U.S. health officials confirmed the first human case of flesh-eating screwworm in August 2025. The U.S. Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC) reported that a patient in Maryland contracted the New World screwworm, Cochliomyia hominivorax, after returning from Central America. The infection originated during travel to either El Salvador or Guatemala, as different reports revealed slight inconsistencies in the official narrative. The confirmation immediately raised concern because this parasite has long been considered eradicated from North America. Authorities stressed that the infection was treatable and that the risk of wider public spread remained low, yet the announcement highlighted a troubling re-emergence of a dangerous parasite.
The CDC stated that while the patient experienced painful tissue infestation, doctors successfully treated the condition by surgically removing larvae and disinfecting the wound. “The infection is rare, but healthcare providers must remain alert to the possibility of travel-related screwworm cases,” the CDC advised. The agency emphasized that there was no evidence of local transmission in the United States and urged veterinarians and physicians to report suspected cases immediately.
What Is the Flesh-Eating Screwworm?
The New World screwworm fly, scientifically known as Cochliomyia hominivorax, belongs to the blowfly family Calliphoridae. Unlike most blowfly species, whose larvae consume dead tissue, the screwworm uniquely attacks living flesh, causing a condition known as myiasis. This parasitic behavior makes it especially dangerous for both animals and humans. The female screwworm lays 250 to 500 eggs on exposed wounds, the navel of newborns, or even natural orifices such as the nose, eyes, or perineum. When larvae hatch, they burrow into healthy tissue, feeding aggressively and “screwing” deeper when disturbed. This invasive feeding habit gives the parasite its chilling name.
The life cycle of C. hominivorax spans around 20 days. Larvae feed inside host tissue for up to seven days before falling to the ground to pupate. Adults emerge within another week. Females mate only once but can lay up to 3,000 eggs in their lifetime, storing sperm for continuous reproduction. Males, however, mate multiple times and typically remain hidden in vegetation until they detect females. This reproductive efficiency and long-range flight ability, up to 200 kilometers in a lifetime, make eradication challenging.
The parasite infects a wide range of warm-blooded hosts, from livestock such as cattle, sheep, and goats to wildlife and, in rare cases, humans. Cold-blooded animals and carrion do not support infestation, distinguishing screwworms from other blowfly species. Without timely treatment, infestations can cause severe tissue destruction, secondary infections, and even death of the host.
Historical Distribution and Threat of Screwworm
The screwworm fly spread across the southern United States, Mexico, Central America, and much of South America, reaching as far south as Uruguay and northern Argentina. It also existed on Caribbean islands such as Cuba, Trinidad, Jamaica, and Hispaniola. In favorable years, the parasite reached as far north as the Canadian border. Its presence devastated livestock industries because infestations left animals weakened, disfigured, or dead, costing millions in economic losses.
By the 1950s, the U.S. Department of Agriculture (USDA) recognized the screwworm as a national agricultural emergency. In response, American scientists developed a revolutionary biological control strategy called the Sterile Insect Technique (SIT). The approach involved mass-rearing screwworm flies, sterilizing males with radiation, and releasing them in infested areas. Because females mate only once, pairing with sterile males ensured no viable offspring. This program achieved remarkable success. After early trials in Curaçao in 1954, USDA eradicated the screwworm from the Southeastern and Southwestern U.S. by the 1960s and officially declared the species eliminated nationwide in 1982.
Subsequent cooperative campaigns eliminated the parasite from Central American countries, including Guatemala, Belize, El Salvador, Honduras, Nicaragua, and Costa Rica, between 1988 and 2000. In 1998, sterile fly releases in Panama created a permanent barrier zone at the Darién Gap, preventing re-invasion from South America. However, the parasite remained endemic in tropical and subtropical parts of South America and some Caribbean islands.
Re-Emergence of Screwworm in Latin America
Despite decades of success, the screwworm has recently staged a comeback. Outbreaks in southern Mexico and Central America in 2024 and 2025 breached the protective barrier in Panama. Reports confirmed widespread animal infestations, prompting the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) to provide emergency support. In July 2025, the U.S. government even halted livestock imports from Mexico to prevent reintroduction of the parasite into American herds.
The USDA warned that a screwworm outbreak in Texas, the country’s largest cattle-producing state, could cause economic damage of up to $1.8 billion annually. Such figures underscore the parasite’s potential to disrupt food security and agricultural livelihoods. In response, USDA is prepared to breed billions of sterile flies in Mexico and release them aerially over infested regions beginning in 2026, continuing the proven SIT strategy.
The Maryland Human Case in Context
The detection of a human case in Maryland in 2025 served as a stark warning. The patient, whose travel history pointed to Central America, became the first confirmed U.S. human screwworm case in decades. While myiasis in humans remains rare compared to animals, the risk increases when travelers return from endemic regions. The U.S. Department of Health and Human Services assured the public that “this is an isolated travel-related infection” and not evidence of a domestic outbreak.
Medical experts described the symptoms as severe pain, visible larvae within flesh, and secondary tissue infections. Treatment required immediate removal of larvae, disinfection, and in some cases, reconstructive surgery. The CDC urged physicians to consider screwworm in the differential diagnosis when evaluating unusual wound infections in travelers from Latin America.
The incident also revealed communication challenges. While some reports identified El Salvador as the country of infection, others traced it to Guatemala. This inconsistency highlighted the difficulty of tracking cross-border parasite movements and reinforced the need for stronger collaboration between regional health authorities.
Broader Implications and Future Response
The reappearance of screwworm in humans, combined with outbreaks in livestock across Latin America, reminds us that eradication is never permanent. Climate change, increased animal trade, and global travel all increase the risk of resurgence. U.S. authorities continue to strengthen surveillance systems at borders, monitor livestock movements, and invest in new technologies for sterile male production.
Researchers are also exploring genetic sterilization methods to make SIT more efficient. While such innovations remain experimental, they hold promise for reducing costs and improving scalability. Until then, the traditional SIT strategy, releasing irradiated sterile males, remains the frontline defense.
The CDC concluded in its advisory that the Maryland case should not alarm the general public but should serve as “a critical reminder for clinicians and veterinarians to remain vigilant.” The USDA echoed this by pledging to “do everything possible to protect U.S. agriculture from the devastating effects of this parasite.”
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Conclusion
The confirmation of the first human screwworm case in the U.S. in decades marks both a medical event and an agricultural warning. Although authorities swiftly treated the patient and contained the threat, the parasite’s resurgence in Latin America and its potential northward spread present ongoing challenges. The case underscores the importance of surveillance, rapid response, and international cooperation. History shows that scientific innovation, such as the sterile insect technique, can achieve eradication, but only constant vigilance can keep this flesh-eating parasite at bay.
