New World screwworm: The flesh-eating parasite returning to the spotlight

Editor’s note: This article originally appeared on ContagionLive.com, a sister publication to dvm360.

For decades, the New World screwworm (Cochliomyia hominivorax; NWS) was considered one of the greatest success stories in veterinary medicine and agricultural public health.¹ Through an unprecedented international eradication campaign utilizing the Sterile Insect Technique (SIT), the parasite was eliminated from the United States in 1966 and later pushed south into Central America.² Many veterinarians and physicians practicing today have never encountered a case firsthand.³

However, NWS has recently returned to national headlines. Since 2023, outbreaks have progressively moved northward through Central America and Mexico, prompting heightened surveillance by the USDA⁴ and the CDC.⁵ In June 2026, USDA confirmed new detections in livestock in Texas, representing the first domestic cases in approximately 6 decades and triggering aggressive containment efforts.⁴

Despite sensational media descriptions of a "flesh-eating parasite," understanding the biology of screwworm is essential. Unlike many fly larvae that feed on dead or decaying tissue, NWS larvae actively consume living tissue, making them uniquely destructive. Their impact extends beyond animal suffering to substantial economic consequences for livestock industries, wildlife conservation, and international trade.

Fortunately, decades of scientific research have provided effective methods for diagnosis, treatment, surveillance, and prevention. Understanding this parasite is critical for veterinarians, physicians, livestock producers, wildlife managers, and the public.

A remarkable history: From agricultural disaster to scientific triumph

The history of NWS illustrates both the devastating consequences of parasitic disease and the power of applied science. Screwworms have likely existed throughout the Americas for thousands of years, affecting wildlife long before the introduction of domestic livestock. With the expansion of cattle ranching across the southern US during the nineteenth and early twentieth centuries, the parasite became one of the livestock industry's greatest challenges.

Annual economic losses reached hundreds of millions of dollars when adjusted for today's economy. Ranchers routinely encountered calves, sheep, goats, horses, and even pets suffering from severe infestations. Animals frequently died if treatment was delayed.

The name "screwworm" does not refer to the adult fly but rather to the behavior of its larvae. As they burrow deeper into living tissue, they appear to twist like a screw being driven into wood. This invasive feeding pattern distinguishes screwworm from common blowflies, whose larvae typically consume only dead tissue.

A revolutionary breakthrough occurred during the 1950s through the work of entomologists, particularly Edward F. Knipling.⁶ They developed the SIT, in which millions of laboratory-raised male flies are sterilized through irradiation and released into the environment. Female screwworm flies mate only once during their lifetime. If that single mating occurs with a sterile male, no viable offspring are produced, causing local populations to collapse over time.

This innovative strategy⁷proved extraordinarily successful. By 1966, NWS had been eradicated from the US. International cooperation subsequently pushed the barrier progressively southward through Mexico and Central America, with ongoing surveillance programs designed to prevent re-establishment.

The current resurgence demonstrates that eradication requires continual vigilance. Once surveillance barriers weaken or populations expand beyond containment zones, the parasite can rapidly spread across favorable environments.

Cause and transmission: Understanding the screwworm life cycle

NWS is caused by the parasitic fly Cochliomyia hominivorax. Importantly, it is not a worm, bacterium, or virus. Instead, disease results from the larval stage of the fly, producing a condition known as myiasis, or infestation of living tissue by fly larvae.

The adult fly resembles an ordinary metallic blue-green blowfly and generally lives only a few weeks. The female's reproductive strategy, however, makes it particularly dangerous.

She actively seeks out warm-blooded animals with even the smallest skin injury. Potential sites include:

• Tick bites
• Surgical incisions
• Branding wounds
• Castration sites
• Dehorning wounds
• Umbilical cords of newborn animals
• Minor scratches
• Natural body openings such as the eyes, nose, ears, mouth, or genital regions

The female deposits approximately 200 to 300 eggs at one time and may lay as many as 3000 eggs during her lifetime.⁸ Within about a day, the eggs hatch into larvae that immediately begin feeding on living tissue rather than dead tissue.

As multiple larvae feed simultaneously, the wound enlarges and deepens. Their backward-facing spines anchor them firmly within tissue, making spontaneous removal difficult. Secondary bacterial infections commonly develop, further worsening tissue destruction.

After feeding for approximately one-week, mature larvae leave the host and drop into the soil, where they pupate. Days later, adult flies emerge, mate, and continue the cycle.

Unlike many infectious diseases, screwworm is not transmitted directly from animal to animal or person to person. Rather, each infestation begins when a female fly deposits eggs onto susceptible tissue. The fly serves as the vector, while the larvae produce the disease.

Humans can become infected, although cases are relatively uncommon and occur primarily in endemic regions or among travelers exposed to affected environments. Risk factors include:

• Open wounds

• Prolonged outdoor exposure
• Close contact with livestock
• Poor wound hygiene
• Medical conditions producing chronic skin lesions

Because even a tiny wound may attract an egg-laying female, awareness and prompt wound care are essential preventive measures.⁸

Animal and human health impacts: Why screwworm Is more than an agricultural problem

One of the most important misconceptions about screwworms is that it simply causes an annoying wound infestation. NWS is an obligate parasite, meaning its larvae require living tissue to complete their development. This biological characteristic makes it significantly more destructive than many other fly species whose maggots feed primarily on dead or decaying tissue.

For livestock producers, the consequences can be severe. Cattle are among the most affected animals because routine management practices—including branding, ear tagging, castration, dehorning, or even tick bites—can create wounds attractive to egg-laying female flies.⁹ Sheep, goats, horses, swine, and companion animals such as dogs may also become infected. Wildlife, including deer and other mammals, can serve as hosts, complicating eradication efforts because infected wild animals can transport the parasite across large geographic areas.

Once larvae hatch, they burrow deeper into the wound while feeding in a screw-like pattern that gives the parasite its common name. The infestation enlarges rapidly, producing extensive tissue destruction, inflammation, bleeding, and a characteristic foul odor. Animals often exhibit obvious signs of distress, including restlessness, isolation from the herd, loss of appetite, and weight loss. As infestation progresses, affected animals may become weak, develop secondary bacterial infections, or die if treatment is delayed.

The economic implications extend well beyond individual animal losses. Producers may face veterinary expenses, decreased weight gain, reduced milk production, reproductive losses, and restrictions on animal movement or international trade. Historically, before eradication in the US, screwworm cost the livestock industry hundreds of millions of dollars annually when adjusted for today's economy. These economic impacts helped drive one of the largest coordinated insect eradication programs ever undertaken.

From a wildlife perspective, screwworm represents a conservation challenge as well. Endangered species are not immune to infestation. During outbreaks, wildlife populations may experience increased mortality, and surveillance becomes considerably more difficult because affected animals are often inaccessible or discovered only after advanced disease.

Although human cases are relatively rare compared with animal infections, they are well documented. Human myiasis typically occurs when flies deposit eggs into untreated wounds or natural body openings such as the nose, ears, or eyes. Individuals living or traveling in endemic areas, agricultural workers, people with chronic wounds, or those unable to maintain adequate wound hygiene may be at greater risk.

Importantly, humans do not "catch" screwworm from another infected person or animal through ordinary contact.¹⁰ Each case develops when an adult female fly deposits eggs directly onto susceptible tissue. Nevertheless, human infections require prompt medical attention because the larvae continue feeding on living tissue until removed.

The broader lesson is that screwworm exemplifies the interconnected nature of One Health, recognizing that human health, animal health, and environmental health are closely linked. An outbreak affecting livestock can influence wildlife conservation, food production, economics, and ultimately public health infrastructure.

Diagnosis and treatment: Early recognition saves lives

Prompt diagnosis is critical because the severity of tissue damage increases rapidly as larvae mature.¹¹ Fortunately, experienced veterinarians and physicians can often recognize screwworm based on characteristic clinical findings, although laboratory confirmation remains important for surveillance and control efforts.

Typical signs include:

• A wound that unexpectedly enlarges rather than heals
• Bloody or serous discharge
• Foul-smelling tissue
• Visible movement within the wound
• Clusters of cream-colored larvae
• Persistent irritation despite routine treatment

Animals may repeatedly lick or bite the affected area, while humans often report pain, itching, or the sensation of movement within the wound.

Laboratory identification generally involves collecting larvae and submitting them to specialized diagnostic laboratories, where entomologists examine their distinctive morphological characteristics. Increasingly, molecular diagnostic techniques such as polymerase chain reaction (PCR) and DNA sequencing provide rapid and highly accurate species identification, allowing authorities to distinguish New World screwworm from other fly species that produce myiasis.¹²

Treatment focuses on two primary goals: eliminating the larvae and managing the wound.¹¹

Mechanical removal of all larvae is essential. Depending on the location and severity of infestation, veterinarians or physicians may manually extract larvae while carefully cleaning the affected tissue. Failure to remove every larva may allow continued tissue destruction.

Following removal, thorough wound cleansing and debridement help eliminate damaged tissue and reduce bacterial contamination. Secondary bacterial infections frequently require antimicrobial therapy based on clinical evaluation. Pain management and supportive care are also important, particularly for severely affected animals.

In veterinary medicine, approved antiparasitic agents and topical insecticides may assist treatment and help prevent reinfestation. Livestock producers should always follow veterinary guidance and applicable regulatory recommendations when selecting treatments.

For human patients, management depends on the location and extent of infestation but similarly centers on complete larval removal, wound care, and treatment of any secondary infection. Early medical evaluations significantly improve outcomes.

Perhaps the most important principle is simple: suspected screwworm cases should be reported immediately to appropriate animal health authorities. Rapid reporting enables surveillance teams to investigate, confirm diagnoses, and implement containment measures before additional flies emerge and spread the infestation.¹³

Prevention and the future: Science, surveillance, and One Health

The history of screwworm demonstrates that prevention is far more effective than treatment. Because each female fly mates only once during her lifetime, disrupting reproduction has proven to be one of the most successful biological control strategies ever developed.¹⁴

The SIT remains the cornerstone of eradication efforts. Millions of laboratory-reared male flies are sterilized through carefully controlled irradiation and then released into affected areas. These sterile males compete successfully with wild males for mates. When females mate with sterile males, they produce no viable offspring, gradually causing the local population to collapse.

Unlike widespread pesticide application, SIT is highly species-specific and environmentally responsible. It targets the reproductive biology of screwworms without broadly affecting beneficial insects or ecosystems. The success of this program has made it a model for integrated pest management worldwide.

Surveillance also plays a central role. Veterinary authorities monitor livestock, wildlife, and animal movement while maintaining diagnostic capacity to identify suspect cases quickly. Producers are encouraged to inspect animals regularly, particularly following procedures that create wounds or during seasons favorable for fly activity.

Practical preventive measures include:

• Prompt cleaning and protection of wounds
• Careful monitoring of newborn animals
• Appropriate fly control programs
• Timely veterinary evaluation of non-healing wounds
• Reporting suspicious infestations to animal health officials
• Following movement restrictions during outbreaks

For travelers and healthcare providers, awareness is equally important. Individuals returning from endemic regions who develop unusual wound infestations should seek medical evaluation, and clinicians should consider myiasis in the appropriate clinical context.

The recent re-emergence of NWS near the US serves as a reminder that infectious and parasitic diseases do not recognize political boundaries. Climate conditions, animal movement, wildlife migration, and international commerce all influence disease ecology. Continuous investment in surveillance, scientific research, international collaboration, and public education remains essential.

Final thoughts: An old threat that demands modern vigilance

The NWS is both an extraordinary biological organism and a significant veterinary and public health challenge. Although often described simply as a "flesh-eating parasite," its true importance lies in its ability to exploit even minor wounds, causing devastating disease in livestock, wildlife, and occasionally humans.

The US once demonstrated that science, innovation, and international cooperation could successfully eliminate this parasite through one of the most remarkable eradication campaigns in history. The recent detections underscore that such achievements require ongoing vigilance rather than complacency.¹⁵

From producers inspecting cattle to veterinarians diagnosing unusual wounds, physicians recognizing rare cases of myiasis, entomologists monitoring fly populations, and public health officials coordinating surveillance, every component contributes to protecting animal agriculture and human health.

Ultimately, the story of screwworm is not merely about a parasite. It is a compelling example of the One Health concept in action—illustrating how human health, animal health, environmental stewardship, and scientific innovation intersect to address emerging biological threats. Continued investment in research, surveillance, and education will be essential to ensuring that this once-eradicated pest does not regain a permanent foothold in the United States.¹⁶

References

1. Will Gottsegen. The Screwworm Is Messing with America’s Beef. The Atlantic.https://www.theatlantic.com/newsletters/2026/06/screwworm-parasite-threatening-cows-beef/687492/ Published June 9, 2026. Accessed June 10, 2026.
2. International Atomic Energy Agency (IAEA). Sterile insect technique. https://www.iaea.org/topics/sterile-insect-techniquePublished Unknown. Accessed June 8, 2026.
3. Rohde R.E. Will Physicians and MLS Professionals be Prepared to Diagnose the Return of Vaccine-Preventable Diseases? “Under the Microscope” – Healthcare Hygiene Magazine. March / April, Volume 8, Number 3/4, pp 12-13. https://www.healthcarehygienemagazine.com/monthly-issues/#flipbook-df_30457/13/ Published March 20, 2026. Accessed June 8, 2026.
4. United States Department of Agriculture (USDA). Animal and Plant Health Inspection Service (APHIS). USDA Confirms Presence of New World Screwworm in the United States. https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-presence-new-world-screwworm-united-states? Published June 3, 2026. Accessed June 8, 2026.
5. Centers for Disease Control and Prevention (CDC). New World Screwworm Outbreak. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html?Published June 4, 2026. Accessed June 8, 2026.
6. Michaeleen Doucleff. How The Screwworm's Sex Life Saved Your Steaks. https://www.kpbs.org/news/2016/06/24/how-the-screwworms-sex-life-saved-your-steaks? Published June 24, 2016. Accessed June 9, 2026.
7. USDA. APHIS. New World Screwworm. https://direct.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm Published June 7, 2026. Accessed June 9, 2026.
8. CDC. New World Screwworm. About New World Screwworm. https://www.cdc.gov/new-world-screwworm/about/index.html? Published February 24, 2026. Accessed June 9, 2026.
9. Leah Douglas. Reuters. US confirms new screwworm cases, as ranchers brace for spread. https://www.reuters.com/world/us/usda-confirms-two-additional-cases-new-world-screwworm-united-states-2026-06-08/? Published June 8, 2026. Accessed June 9, 2026.
10. N.C. State Home. Veterinary Medicine News. New World Screwworm in the U.S.: What It Means for North Carolina. https://cvm.ncsu.edu/news/new-world-screwworm-in-the-u-s-what-it-means-for-north-carolina/ Published June 8, 2026. Accessed June 9, 2026.
11. CDC. New World Screwworm. Clinical Overview of New World Screwworm. https://www.cdc.gov/new-world-screwworm/hcp/clinical-overview/index.html Published February 24, 2026. Accessed June 9, 2026.
12. Palinski, R., Hicks, J., Alfred, J.T. et al. Validation of a molecular workflow for Cochliomyia hominivorax (New World screwworm) identification in field samples. Sci Rep (2026). https://doi.org/10.1038/s41598-026-54866-6 Published June 3, 2026. Accessed June 8, 2026.
13. USDA. APHIS. Report Suspected Cases of Screwworm. https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm/report-suspected-cases-screwworm Published June 8, 2026. Accessed June 9, 2026.
14. USDA. APHIS. Stop Screwworm. Unified Government Response to Protect the United States. https://direct.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm Published June 9, 2026. Accessed June 9, 2026.
15. American Veterinary Medical Association (AVMA). AVMA News. Officials call for veterinary vigilance as screwworms move closer to US. By R. Scott Nolen and Malinda Larkinhttps://www.avma.org/news/officials-call-veterinary-vigilance-screwworm-moves-closer-us Published September 22, 2025. Accessed June 9, 2026.
16. Rohde R.E. Re-emerging Diseases and the Need for Diagnostic Readiness. American Society of Microbiology, Bugs and Drugs article. https://asm.org/articles/2026/april/re-emerging-diseases-and-the-need-for-diagnostic-r Published April 24, 2026. Accessed June 8, 2026.