The Flesh-Eating Screwworm: Rising Threat to Livestock, Beef Production, and Public Health

The flesh-eating screwworm (Cochliomyia hominivorax), also known as the New World screwworm, has re-emerged as a major public health and agricultural concern in the Americas. This parasitic fly, once eradicated from North and Central America through large-scale control programs, is now spreading rapidly in parts of Central America and Mexico, raising alarms among scientists, governments, and livestock producers alike. In August 2025, the screwworm made international headlines when the first travel-associated human case in the United States was confirmed in Maryland.

What is the Screwworm?

Unlike most maggots, which feed on dead tissue, screwworm larvae are unique in their ability to consume living flesh. Female flies can lay up to 300 eggs at a time on wounds or natural body openings of warm-blooded animals, including humans. Within hours, the eggs hatch into larvae that burrow into tissue like a corkscrew, causing severe pain, tissue destruction, and even death if untreated.

After feeding for 3–7 days, the larvae drop to the soil, pupate, and emerge as adult flies, continuing the cycle. Each female fly mates only once but can lay about 3,000 eggs in her 30-day lifespan, making population growth extremely rapid.

Human Infections: Rare but Serious

Human cases of screwworm infestation, known as myiasis, are rare but alarming when they occur. Eggs may be laid in open wounds, or in sensitive areas such as the ear, nose, mouth, or eyes.

Symptoms include:

• Painful, non-healing wounds

• A foul odor from the infected site

• Visible larvae or the sensation of “movement” under the skin

There is no drug treatment to kill the larvae. Removal requires manual or surgical extraction, followed by wound care and antibiotics.

Recent examples highlight both the risks and rarity of human cases:

• In Maryland (2025), a traveler returning from El Salvador became the first confirmed U.S. case linked to the outbreak; he has since recovered.

• In Canada (2024), an elderly man required surgery after doctors removed 30–40 larvae from his leg wound sustained in Costa Rica.

• In Costa Rica (2024), a person with disabilities tragically died from infestation, the first fatality in decades.

Health experts stress that while the risk to the general public remains extremely low, travelers to outbreak zones should be cautious.

Current Outbreak Regions

Historically, screwworms ranged widely across the Americas but were eradicated from the U.S., Mexico, and Central America by the early 2000s through mass control programs. Since 2023, however, populations have resurged dramatically:

• Panama: Over 6,500 animal cases in 2023.

• Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador: Ongoing outbreaks with rising human and animal cases.

• Mexico: By August 2025, more than 5,000 animal cases in a single month were confirmed, a 53% increase from July.

This northward expansion is alarming because while adult flies don’t typically travel more than 12 miles, infected livestock and pets can carry larvae across borders. Experts warn that if screwworms re-establish in the U.S., they could persist year-round in warm states like Texas or Florida.

Impact on Livestock and Beef Production

While human cases capture headlines, the greatest danger lies with livestock, especially cattle.

Animal health impacts:

• Deep, ulcerating wounds that attract repeated egg-laying

• Severe pain, weight loss, reduced fertility, and lower milk yields

• Death if infestations go untreated

Economic and trade consequences:

• Historically, screwworms caused hundreds of millions of dollars in losses annually in the U.S. before eradication.

• The current outbreak has already cost Mexico $1.3 billion in livestock losses.

• The USDA estimates that if screwworms were to re-establish in the U.S., losses could surpass $100 billion.

• Trade restrictions: The U.S. and other countries have placed bans on imports of cattle, horses, and bison from affected regions, disrupting supply chains and driving up beef prices.

As veterinary entomologist Sonja Swiger of Texas A&M put it: “It is a game changer for livestock impact, and those are the animals we can control. But if it spills into wildlife like deer, the impact will be huge.”

How It Is Controlled

Sterile Insect Technique (SIT)

The most effective weapon against screwworm is the Sterile Insect Technique (SIT). In this approach, millions of male flies are reared in specialized facilities, sterilized by radiation, and then released into the environment. When these sterile males mate with wild females, no viable offspring are produced, gradually collapsing the population.

• SIT eliminated screwworms from the U.S. by 1966, Mexico in the 1970s, and Central America by the early 2000s.

• It successfully contained a 2016 Florida Keys outbreak in Key deer.

• Today, SIT programs are being scaled up massively:

  • Facilities in Panama continue to produce sterile males.

  • The U.S. is funding new and retrofitted plants in Texas and Mexico.

  • Weekly releases now reach hundreds of millions of sterile flies.

Emerging Strategies

• Genetic engineering: Scientists are exploring modified screwworms that pass on lethal mutations to their offspring.

• Fast-tracked drugs: The FDA has begun working with manufacturers to repurpose or accelerate approval of antiparasitic drugs for livestock infestations.

Surveillance and response

• Enhanced border inspections, detector dogs, and mounted patrols are deployed in high-risk areas.

• Veterinary authorities are urging ranchers to inspect wounds daily, treat infestations promptly, and report suspicious cases.

Public health measures

• The CDC has issued alerts for healthcare providers, advising them to recognize symptoms of myiasis in travelers.

• Awareness campaigns stress the importance of covering wounds, using insect repellents, and seeking immediate care for non-healing lesions.

Lessons from the Past

The resurgence of the screwworm is a reminder of how fragile eradication victories can be. Experts note that the 2016 Florida case was eliminated within months thanks to rapid SIT deployment, proof that swift, coordinated responses work.

However, the scale of today’s outbreak, combined with the parasite’s spread into Mexico’s cattle heartland, presents a larger challenge. If wild populations gain a foothold in North America, eradication will be far harder and costlier than in the past.

    For now, the risk to humans remains very low, and isolated cases are treatable. But for livestock industries, the screwworm’s return poses a serious economic and food security crisis, with billions of dollars and the stability of beef production at stake.

Governments, scientists, and ranchers are mobilizing with sterile insect programs, advanced surveillance, and new biotechnologies. The success of these efforts will determine whether the Americas can once again eliminate this devastating parasite, or whether it will become a permanent burden on animal health and agriculture.