Screw Worm Detected 25 Miles from U.S. Border: Causes, Symptoms, and Control

Screw Worm Detected 25 Miles from U.S. Border, Sparking Vigilance

A New World screwworm (Cochliomyia hominivorax) was found in a 5-year-old goat in Coahuila, Mexico, just 25 miles from the Texas border on Thursday, marking the closest detection to U.S. soil since at least September. The USDA has tracked 32 cases in Coahuila, 19 still active, and over 26,216 total cases across Mexico, with upward of 2,700 active. Despite the proximity, officials stress that the risk to U.S. livestock and humans remains very low.

The pest “is not currently present” in the United States, and “the current risk to livestock, other animals, and people in the United States remains very low,” the USDA notes on its dedicated webpage.

Monitoring within 400 miles of the border has intensified. In April, a detection 60 miles from the border was reported, and Texas officials have warned outdoor enthusiasts since late 2024. The first human case in the U.S. occurred last year in a traveler returning from El Salvador to Maryland, who has since recovered without further transmission.

• Detection in a 5-year-old goat in Coahuila, 25 miles from Texas, on June 3, 2026.
• 32 cumulative cases in Coahuila, 19 active; 26,216 total in Mexico, ~2,700 active.
• First human case in U.S. confirmed in 2025 (traveler from El Salvador).
• USDA updates cases twice weekly; risk to U.S. deemed very low.

Understanding Screw Worm Infestation: Symptoms and Impact on Hosts

Screw worm larvae feed on living tissue, creating deep, expanding wounds that attract further egg-laying and can lead to secondary infections and death if untreated. Infestations cause severe animal suffering and economic losses through reduced weight gain, milk production, and treatment costs.

Human infestations, though rare, occur when flies deposit eggs on open wounds or mucous membranes, causing painful myiasis requiring medical intervention.

The economic toll on livestock can be devastating. A single untreated case can spread within a herd, as adult flies are attracted to wounds from other causes. Infested animals often require repeated cleaning, insecticide application, and isolation. In endemic regions, screw worm outbreaks have historically cost the cattle industry millions annually.

• Larvae burrow into living tissue, causing necrotic wounds.
• Secondary bacterial infections common; mortality high without treatment.
• Reduced productivity: lower milk yield, weight gain, and fertility.
• Human myiasis: rare but painful, usually associated with poor wound care.

Modern Control Methods: Sterile Insect Technique and Wound Management

The sterile insect technique (SIT) is the cornerstone of large-scale screw worm control. Millions of sterilized male flies are released to mate with wild females, producing no offspring and driving population collapse. SIT successfully eradicated the pest from the U.S. and parts of Central America in the 1960s–70s, but sustained funding and cross-border cooperation are essential to prevent reintroduction.

Current outbreaks in Mexico underscore the need for continued surveillance. Wound management—cleaning lesions, applying insecticide, and preventing re-infestation—remains critical for treating individual animals and controlling local outbreaks. The USDA and Mexico’s agriculture department collaborate on monitoring and response, but budget constraints and movement of infested animals complicate efforts.

The sterile insect technique is the most effective control method, but it requires continuous application and international coordination.

• SIT: release of sterilized males to break reproduction cycle.
• Wound treatment: debridement, insecticide (e.g., coumaphos), and protection from flies.
• Surveillance: monitoring traps and reporting cases within 400 miles of border.
• Public education: ranchers, hunters, and veterinarians must recognize early signs.

Key Takeaways

• Screw worm (Cochliomyia hominivorax) is a parasitic fly that infests warm-blooded animals, causing potentially fatal myiasis.
• Recent detections in Mexico near the U.S. border highlight the ongoing risk of reintroduction despite past eradication.
• Sterile insect technique is the most effective large-scale control method, but requires continuous application.
• Early detection and proper wound care are essential to prevent severe infestations in livestock and humans.
• The current risk to the U.S. remains very low, but vigilance through monitoring and public awareness is critical.
• Cross-border collaboration between the U.S. and Mexico is vital to manage and contain outbreaks.