense:
“Nothing replaces daily checks and good wound care. An ounce of prevention is definitely worth a pound of cure.”
Report Immediately
Screwworm is a reportable disease, meaning any suspected case must be reported immediately to a veterinarian or state animal health officials. Prompt reporting allows rapid containment, treatment, and prevents spread to neighboring farms.
Though the New World screwworm is not currently established in the U.S., awareness and quick action remain key to keeping it that way. By maintaining strong biosecurity and observation habits, producers can help ensure herds stay healthy and protected from this dangerous parasite.
