The New World screwworm threat could be the top livestock health topic of 2025. Although it is predominantly found in Central and South America, it has appeared sporadically in the United States since national eradication efforts were implemented in the 1960s. The last outbreak occurred in the Florida Keys in 2016.
Despite its name, the New World screwworm (Cochliomyia hominivorax) is not a worm; it’s a fly. But the screwworm is not your average fly.
A female screwworm only lays her eggs in open wounds, including castration sites and lacerations, even those from tick bite wounds, and other potential entry points such as mucous membranes. Each female can lay about 200 eggs. The eggs then hatch into larvae, burrowing into the live tissue as they develop before eventually dropping out of the animal and burrowing into the ground. Once in the ground, the pest goes into the pupal stage of its life cycle before again emerging as a fly.
“Screwworm maggots feed exclusively on live tissue, making it especially devastating. Left untreated, infestations can become debilitating, and if the larvae reach vital organs, they can be fatal,” says Dr. Thach Winslow, beef cattle technical consultant for Elanco Animal Health.
There are two ways the New World screwworm moves geographically - through natural migration and through the movement in livestock and other animal hosts.
“If the screwworm has a host source available, it is satisfied to stay in a local area where it can infect live animals. But if that becomes less available, the screwworm will travel 10 to 12 miles to find an adequate host to lay its eggs,” Winslow says. “The more rapid movement from Central America into Mexico has been the direct result of those larvae taking a ride with cattle and other livestock.”
Prioritizing wound care
Should New World screwworm reach the U.S., wound management and pest control are ranchers’ first methods of defense. We’re working with our customers to help prevent or mitigate the threat.
“We need to be diligent in caring for wounds related to castration, branding, and dehorning. We need to focus on newborn calf checks, because the navel cord can be an entry point for this pest,” Winslow says.
Good tick and fly control is also crucial to reducing the bite wounds that could serve as entry points for screwworm.
The importance of surveillance
This evolving threat highlights the need for on-farm vigilance and individual producer action alongside government oversight and recommended responses.
“Be mindful and monitor herds,” Winslow says.
When surveilling for potential New World screwworm populations, Winslow notes that producers should be aware of the secondary screwworm, a lookalike pest.
“It’s the same genus, but a different species, and it looks very similar to the screwworm and is present in the United States. However, it feeds on nothing but dead tissue. In comparison, the New World screwworm feeds only on live tissue,” he says.
Identify and report
The next important step is early detection.
“Animals that appear off—isolated, lethargic, or disinterested—warrant a closer look,” Winslow says. “Even small wounds from tick or horn fly bites can become entry points.”
When examining the animal, look for wounds with a foul odor—this can signal an infestation.
Screwworm eggs may resemble small white shingles on a wound’s surface. And once eggs hatch, the wound area may appear larger and more visually prominent.
If you suspect a screwworm infestation, contact your veterinarian immediately. You’ll also need to contact your state veterinarian and collect samples, if possible, preserving them in an alcohol-filled container.
“The screwworm larvae (maggots)are both easier to collect and to identify than flies. If the larvae are not found feeding in live animal tissue, they are not screwworms,” Winslow says.
Above all, producers must remain vigilant. Early detection and prompt reporting are essential to stopping the spread of this invasive pest if it were to cross into the United States. Through responsible care, treatment, and collaboration with veterinarians, we can reduce the potential risk to herds and help prevent the New World screwworm from becoming a national problem again.
For updated information about the New World screwworm issue, please monitor the websites and informational bulletins, including those published by USDA, the Texas Animal Health Commission, the Florida Department of Agriculture, Texas ACM AgriLife, and Texas Tech.
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