Screwworm Explained: Everything You Need To Know About This Flesh-Eating Pest
Most pests invade homes. The screwworm invades living tissue.
Unlike ordinary maggots that feed on dead or decaying material, the flesh eating screwworm attacks healthy, living flesh. This single difference has made it one of the most economically significant livestock pests in modern history, costing ranchers millions of dollars before widespread eradication efforts nearly eliminated it from North America.
What Is Screwworm?
Today, reports of screwworm Texas continue to draw national attention whenever a new case of screwworm is identified in Central America or near the U.S. border. That’s because even a single introduction has the potential to threaten cattle, horses, sheep, goats, wildlife, pets, and in rare cases, people.
Whether you’re wondering what is screwworm, do screw worms infect humans, or how to get rid of screwworms, understanding its biology is the first step. The more you know about how the New World screwworm spreads, the easier it becomes to understand why governments, veterinarians, ranchers, and animal health officials respond so aggressively whenever one is discovered.
Despite its name, a screwworm is not actually a worm. It is the larval stage of the New World screwworm fly (Cochliomyia hominivorax), a metallic blue-green blow fly found primarily in parts of South America, the Caribbean, and regions of Central America.
Adult flies don’t cause the damage.
The danger begins when a female fly lays eggs on the edge of an open wound, fresh surgical incision, tick bite, branding mark, dehorning site, or even the moist tissue around the eyes, nose, or navel of newborn animals.
Within hours, the eggs hatch into screwworm larvae, which immediately begin to feed on living tissues rather than dead tissue. As the nws larvae grow, the larvae burrow deeper into the wound using rows of backward-facing spines that make removal increasingly difficult.
This behavior separates screwworms from nearly every other fly species found in North America.
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Why Screwworm Is Different From Every Other Fly
Most fly larvae play an important ecological role by breaking down dead animals and decaying organic material. Screwworms do the exact opposite.
Instead of waiting for tissue to die, they actively consume healthy flesh. Each additional female that deposits eggs in the same wound can dramatically increase the amount of tissue damage, creating larger wounds that attract even more flies.
Without treatment, infestations become progressively worse. Secondary bacterial infections often follow, and severely affected warm blooded animals including cattle, horses, deer, dogs, and wildlife may die from tissue destruction, infection, or blood loss.
This unique feeding behavior explains why the screwworm became one of the most feared agricultural pests in the Western Hemisphere.
The Life Cycle That Made Screwworm So Dangerous
Understanding the life cycle helps explain why screwworm spreads so efficiently.
It begins when an adult female locates an open wound. After she lays eggs, they typically hatch within 12 to 24 hours.
The newly emerged larvae immediately begin feeding on living tissue for several days while continuously enlarging the wound. Once fully developed, the larvae drop to the ground and pupate in the soil before emerging as adult flies capable of repeating the cycle.
Unlike many nuisance flies, the damage occurs almost entirely during the larval stage.
Because females usually mate only once during their lifetime, scientists realized decades ago that interrupting reproduction—not killing every fly—could collapse an entire population.
That discovery changed pest management forever.
The Brilliant Strategy That Nearly Eliminated Screwworm
Few people realize that one of the greatest pest control success stories in history wasn’t a pesticide—it was genetics.
Beginning in the 1950s, researchers developed the Sterile Insect Technique (SIT), a revolutionary approach that relied on the mass release of sterile male flies.
Millions of sterilized males were raised in specialized facilities, exposed to radiation that rendered them infertile, and released over infested regions.
Because female screwworm flies mate only once, any female that mated with a sterile male produced no offspring.
Repeated releases caused populations to collapse generation after generation without introducing large quantities of insecticides into the environment.
Today, the sterile insect technique sit remains one of the most successful examples of biological pest management ever implemented and forms the foundation of ongoing eradication programs throughout the Americas.
Why A Single Case Still Matters Today
Modern surveillance systems are designed to detect even one case of screwworm.
When an infestation is confirmed, veterinarians immediately notify the appropriate state animal health official and federal partners. Additional trapping, livestock inspections, and movement restrictions may follow to prevent the pest from becoming established.
The reason is simple.
History has shown that unchecked nws infestation can spread rapidly through livestock populations, creating enormous economic loss for ranchers and entire livestock industries.
For states with large cattle populations, including Texas, maintaining surveillance remains an essential part of protecting animal agriculture.
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Get a QuoteFrequently Asked Questions About Screwworms
Get your questions answered with these screwworm FAQs.
Contact usDo Screw Worms Infect Humans?
One of the most common online questions is: Do screw worms infect humans?
The answer is yes—but human cases are uncommon.
Like animals, people generally become infected only when adult flies deposit eggs into an existing wound or vulnerable tissue. Travelers returning from endemic regions occasionally account for reported human cases.
Prompt medical treatment is important because the larvae continue feeding until they are removed.
For pet owners and livestock producers, veterinarians should always evaluate suspected screwworm infections immediately.
Can Pest Control Companies Eliminate Screwworm?
This is where many people are surprised.
Unlike ants, termites, or mosquitoes, screwworm pest control isn’t about treating a house—it’s about protecting animals and preventing the fly from completing its life cycle.
If you suspect an active screwworm infection in a person or animal, the first call should be to a physician or veterinarian. Livestock owners should also contact local or state animal health officials when appropriate.
However, professional pest management still plays an important supporting role by helping property owners reduce nuisance fly populations, identify conducive conditions around barns, kennels, and animal facilities, and develop integrated fly control strategies that support overall biosecurity.
How To Get Rid Of Screwworms?
If you’re searching for how to get rid of screwworms, the answer depends on where they’re found.
An active screwworm infestation requires immediate veterinary or medical care to remove the larvae and properly treat the wound. Attempting to manage an active infestation with household pesticides or DIY methods is not appropriate.
Long-term prevention focuses on wound management, routine livestock inspections, prompt veterinary care, fly population monitoring, and continued support for regional eradication efforts.
Understanding the biology of screwworm is ultimately the best defense against its return.
Defend Your Home From Unwanted Pests
The screwworm remains one of the most fascinating—and consequential—pests ever encountered in North America. At Impact Pest Control, we're committed to helping homeowners, ranchers, and business owners understand the pests that affect their property, even when the solution begins with education. The more informed you are, the better prepared you'll be to protect your family, your animals, and your investment.