A squirming larva that screws itself deeper into living flesh as it feeds? Yeah, a screwworm sound like something a horror writer invented after a very bad camping trip.
Unfortunately, they’re real. Even more unfortunately, they’re flies.
The New World Screwworm (NWS) is a parasitic fly whose larvae (maggots) can infest wounds and feed on living tissue in warm-blooded animals. That includes livestock, wildlife, pets and, in rare cases, humans. Little overachievers, really.
So, can humans get screwworm? Yes, but human cases are rare. The bigger concern is what these insects can do to animals, especially livestock, and why fast identification and control matter so much.
In this article, we explore:
A Screwworm is a parasitic flies whose larvae feed on living tissue. According to the Centers for Disease Control and Prevention (CDC), the fly lays its eggs in open wounds and body openings, including the eyes, ears, nose, mouth or genitals. (If your whole body just tried to leave the room, that’s a normal response.) A wound as small as a tick bite or scratch can attract a female fly.
The New World screwworm, known scientifically as Cochliomyia hominivorax, is the species causing concern in the Americas. Adult flies look similar to other flies, but their larvae are what make them dangerous.
Once the eggs hatch, the larvae burrow into tissue and start feeding. And no, they don’t politely stick to dead or decaying material like some other fly larvae. New World screwworm larvae feed on living tissue, which is why infestations can get severe fast.
The name “screwworm” comes from the larvae’s appearance and behavior. As they feed, they burrow into tissue like a screw twisting into a surface — which is exactly as unpleasant as it sounds.
Screwworm larvae are dangerous because they feed on living flesh and can make wounds worse within days.
Many people hear “maggots” and think of insects feeding on dead tissue. New World screwworm are different. Their larvae infest live animals and enlarge wounds as they feed. Without treatment, infestations can lead to pain, tissue damage, secondary infections and even death in severe cases.
Basically, if there’s an open wound, this fly sees a five-star resort. Horrible? Yes. Biologically fascinating? Also yes.
That’s what makes the screwworm such a serious threat: They don’t just show up after damage has already happened; they actively make that damage worse.
Screwworm symptoms can include a painful wound that worsens, visible larvae, bleeding, foul odor and signs of infection.
In animals, screwworm infestations often begin around a wound. At first, the problem may look like an ordinary injury. But as larvae feed, the wound gets larger, deeper and more irritated.
According to the CDC, common signs of New World Screwworm infestation may include:
In livestock and pets, behavioral changes can also be a clue. Animals may lick, bite, scratch, isolate themselves or act unusually uncomfortable.
Because infestations can worsen quickly, suspected screwworm cases should be taken seriously. For animals, that means contacting a veterinarian or animal health authority. For people, that means seeking medical care. A healthcare provider is the only one that should be removing any maggots or egg masses, so spare yourself the trauma and don’t try it yourself.
Screwworms are a serious cattle concern because infestations can spread quickly, harm animal health and create major costs for livestock producers. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (USDA APHIS) notes that another New World Screwworm incursion in the U.S. could cost millions of dollars from livestock losses, trade embargoes and eradication work.
Cattle are also more at risk because they’re vulnerable to frequent small wounds. Bites, scratches, branding sites, dehorning wounds, surgical sites and everyday injuries can all attract female screwworm flies. Once larvae hatch, they can damage tissue and worsen the wound.
USDA APHIS maintains a confirmed detections dashboard for New World Screwworm animal and wild fly detections in the United States. Because case counts change quickly, the dashboard is the best place to find the latest details, including location, the animals affected and when each case was confirmed.
For ranchers and farmers, the risk isn’t just one sick animal. Screwworm infestations can lead to:
Cattle also can’t walk into the house and say, “Hi, something deeply unsettling is happening over here.” That’s where livestock producers, veterinarians, inspectors and entomologists come in.
Entomologists help identify screwworm by examining larvae, adult flies and infestation patterns to confirm the species and guide response efforts.
Proper identification matters because not every maggot in a wound is a New World Screwworm. Different fly species can look similar, especially to someone who doesn’t spend their career thinking about morphology.
Mature New World screwworm larvae have spines that wrap around the body in a spiral pattern, which is where the name “screwworm” comes from. But confirming the species usually requires a trained entomology specialist.
Entomologists may use features such as larval shape, body spines and internal structures to identify the species. Their work supports:
This is also where medical entomology comes into the picture. Medical entomology focuses, in part, on insects and arthropods that affect human and animal health. Screwworms are a textbook case: They connect insect biology with wounds, disease risk, animal welfare and public health response.
Screwworm control depends on fast detection, proper treatment, surveillance, movement controls and sterile insect release.
A strong response may include:
One of the most important tools used against New World Screwworm is the sterile insect technique. It helped to remove the NWS fly population from the U.S. in 1966.
The basic idea is surprisingly elegant for something involving millions of flies: Sterile male flies are produced and released into targeted areas. Because a female screwworm mates only once, when they mate with a sterile male, no viable offspring are produced. Over time, this reduces the population and helps stop the pest from spreading.
Essentially, it’s pest control by way of reproductive sabotage.
This is one reason entomology matters so much in real-world pest management. Effective control depends on understanding how the insect lives, travels, reproduces and interacts with animals and people.
If this disturbing-yet-fascinating insect left you wanting more, the University of Florida’s online graduate entomology programs allow students to study insect science in a flexible format built for working professionals.
Through UF’s online graduate entomology programs, you can explore topics related to insect biology, pest management, medical entomology, ecology or beekeeping while learning from faculty with deep expertise in the field.
If you’re curious about the wide world of entomology, UF’s entirely online programs offer a way to study the small organisms behind some very big questions. Check out UF’s online graduate entomology programs to find the one that best suits your career goals.
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