Key Points
- New World screwworm (NWS) is a parasitic infestation caused by Cochliomyia hominivorax blowfly larvae, which consume healthy, living flesh of warm-blooded animals.
- NWS infestation poses a significant threat to global and local food supplies, as well as regional economic stability.
- Implementation of the sterile insect technique eradicated NWS in the U.S., Mexico and most of Central America by the early 2000s.
- In 2023, outbreaks of NWS began spreading north through Central America and Mexico again.
- Transboundary cooperation will continue to be vital to slow the spread of disease and protect humans and animals alike.
New World screwworm (NWS) is a parasitic infestation caused by Cochliomyia hominivorax blowfly larvae burrowing into the living tissue or flesh of warm-blooded animals. It is endemic in parts of the Caribbean and most of South America, where annual economic losses caused by NWS have been estimated to be ~$3.6 billion. The parasite poses a significant threat to global and local food supplies, as well as regional economic stability, and is listed by multiple international organizations, including the World Health Organization (WHO), as 1 of the 6 priority transboundary animal diseases (TADs) to be eliminated in the Americas.
In an brilliant display of international collaboration, scientists and diplomats from North and Central America, temporarily eradicated NWS in the United States, Mexico and most of Central America by the early 2000s. The invention, utilization and international adoption of the Sterile Insect Technique took decades of cooperation and effort. Then, in 2023, new outbreaks of NWS, once again, began spreading northward through Central America and Mexico, and in August 2025, the first human case of NWS in the U.S. in almost a decade was reported in Maryland. Moving forward, transboundary cooperation will continue to be vital to slow the spread of disease and protect humans and animals alike.
New World Screwworm Myiasis
C. hominivorax is unusual in that its larvae only consume healthy, living flesh and never dead tissue. NWS infestation, or myiasis, can affect any warm-blooded animal, including humans. The name screwworm comes from the eating habit of the larvae. Once they invade an open wound, the larvae burrow into it like a screw into wood, tearing at the host's tissue as they eat into the flesh.
The infection cycle begins when adult screwworm flies, which are approximately the size of a regular house fly with reddish orange eyes, metallic blue or green bodies and 3 dark stripes along their backs, lay their eggs at the wound sites of warm-blooded animals. Any open wound can be infected, including cuts, newborn navels and even an injury as small as a tick bite.
NWS infestations can cause spontaneous abortion in animals, decrease milk production, reduce body weight and hinder future fertility. As more maggots hatch and feed on living tissue, the wound becomes larger and deeper. This increases the chances of sequelae and further infestation as the wound attracts additional flies. The resulting infestation scars reduce leather quality, further lowering the value of many livestock.
Treating infected animals is expensive, highly labor-intensive and unpleasant. The animals require quarantine, anti-parasitic medications and the physical removal of hundreds of larvae and thorough wound disinfection. An infestation can kill infected cattle within 10 days, so treatment must be timely and consistent. Additional complications arise because, after decades of eradication, most cattle ranchers no longer have the experience or tools to diagnose and treat NWS. Therefore, infection prevention remains the best solution. Traditionally, NWS prevention involved chemical pesticide treatments not only for animals, but soil and plantlife as well, risking further environmental harm.
Eradication Through Sterile Insect Technique
The search for an alternate preventive to chemical pesticide treatments began in the late 1930s with scientists working at the U.S. Department of Agriculture (USDA) Laboratory in Menard, Texas. At that time, the screwworm was devastating livestock herds, including red meat and dairy supplies across the Southern U.S., Mexico, Central America and South America. Edward Knipling developed the theory of autocidal control-breaking the reproductive cycle. His colleague Raymond Bushland's enthusiasm for Knipling's theory sparked the pair to search for a way to rear flies in a "factory" setting, and to find an effective way to sterilize them.
Their solution, the Sterile Insect Technique (SIT), is one of the most effective and environmentally friendly pest control methods currently available. It involves mass breeding insects and then exposing them to ionizing radiation. The infertile male flies are then released to mate with wild females, resulting in no offspring from that pairing. This strategy is particularly effective because female C. hominivorax adults only mate once, whereas males can mate many times. This means that 1 sterile male fly can effectively prevent high numbers of future larvae. To remain competitive with their fertile counterparts, it is important that the sterile males are not impaired in any way. Thus, healthy breeding practices are important. 
This method differs from other conventional forms of pest control in a variety of ways. Because the flies are released into their native environment, there is no concern about introducing an invasive species and damaging the existing ecosystem. Additionally, sterile insects cannot self-replicate, so they cannot establish themselves in the environment. Unlike pesticides or other chemical controls, SIT is species-specific and, therefore, minimizes downstream effects on other species.
The first trial to eliminate NWS in the wild occurred in 1954 on the island of Curaçao, north of Venezuela. Within 4 months, the New World screwworm fly was successfully eradicated from the island. This success led to the adoption of the technique in the continental U.S., and C. hominivorax has been considered extinct there since 1966. In the years since this first proof of concept, researchers have continued to develop more competitive strains of screwworm flies and improved fly cultivation processes.
In 1991, Knipling and Bushland's technique halted a serious outbreak of NWS in northern Africa. Both Bushland and Knipling received worldwide recognition for their leadership and scientific achievements, including the 1992 World Food Prize. The technique was hailed by former U.S. Secretary of Agriculture Orville Freeman as "the greatest entomological achievement of the 20th century."
Eradication efforts saw further success in many Caribbean islands and Central American countries, including Puerto Rico, the Virgin Islands, Guatemala, Honduras and others. The first sterile insects were released in Panama in 1998, and that country was declared screwworm-free in 2006.
Successful eradication in Panama was crucial because the unique geography of Panama creates a natural barrier for screwworm as well. Northbound travel from South America requires crossing the difficult terrain of the Darien Gap, a remote landbridge composed of dense, mountainous rainforest between the northern and southern continents. Continued sterile fly releases act as a secondary living insect barrier by preventing new fly populations from taking hold north of the Isthmus of Panama.
The release of sterile insects has proved to be an effective control against a variety of other insect vectors, including Aedes mosquitoes, which carry dengue virus, the tsetse fly, which causes African sleeping sickness, and various fruit flies and moths responsible for food crop losses around the world.
The Case for Deliberate Extinction
The risks posed by destructive parasitic organisms, like screwworm, tsetse flies and others, to public health, ecosystems and the global food supply raise additional ethical questions about their continued existence. A 2025 publication in the journal Science discusses whether the deliberate extinction of a species might be ever be warranted. The authors lay out their criteria for such an extreme step using NWS as one of their examples.
NWS is a candidate for elimination because it poses a significant threat to public health and food security due to its broad host range and the extreme suffering and death the larvae cause for other animals and humans. The difficulty in treating an infestation and the speed at which severe damage occurs are important decision-making factors. Additionally, the screwworm can be a threat to the survival of other species. A 2016 outbreak in the Florida Keys killed at least 135 endangered key deer out of a population of approximately 1,000. Mortality rates this high in already low populations can have a profound effect on the future for that species. 
Further, eradication would not have substantial negative environmental impacts, as screwworms are not pollinators and do not fulfill a unique feeding niche for another species.
Even as the authors lay out a justification framework for the complete eradication of a species, they caution about the ethical and ecological implications of taking such a step, stating that intentional extinction should only be considered in extreme and rare circumstances.
The Need for International Cooperation
The effects of screwworm infestation are not limited to single communities, and the issue (as well as their future existence) should be evaluated on an international scale. Before eradication, screwworm populations would overwinter in Mexico and Central America. This meant that a U.S.-Mexico partnership was crucial for success. Efforts in these countries relied on an increased concentration of sterile flies, first from a production facility in Mission, Texas, and later from a facility in Chiapas, Mexico. In 1972, a severe outbreak in the U.S. demonstrated the continued risk of spread due to the flies' flight range (12 miles in a fly's lifetime) and the fact that they can be transported long distances while burrowed inside their hosts. The Screwworm Eradication Program Agreement between the U.S. and Mexico was signed the same year.
As a result, North and Central America successfully eradicated NWS until 2023, when the flies suddenly overpowered the insect barrier and the difficult terrain of the Darien Gap at the southern tip of Panama. The exact cause of this breach is unclear but is most likely due to multiple factors, including interruptions in sterile fly production due to the COVID-19 pandemic and illegal cattle imports, as well as the challenges involved in surveillance of the gap's difficult geography.
Regaining and maintaining control will require just as much international cooperation as was required during the original eradication. A facility specially designed to breed and sterilize screwworms in Panama releases 100 million sterile flies every week, but more factories will be needed to stop the spread as the parasites continue to migrate north through Mexico. To prevent the infection of cattle in the U.S., cattle imports from Mexico were halted in May 2025.
After increased release efforts on the part of the Mexican government, the NWS prevention program faced a tragic setback in June 2025 when a plane carrying sterile flies crashed in southern Mexico. Both Guatemalan pilots and a Mexican agronomist engineer were killed. More frequent fly releases are part of a strategy coupled with a $21 million investment to convert a fruit fly factory in Mexico to produce more sterile screwworms. Once the conversion is complete, the factory will be able to release between 60-100 million sterile flies per week. The USDA has stated the desire to re-open the border to cattle imports by the end of 2025 and is continuously reevaluating the situation.
Not Just Livestock
Cattle imports are of particular concern due to multi-year drought conditions that have reduced available grazing lands in the U.S. Cattle herd numbers are currently lower than they have been since the 1950s, and the ban on imported Mexican cattle is expected to drive already high prices even higher on related products. For families already facing food insecurity, rising food costs will increase risk for downstream health effects, such as chronic disease and childhood developmental problems. NWS not only infects livestock; pets, wildlife and humans are also vulnerable. People who spend a lot of time outdoors, e.g., construction or agricultural laborers, as well as people who sleep outside, have increased chances of infection.
Containing the spread of NWS illustrates the importance of One Health collaborations and solutions, which according to T Rubin et al., are most successful when interdisciplinary teams with shared goals, values and a unified task are faced with a shared and imminent threat. These efforts often require partnerships with unfamiliar disciplinary, organizational and national cultures; thus, a foundation of mutual trust and respect is necessary. 
Key factors that made NWS an excellent candidate for collaborative eradication efforts include the fact that the effects of infestation are clearly observed and heavily felt (both from a health and economic standpoint). Additionally, the timeline for outbreaks is easily understood. Such clarity of the problem and potential solutions establishes an ideal situation for multiple nations, agencies and people to work together to prevent NWS spread.
And in this case, international cooperation has proved to be cost-effective and highly successful. For example, the cost-benefit ratio for the joint Costa Rica-U.S. screwworm eradication program ranged from 4.8-12.8, meaning that for the $41 million price tag, Costa Rica has seen benefits ranging from $168 million to $448 million-an excellent return on investment for both the Costa Rican and American governments.
As Nobel Prize winner Ahmed Zewail, Ph.D., noted, "The soft power of science has the potential to reshape global diplomacy." International partnership development and maintenance have the potential not just to stop the spread of NWS, but also to address many other health and social threats.
NWS are unique in that they eat only living, healthy flesh, but the larvae of other species of flies, also known as maggots, have been used for centuries to promote wound healing and prevent infection.
