USDA APHIS has expanded its sterile fly release program along the Texas-Mexico border in response to New World Screwworm (NWS) activity. For veterinarians in border states and those seeing patients that have traveled internationally, understanding this parasitic threat is essential for early detection and reporting.
What is New World Screwworm?
New World Screwworm (Cochliomyia hominivorax) is an obligate parasite of warm-blooded animals—meaning larvae can only develop in living tissue. Unlike other fly species that target dead tissue, NWS larvae feed on healthy flesh, causing progressive, destructive wounds that can be fatal if untreated.
The U.S. eradicated NWS in 1966 through the Sterile Insect Technique (SIT), and USDA APHIS has maintained a biological barrier along the Darien Gap in Panama ever since. However, periodic incursions near the southern border require ongoing vigilance.
Life Cycle and Clinical Presentation
Transmission
- Female flies deposit eggs (150-500 per batch) at the edges of wounds, body openings, or on mucous membranes
- Any wound can serve as an entry point: castration sites, tick bites, dog bites, dehorning wounds, or even the umbilicus of newborns
- Eggs hatch within 12-24 hours
Clinical signs
- Wound that fails to heal or progressively enlarges
- Serosanguinous discharge with foul odor
- Visible larvae within the wound (white to pink, up to 15mm)
- Tissue destruction extending beyond the original wound margins
- Pain, restlessness, and loss of appetite
- Secondary bacterial infection
- Death in severe or untreated cases (from toxemia or tissue destruction)
Key differentiator: Unlike facultative myiasis (blowfly strike), screwworm larvae burrow into healthy tissue rather than remaining on the surface of necrotic tissue.
At-Risk Animals
All warm-blooded animals are susceptible, including:
- Cattle, sheep, goats, horses, and pigs
- Dogs and cats
- Wildlife (deer are significant sentinels)
- Humans (rare but documented)
Newborn animals and any animal with open wounds are at highest risk.
Why the Current Alert Matters
The USDA’s expansion of sterile fly releases along the Texas border indicates active surveillance and control efforts in response to detected cases in Mexico. For U.S. veterinarians, this means:
1. Travel history matters
Ask about recent travel—particularly to Mexico, Central America, South America, or the Caribbean. Pets accompanying owners on international trips are at risk.
2. Report suspicious cases immediately
New World Screwworm is a reportable foreign animal disease. If you suspect NWS:
- Collect larvae samples (preserve in 70% alcohol or saline)
- Contact your State Veterinarian or USDA APHIS immediately
- Do not wait for laboratory confirmation to report
USDA hotline: 1-866-536-7593
3. Examine all wounds carefully
Any non-healing or worsening wound in an animal with potential exposure history warrants close inspection for larvae.
Geographic Considerations
Veterinarians in Texas, New Mexico, Arizona, and California should maintain heightened awareness. However, given the mobility of pets and livestock:
- Animals transported from endemic areas can present anywhere
- Wildlife movement doesn’t respect state lines
- Pet travel to Mexico, Caribbean destinations, or South America creates exposure risk
Treatment
If NWS is confirmed or strongly suspected:
- Physical removal of all larvae from the wound
- Wound debridement and irrigation
- Larvicidal treatment (ivermectin-based products are commonly used)
- Wound protection to prevent reinfestation
- Systemic antibiotics for secondary infection
- Supportive care
Note: Treatment should be coordinated with regulatory authorities, as NWS cases trigger official response protocols.
Client Communication
For clients traveling with pets to endemic regions:
- Inspect animals daily for wounds during and after travel
- Seek immediate veterinary care for any wounds
- Be aware that even minor injuries can become entry points
- Consider postponing travel with animals if active outbreaks are reported
For livestock producers near the border:
- Timing of procedures (castration, dehorning) may need adjustment
- Enhanced wound management protocols
- Report any unusual wound presentations
Resources
- USDA APHIS: Screwworm program updates and reporting
- CFSPH Iowa State: Technical factsheets and images
- State Veterinarian offices: Local reporting and guidance
- WOAH: International disease status
Key Takeaways
- New World Screwworm causes myiasis in living tissue—wounds worsen rather than heal
- The U.S. is officially free of NWS, but border surveillance is ongoing
- Travel history to Mexico, Central/South America, or Caribbean is a risk factor
- Larvae in wounds + travel history = report immediately
- Early detection protects individual animals and U.S. livestock industries
Vigilance at the clinical level supports the broader eradication effort that has kept NWS out of the United States for decades.
This post is for educational purposes and reflects information available as of February 2026. For current disease status and reporting guidance, consult USDA APHIS and your State Veterinarian.