On Sunday, August 24, 2025, health authorities in the United States confirmed a chilling milestone: the first documented human case of New World screwworm, a rare and deadly flesh-eating parasite, in a patient who had recently returned from El Salvador. The announcement, made by the Maryland Department of Health in collaboration with the U.S. Centers for Disease Control and Prevention (CDC), has sent ripples of concern through both public health and agricultural sectors. While the immediate risk to public health is deemed low, the detection of this parasitic infection, caused by the larvae of the screwworm fly (Cochliomyia hominivorax), raises significant alarms due to its devastating potential for livestock and wildlife. This article explores the details of the case, the biology and dangers of the screwworm, the response from health and agricultural authorities, and the broader implications for public health, agriculture, and climate-driven disease spread in the United States and beyond.
The Case: A Traveler’s Tale of Parasitic Infection
The identification of the New World screwworm in a human patient marks a rare and alarming event in the United States, where the parasite was eradicated from domestic animal populations decades ago. According to the Maryland Department of Health and the CDC, the patient, whose identity remains confidential, had recently traveled to El Salvador, a country where screwworm remains endemic. The patient sought medical attention upon returning to the U.S., presenting with symptoms consistent with a screwworm infestation, likely stemming from an open wound that became a breeding ground for the parasite’s larvae.
Andrew G. Nixon, a spokesman for the U.S. Department of Health and Human Services, addressed the case in a statement to Reuters on August 24, 2025: “The risk to public health in the United States from this introduction is very low.” This reassurance reflects the rarity of human screwworm infections, which typically require specific conditions—such as an open wound in a tropical or subtropical environment—for the parasite to take hold. However, the case underscores the potential for imported infections in an increasingly interconnected world, where international travel can introduce exotic pathogens to new regions.
The patient’s condition required immediate and meticulous treatment, as screwworm infestations, known scientifically as myiasis, are both painful and potentially life-threatening if left untreated. The treatment process involves the painstaking manual removal of larvae from the affected tissue, followed by thorough disinfection to prevent secondary infections. The screwworm’s name derives from its unique and gruesome behavior: when disturbed, the larvae use their sharp, screw-like mouthparts to burrow deeper into living tissue, making complete eradication challenging. Health officials have not disclosed the patient’s current condition, but the case is being closely monitored to ensure no further spread occurs.
The Biology of the New World Screwworm
The New World screwworm (Cochliomyia hominivorax) is a parasitic fly native to the Americas, particularly prevalent in tropical and subtropical regions of South and Central America. Unlike other flies that feed on decaying matter, screwworm larvae are obligate parasites, meaning they require living tissue to survive. The adult female fly lays hundreds of eggs—often 200 to 400 at a time—in open wounds, sores, or mucous membranes of warm-blooded animals, including humans, livestock, and wildlife. Within 24 hours, the eggs hatch into larvae that begin feeding on the host’s living tissue, causing severe pain, tissue destruction, and, in severe cases, death due to systemic infection or organ failure.
The larvae’s sharp, barbed mouthparts allow them to burrow deep into flesh, creating tunnels that can lead to extensive tissue damage. This behavior, combined with their rapid reproduction cycle, makes screwworm infestations particularly dangerous. In humans, infections are rare and typically occur in individuals with untreated wounds exposed to the flies in endemic areas. Symptoms include intense pain, swelling, foul-smelling discharge, and systemic effects like fever or sepsis in advanced cases. In animals, the impact is far more widespread, as screwworms can infest livestock such as cattle, sheep, and goats, as well as wildlife like deer and dogs, leading to significant economic and ecological consequences.
The screwworm’s life cycle is rapid, with larvae maturing within 5 to 7 days before dropping to the ground to pupate and emerge as adult flies. This efficiency, combined with the female fly’s ability to lay thousands of eggs over her lifetime, enables screwworm populations to grow explosively if not controlled. Historically, screwworm infestations have caused catastrophic losses in the livestock industry, with untreated cases leading to mortality rates as high as 50% in affected herds.
Public Health Implications: A Low but Persistent Risk
While the Maryland case is the first confirmed human screwworm infection in the U.S., health authorities have emphasized that the risk to the general public remains low. Human infections require direct exposure to the fly, typically in regions where it is endemic, and are unlikely to occur in the U.S. due to robust sanitation, healthcare access, and the absence of established screwworm populations. The patient’s travel history to El Salvador, where screwworm is a known issue, likely explains the infection, highlighting the role of international travel in introducing rare pathogens.
The CDC and Maryland Department of Health are working together to monitor the situation, ensuring that no secondary cases emerge. Contact tracing is underway to identify individuals who may have been exposed to the patient, though the risk of human-to-human transmission is negligible, as screwworm is not contagious. Instead, the primary concern is preventing the introduction of the parasite into U.S. animal populations, where it could establish a foothold and spread rapidly.
Public health officials are also raising awareness about the importance of wound care for travelers visiting endemic areas. Simple precautions, such as keeping wounds clean and covered, using insect repellent, and seeking prompt medical attention for signs of infection, can significantly reduce the risk of screwworm infestation. The case serves as a reminder of the need for vigilance in an era of global mobility, where diseases once confined to specific regions can cross borders with ease.
Agricultural Threat: A Potential Economic Catastrophe
While the human health risk is low, the detection of screwworm in the U.S. has sounded alarm bells in the agricultural sector, particularly the cattle industry. The U.S. Department of Agriculture (USDA) estimates that a screwworm outbreak in Texas alone could cost the state’s economy approximately $1.8 billion due to livestock losses, reduced productivity, and the costs of containment and eradication. Nationally, the impact could be even more severe, given the importance of cattle to the U.S. economy, with beef and dairy industries contributing billions of dollars annually.
Screwworm infestations in livestock are devastating because they target healthy animals, often entering through minor wounds caused by routine procedures like branding, dehorning, or castration. Once established, the larvae cause rapid tissue destruction, leading to weight loss, reduced milk production, and high mortality rates if untreated. The parasite’s ability to spread quickly among herds makes it a formidable threat, capable of decimating entire populations in a matter of weeks.
The U.S. has a history of successfully combating screwworm, having eradicated it from domestic animal populations by the 1960s through an innovative sterile insect technique (SIT). This method involves releasing sterilized male flies into the environment, which mate with wild females but produce no viable offspring, gradually reducing the population. The success of SIT in the U.S., and later in Central America, is considered one of the greatest achievements in veterinary science, saving billions of dollars and protecting livestock industries across the region.
However, the reintroduction of screwworm poses a renewed threat, particularly as global temperatures rise, enabling the parasite to expand its range northward. Scientists have warned that climate change could facilitate the spread of screwworm from South and Central America into the southern U.S., where warmer conditions create a favorable environment for the fly. No animal cases have been confirmed in the U.S. in 2025, but the human case has heightened concerns about the potential for undetected introductions, particularly in states like Texas, Florida, and Arizona, which share ecological similarities with endemic regions.
Government Response: Proactive Measures to Mitigate the Threat
In response to the human case and the broader risk to agriculture, U.S. authorities are taking proactive steps to prevent a screwworm outbreak. Agriculture Secretary Brooke Rollins announced plans to construct a sterile fly facility in Texas, aimed at producing and releasing sterilized male screwworm flies to suppress potential populations. The facility, modeled on successful programs in Panama and other regions, will bolster the U.S.’s ability to respond quickly to any signs of screwworm reintroduction. “We are committed to protecting our livestock and wildlife from this devastating parasite,” Rollins said in a statement on August 23, 2025. “The sterile fly facility will be a critical tool in ensuring that screwworm does not regain a foothold in the United States.”
The sterile insect technique, pioneered by entomologist Edward F. Knipling in the 1950s, relies on mass-rearing male flies, sterilizing them with radiation, and releasing them in large numbers to outcompete wild males. This approach disrupts the reproductive cycle, as female flies mate with sterile males and produce no viable eggs. The method proved highly effective in eradicating screwworm from the U.S. by 1966 and from Central America by the early 2000s, with ongoing programs maintaining a barrier zone in Panama to prevent reinvasion from South America.
The USDA is also enhancing surveillance efforts, working with state agricultural departments to monitor livestock and wildlife for signs of screwworm. Veterinarians and farmers are being urged to report any unusual wounds or larval infestations immediately, as early detection is critical to preventing outbreaks. The agency has issued guidelines for wound management in livestock, emphasizing the importance of prompt treatment and fly control measures, such as insect repellents and physical barriers.
Climate Change and the Northward Spread of Screwworm
The detection of screwworm in the U.S. has intensified concerns about the role of climate change in driving the spread of tropical diseases. Rising global temperatures, coupled with changes in precipitation patterns and ecosystems, are expanding the range of pests and pathogens, including the screwworm fly. Historically confined to tropical and subtropical regions, screwworm has been detected in new areas in recent years, including parts of South America previously unaffected. Scientists warn that warmer winters and extended warm seasons in the southern U.S. could create conditions conducive to screwworm establishment, particularly in states like Texas, where large cattle populations provide ample hosts.
Climate change also exacerbates the challenges of controlling screwworm. Higher temperatures accelerate the fly’s reproductive cycle, allowing populations to grow more rapidly. Changes in wind patterns and increased international trade and travel further heighten the risk of introduction, as infected animals or humans can inadvertently carry the parasite across borders. The Maryland case, linked to travel from El Salvador, underscores this risk, highlighting the need for global cooperation in monitoring and controlling screwworm.
Economic and Ecological Stakes
The potential reintroduction of screwworm into the U.S. carries significant economic and ecological stakes. The cattle industry, a cornerstone of the U.S. agricultural economy, faces the greatest risk, as screwworm infestations can decimate herds and disrupt supply chains. The USDA’s $1.8 billion estimate for a Texas outbreak accounts for direct losses, such as dead livestock and reduced productivity, as well as indirect costs, including trade restrictions and consumer confidence. An outbreak could also affect international markets, as countries may impose bans on U.S. beef exports to prevent the spread of the parasite.
Ecologically, screwworm poses a threat to wildlife, particularly species with limited access to veterinary care. Deer, wild boars, and other mammals are susceptible to infestation, which can lead to population declines and disrupt ecosystems. The loss of wildlife could have cascading effects on biodiversity, predator-prey dynamics, and habitat stability, particularly in regions already stressed by climate change and habitat loss.
Public Awareness and Prevention Strategies
Raising public awareness is a critical component of preventing screwworm spread. The CDC and USDA are collaborating on educational campaigns to inform travelers, farmers, and veterinarians about the risks and symptoms of screwworm. For travelers to endemic areas, such as parts of Central and South America, the Caribbean, and the Horn of Africa, recommendations include protecting wounds with bandages, using insect repellent, and seeking medical attention for any signs of infection. Farmers are being advised to inspect livestock regularly, maintain hygiene in barns and pastures, and report suspicious cases to authorities.
The human case in Maryland also highlights the importance of international health cooperation. The CDC is working with counterparts in El Salvador and other endemic countries to share information and strengthen surveillance. Global initiatives, such as the Food and Agriculture Organization’s (FAO) screwworm control programs, aim to reduce the parasite’s prevalence in high-risk regions, minimizing the likelihood of introductions to non-endemic areas like the U.S.
The Broader Context: Emerging Infectious Diseases
The screwworm case is part of a broader trend of emerging infectious diseases driven by globalization and environmental change. In recent years, the U.S. has faced outbreaks of diseases like West Nile virus, Zika, and chikungunya, all of which have been linked to travel and climate factors. The reemergence of screwworm underscores the need for robust surveillance systems, rapid response mechanisms, and international collaboration to address these threats.
Public health and agricultural authorities are also exploring technological innovations to combat screwworm and similar pathogens. Advances in genetic engineering, such as gene drives to reduce fly populations, and improved diagnostic tools could enhance control efforts. However, these approaches must be balanced with ethical and ecological considerations to avoid unintended consequences.
Conclusion: A Call to Action
The detection of the New World screwworm in a human in the United States on August 24, 2025, is a sobering reminder of the interconnectedness of global health and agriculture. While the immediate risk to public health is low, the potential for screwworm to reestablish itself in U.S. livestock and wildlife populations demands urgent action. The case highlights the challenges of managing emerging infectious diseases in a warming world, where climate change and global travel create new pathways for pathogens.
The response from health and agricultural authorities, including plans for a sterile fly facility in Texas and enhanced surveillance, demonstrates a proactive approach to mitigating the threat. However, the success of these efforts will depend on sustained investment, public awareness, and international cooperation. For farmers, veterinarians, and travelers, vigilance is key, as early detection and prevention are critical to stopping screwworm in its tracks.
As the U.S. navigates this rare but serious threat, the screwworm case serves as a wake-up call, urging stakeholders to strengthen defenses against emerging diseases and protect the nation’s health, economy, and environment. The lessons learned from this incident could shape future strategies for managing parasitic threats, ensuring that the U.S. remains prepared for the challenges of a rapidly changing world.
