A screwworm infestation occurs when the larvae of screwworm flies burrow into the living tissue of a warm-blooded animal or human and feed on it from the inside. Unlike most maggots, which only consume dead tissue, screwworm larvae eat healthy flesh, earning them the nickname “flesh-eating.” The entire life cycle takes about three weeks, but untreated infestations can cause severe tissue destruction, secondary infections, and death in animals.
How Infestations Start
Female screwworm flies are drawn to the smell of any open wound on a warm-blooded host. The wound doesn’t need to be large. A tick bite, a scratch from barbed wire, or the raw umbilical stump of a newborn calf is enough to attract a female. She lays her eggs in a neat, shingle-like mass along the wound’s edge. A single female can lay up to 3,000 eggs over her lifetime.
The flies also target natural body openings: nostrils, eyes, ears, mouth, and genitals. This means an animal or person doesn’t necessarily need a visible wound to become infested. Within a few hours of being laid, the eggs hatch into tiny larvae that immediately burrow into the surrounding tissue and begin feeding. Their bodies are ringed with small spines that make them look like screws drilling into wood, which is how the insect got its name.
What the Larvae Do Inside a Wound
Once embedded, the larvae feed on living soft tissue for about seven days. During that time, they can grow up to 15 millimeters long. The wound doesn’t heal. Instead, it expands and deepens as more tissue is consumed, often attracting additional female flies to lay eggs in the same site. This “re-infestation” cycle can turn a small puncture into a gaping, infected cavity in a matter of days.
After roughly a week of feeding, the mature larvae drop out of the wound, fall to the ground, and burrow into the soil to pupate. Depending on temperature and humidity, the adult fly emerges from the soil anywhere from 7 to 54 days later. Females mate once, about three days after emerging, and then begin searching for wounds to start the cycle again.
Signs of Screwworm Infestation
The hallmark sign is a wound that refuses to heal and gets progressively worse for no obvious reason. Other signs include:
- Pain and restlessness in the affected animal, or localized pain in humans
- Bleeding or oozing from a wound that should be closing
- A strong, foul smell from the wound site
- Visible maggots or the sensation of movement inside the wound
In livestock, infested animals often stop eating, isolate from the herd, and show obvious distress. In humans, infestations can occur anywhere on the skin and also inside the nose, mouth, eyes, ears, or genital area. Because the initial wound may be tiny, people sometimes don’t notice an infestation until pain, swelling, or the smell becomes impossible to ignore.
Treatment in Humans
Human cases are treated by physically removing the larvae, sometimes under local or general anesthesia. The goal is to extract every larva completely, because fragments left behind trigger intense inflammation. Agents like hydrogen peroxide or chloroform in oil are sometimes applied to the wound to help kill larvae and make removal easier. Suffocating substances like petroleum jelly, mineral oil, or beeswax can also be used to cut off the larvae’s air supply, encouraging them to back out of the tissue on their own.
After removal, the wound is cleaned, dead tissue is trimmed away, and antibiotics are used if infection has set in. Antiparasitic medications taken by mouth have been used successfully in some cases, particularly when larvae are in hard-to-reach locations like inside the nasal passages.
Treatment in Animals
In livestock, most infested wounds are treated with a topical larvicide, a chemical that kills the larvae in place. The wound is then left open to heal on its own rather than being stitched closed, and treatment may need to be repeated. Studies in sheep have shown that topical antiparasitic treatments are significantly more effective than injectable ones, especially against older, larger larvae.
For dogs, cats, and other small animals, larvae are typically removed by hand under sedation or anesthesia. This is often combined with antiparasitic drugs to kill any larvae the veterinarian can’t physically reach. Some newer flea-and-tick medications have shown promise in killing screwworm larvae within 24 hours, though dead larvae sometimes remain in the wound and need to be flushed out.
Infested animals must be quarantined on a hard surface like concrete so that any larvae dropping from the wound can be found and destroyed before they pupate in the soil and produce a new generation of flies.
Where Screwworms Are Found Today
The New World screwworm was eradicated from the United States in 1966 through a massive government program, and a small outbreak in the Florida Keys was eliminated in 2017. Today, the fly remains present in parts of South America, the Caribbean, and Mexico. A related species, the Old World screwworm, occurs in parts of Africa, the Middle East, and Southeast Asia.
As of mid-2026, active screwworm cases in northern Mexico have been detected as close as 2 miles from the U.S. border. Cases in the Mexican states of Chihuahua, Coahuila, and Nuevo León have involved cattle, horses, and dogs. The USDA tracks every confirmed case within 100 miles of the border and updates its public map twice weekly. Earlier in 2026, screwworm-infested zones were declared in parts of Texas and New Mexico, though those zones were released by late August after the threat was contained.
Mexico’s animal health service classifies a case as “active” at diagnosis, then revisits after 15 days. If inspectors find no new wounds or larvae, the case becomes inactive.
How the U.S. Keeps Screwworms Out
The primary defense is the sterile insect technique, one of the most successful pest control programs in history. A facility in Pacora, Panama, jointly run by the USDA and Panama’s agriculture ministry, produces more than 100 million sterile screwworm pupae per week. These pupae are exposed to gamma radiation, which makes the resulting flies unable to reproduce but doesn’t affect their ability to survive or mate normally.
Sterile flies are released by aircraft over target areas in Central America and along the border zone, creating a biological barrier. Because female screwworm flies mate only once in their lives, a female that mates with a sterile male will lay eggs that never hatch. Over time, this drives the wild population down. Ground releases from trucks or stationary crates supplement the aerial drops in areas that need targeted coverage.
To tell sterile flies apart from wild ones, the USDA marks pupae with a fluorescent dye that carries over to the adult flies and glows under UV light. This lets officials quickly rule out sterile flies when they show up in traps or are reported by the public.
Preventing Infestations in Livestock
If you raise animals in or near a screwworm-active area, wound management is the single most important preventive step. Treat the umbilical cords of all newborns with an approved insecticide immediately after birth. Any wound on any animal, no matter how small, should be treated promptly.
Postpone procedures that create open wounds. Dehorning, branding, castration, ear notching, tail docking, and shearing all create exactly the kind of injuries that attract egg-laying females. If these procedures can’t wait, apply insecticide to the wound site and monitor the animal closely in the days that follow. Routine inspection of your herd for unexplained wounds, restlessness, or foul-smelling sores helps catch infestations before they spiral. Any animal you suspect is infested should be seen by a veterinarian immediately and isolated on a hard surface until treatment is complete.