A screwworm is the larval stage of a parasitic fly that feeds on the living flesh of warm-blooded animals, including humans. Unlike common blowflies that only lay eggs on dead tissue, the New World screwworm (Cochliomyia hominivorax) is an obligate parasite, meaning its maggots can only survive by burrowing into and consuming live tissue. This distinction makes it one of the most destructive parasitic insects in the Western Hemisphere, responsible for billions of dollars in livestock losses before a landmark eradication campaign wiped it out across most of North America.
How Screwworms Differ From Other Maggots
The term “screwworm” refers specifically to the larvae of the fly Cochliomyia hominivorax, a member of the blowfly family. The name comes from the larva’s appearance: rows of small, pointed spines encircle each body segment, giving it a screw-like look. Mature larvae range from about 6.5 to 17 mm long, tapering to a point at the head end and blunt at the rear.
The critical distinction is between “primary” and “secondary” screwworms. The primary screwworm (C. hominivorax) targets living tissue exclusively. It doesn’t wait for an animal to die. A secondary screwworm (Cochliomyia macellaria) looks similar but feeds mainly on dead or decaying tissue, making it far less dangerous. When people talk about screwworms as a threat, they mean the primary species.
One identifying feature unique to primary screwworm larvae is their dark breathing tubes, which are pigmented enough to be visible through the body wall. This trait helps labs confirm an infestation quickly, which matters because every confirmed case triggers an emergency response.
How an Infestation Happens
A female screwworm fly is attracted to any wound on a warm-blooded animal, no matter how small. A tick bite, a scratch from barbed wire, even the fresh navel of a newborn calf can be enough. She lays her eggs at the wound’s edge, and once they hatch, the larvae burrow headfirst into the living tissue and begin feeding.
The wound deteriorates rapidly over just a few days. As the larvae eat and grow, the lesion deepens and widens, producing a foul-smelling odor and often bleeding from the open sore. The smell attracts more female flies, which lay additional eggs in the same wound. Left untreated, a single small cut can become a massive, life-threatening cavity of destroyed tissue. Livestock can die within 7 to 14 days if heavily infested.
Bacteria frequently colonize the wound alongside the larvae, potentially causing fever, chills, and systemic infection. In animals, behavioral signs include restlessness, loss of appetite, and isolation from the herd.
Symptoms in Humans
Screwworm infestations in humans are less common than in livestock but follow the same pattern. Any open wound or sore can serve as an entry point. The larvae can also establish in the ears, nose, eyes, or mouth. Symptoms reported by the CDC include painful skin wounds that worsen noticeably within a few days, a strong foul smell from the infestation site, visible bleeding, and the sensation of larvae moving within the wound. Some people can actually see the maggots in the affected area. Secondary bacterial infection can follow, bringing fever and chills.
Treatment requires physical removal of every larva and egg, sometimes surgically. The wound is then re-examined 24 to 48 hours later to confirm nothing was missed. Any remaining larvae must be found and destroyed, because even a few surviving maggots that mature into adult flies could seed a new population in the local environment.
Where Screwworms Exist Today
Screwworms are not currently present in the United States, but they remain active in parts of Central and South America. As of early 2026, Mexico has reported active cases in several states, including Nuevo León, Tamaulipas, San Luis PotosÃ, and Querétaro. The USDA continues to release roughly 100 million sterile insects per week in Mexico to keep the fly from spreading northward, and closely adjusts these efforts based on where new cases appear.
The risk of reintroduction to the U.S. is not zero. Any animal transported from an affected region could carry larvae, and a single pregnant female fly establishing in a warm climate could spark a local outbreak.
The Eradication Campaign That Changed History
Before eradication, screwworms cost American ranchers staggering sums. In the 1950s and 1960s, producers in the southwestern United States lost between $50 and $100 million annually (not adjusted for inflation). Texas alone faced costs of over $132 million per year in 1976 dollars for managing outbreaks. Adjusted for modern inflation, an outbreak on that scale could cost Texas producers $732 million per year and drain $1.8 billion from the state economy.
The solution came from a remarkably elegant biological strategy called the sterile insect technique, developed by USDA scientists Edward Knipling and Raymond Bushland. The concept: flood the environment with male screwworm flies that have been sterilized by exposing their pupae to gamma radiation. The radiation damages chromosomes in the sperm, so when a sterilized male mates with a wild female, her fertilized eggs fail to develop. The embryo dies during early cell division and the eggs never hatch.
The key is overwhelming numbers. To drive a population toward extinction, the program had to maintain a ratio of at least 10 sterile males for every wild male in the ecosystem. At peak production, facilities were churning out 150 million sterile flies per week. Generation after generation, wild females mated mostly with sterile males, producing fewer and fewer offspring until the population collapsed entirely. The United States was declared officially free of screwworms in 1966.
By 1996, the annual economic benefit to U.S. livestock producers from eradication was estimated at $796 million, with roughly $2.8 billion in wider economic benefit. The program remains one of the most successful pest eradication efforts in history, and it continues today along Mexico’s southern border as a living barrier against reinfestation.
Why Screwworms Still Matter
Even though screwworms have been absent from the U.S. for decades, the ongoing release of sterile flies is the only thing maintaining that status. The fly thrives in warm, humid climates and reproduces quickly. A lapse in the sterile insect program, or a shift in the fly’s range driven by changing conditions, could bring screwworms back to the southern United States. In 2016, a screwworm outbreak in the Florida Keys among endangered Key deer demonstrated how quickly the fly can appear in unexpected places and how rapidly agencies must respond to contain it.
For travelers to Central and South America, the practical concern is straightforward: keep wounds clean and covered. Any open sore that worsens rapidly, develops a foul smell, or shows signs of maggot activity in a region where screwworms are present warrants immediate medical attention.