New World Screwworm: What Consumers and Cattle Producers Need to Know
- darren wong
- Jun 26
- 5 min read
Updated: Jul 14
This post includes expert insights from Callahan Grund, Executive Director of U.S. CattleTrace.
The New World screwworm may sound like a relic of the past, but it's once again becoming a real concern for livestock producers and a topic of growing misinformation among consumers. Here’s what you need to know: whether you're buying beef at the grocery store or managing a ranch.
What Is the New World Screwworm?
The New World screwworm is the larval stage of a fly called Cochliomyia hominivorax. Unlike common maggots that feed on dead tissue, screwworm larvae feed on living flesh. The adult fly lays its eggs in open wounds or mucous membranes of warm-blooded animals including cattle, wildlife, pets, and even, in rare cases, humans.
How to Detect Screwworm Infestation
Early detection is key. Watch for:
Restlessness or animals licking and biting at wounds
Visible larvae in wounds
Foul-smelling or slow-healing injuries
A veterinarian can identify the larvae, and confirmation is done through lab testing. Prompt action can save animals and prevent the spread.
What Does Screwworm Look Like?
Larvae: Up to 17 mm long, cylindrical, with dark tracheal tubes inside their bodies.
Adult Flies: Resemble houseflies but have bright orange eyes and striped thoraxes.
Can Screwworm Affect Other Livestock?
Yes, screwworms can infest any warm-blooded animal, including:
Cattle
Horses
Goats
Sheep
Swine
Wildlife
Pets
Humans (rare)
What Happens If It's Left Untreated?
If not detected and treated quickly, screwworm infestation can cause:
Painful tissue destruction
Secondary infections
Death in severe cases
Spread to other animals via fly reproduction
What Does This Mean for Consumers?
While screwworm poses almost zero direct threat to food safety, it can impact:
Supply chains if livestock are quarantined or culled
Beef prices, which could rise with reduced supply
Public perception, as sensational headlines may not reflect actual risk
Could Prices Increase?
Currently, prices remain stable. However, should the screwworm reach U.S. livestock herds in significant numbers, supply could drop, affecting prices in the future.
Is It Dangerous to Humans?
Infection in humans is rare, but possible typically through untreated wounds. Prompt medical attention makes outcomes manageable.
Economic Impact of Screwworm
A widespread outbreak could cost millions to billions in:
Animal deaths
Veterinary treatments
Lost productivity
Quarantine protocols
Trade and border disruptions
Why Are We Talking About It Now?
Since 2023, screwworm has moved north from Panama into southern Mexico just 700 miles from the U.S. border. Two main factors contributed:
Migration surges through the Darién Gap
Reduced effectiveness of sterile male fly programs due to biological fatigue
How Is It Being Managed?
Sterile Insect Technique (SIT): Sterile flies disrupt reproduction. The U.S. and COPEG produce 100+ million sterile flies weekly.
Surveillance: Ground teams inspect and report new cases.
USDA Response: $21 million is being invested to retrofit a fly facility in Mexico (est. 6+ months to scale).
Border Closures & Restrictions
As of now, the USDA has closed the southern border to live animal imports. This decision will be reviewed every 30 days and could expand depending on risk assessments.
USDA Update: New Facility & Five-Pronged Plan to Stop Screwworm
Updated June 18, 2025
U.S. Secretary of Agriculture Brooke L. Rollins has announced a robust national response to the New World Screwworm threat, including the launch of a new sterile fly dispersal facility in South Texas. USDA is activating a five-pronged strategy to protect U.S. livestock and prevent the pest’s northward spread:
Stop the Pest in Mexico
$21 million invested in expanding a sterile fly production facility in Metapa, Mexico, increasing output to 160 million flies per week.
Ongoing USDA audits and support for Mexico’s detection, surveillance, and biosecurity efforts.
Secure the U.S. Border
Strategic trapping, real-time awareness campaigns, and livestock inspection along the border.
Increased cooperation with CBP and state agencies to intercept illegal or stray animals.
Maximize Emergency Readiness
Updated state emergency plans, training, and fast access to treatments.
Streamlining regulatory barriers to make critical supplies more accessible.
Take the Fight to the Screwworm
New dispersal facility planned at Moore Air Base for 2025.
Future expansion could boost U.S.-based sterile fly production by up to 300 million flies/week.
Innovate Toward Eradication
Advancing next-generation technology for sterile insect production, improved traps, and genetic control.
Partnering with land-grant universities in border states for research, outreach, and monitoring.
USDA will also host four public listening sessions to gather stakeholder input on eradication tools, fly production capacity, and emerging innovations.
Leaders React
Texas officials and national ag groups have voiced strong support for USDA’s plan, calling it a vital step to safeguard animal health, the economy, and consumer confidence.
What Cattle Producers Need to Know
Why Is This a Serious Threat?
Screwworms can:
Infect any animal with an open wound
Cause severe productivity losses
Jeopardize herd health and marketability
Require costly treatment and biosecurity upgrades
How to Prevent Screwworm Infestation
Inspect livestock daily for open wounds
Sanitize equipment and facilities
Use topical larvicides (e.g., permethrin) and wound sprays
Cover wounds from branding, tagging, castration, and clipping
Consult a veterinarian if you suspect screwworms
What Actions Should Producers Take?
Maintain a close relationship with your veterinarian
Report suspicious cases to state animal health officials
Stay informed via trusted sources
Long-Term Solutions for Producers
Beyond daily vigilance, one of the most proactive steps producers can take is participating in value-added certification programs like CARE Certified or the Secure Beef Supply Plan. These programs aren’t just about market premiums they’re rooted in the very practices that help protect against threats like New World screwworm: proper sanitation, humane animal handling, consistent record-keeping, and strong veterinary partnerships. By aligning with these programs, you're not only boosting the resilience of your operation, but you’re also building trust with consumers and helping create a more secure, sustainable beef supply chain.
Looking to help protect your operation?
866.395.5883
Where to Get Reliable Information
Avoid misinformation and stay up to date through:
Final Thoughts
Screwworms are a real threat, but they are also a manageable one. With informed vigilance from producers and accurate understanding by consumers, we can help stop the spread, protect livestock, and keep the food supply secure.
Knowledge is the best defense, let’s work together to stop misinformation and safeguard animal health.
About the Contributor Callahan Grund Executive Director, U.S. CattleTrace cgrund@uscattletrace.org (785) 821-1082 Callahan Grund leads national traceability efforts as Executive Director of U.S. CattleTrace. With experience in livestock policy, biosecurity, and producer relations, he brings practical insight to disease preparedness. Grund remains active in the cattle industry, running a small herd with his wife near Wamego, Kansas.