First U.S. Case of New World Screwworm in Nearly 60 Years Confirmed in Texas
The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (USDA APHIS) has confirmed the detection of New World screwworm (Cochliomyia hominivorax) in a three-week-old calf in La Pryor, Zavala County, Texas. The larvae were identified in the animal's umbilical area. This is the first confirmed case in the United States since the pest was declared eradicated in 1966.
No further detections have been reported as of the announcement. The infected animal is undergoing treatment, which includes manual removal of larvae, wound disinfection, antibiotics, and insecticides.
Response and Quarantine
USDA and Texas officials have initiated a containment and eradication response. An infested zone has been established around the detection site, with one source citing a radius of 20 kilometers (approximately 12.4 miles) and another citing 12 miles. The measures include:
- Quarantine enforcement prohibiting the movement of animals, including pets, within the zone.
- Movement controls and increased surveillance.
- Activation of an Incident Command Team in coordination with the Texas Animal Health Commission.
- An increase in trap placement along the U.S.-Mexico border.
Background and Spread
New World screwworm is a parasitic fly native to South America and parts of the Caribbean. Female flies lay eggs in open wounds, scratches, or mucous membranes of warm-blooded animals, including humans. The larvae burrow into and feed on living tissue, which can lead to tissue destruction and death if left untreated.
The pest was eradicated in the United States in 1966. It later reappeared in Central America and spread into Mexico, where detections increased significantly beginning in 2023. According to the USDA, screwworm was detected last week in Mexico, 25 miles from the U.S. border.
- Since September 2024, at least 26,216 animal cases have been identified in Mexico, with over 2,700 currently active.
- Between mid-July and mid-August 2025, Mexico reported a 53% increase in animal cases.
- Mexico confirmed 41 human cases of New World screwworm in 2024, primarily in the state of Chiapas.
In August 2024, the U.S. Department of Health and Human Services announced the first confirmed human case in the U.S. in a patient who had returned from El Salvador to Maryland. The individual recovered, and no transmission to others or animals was found.
Causes
Experts have suggested several possible factors for the resurgence of the pest, including:
- Disruptions to sterile-fly release programs during the COVID-19 pandemic
- Increased movement of livestock and people
- Favorable weather conditions
Economic Impact
The USDA estimates that an outbreak in the U.S. could result in $1.8 billion in potential losses to the Texas economy.
The United States halted cattle imports from Mexico one year ago due to the pest's spread, a measure that has contributed to rising beef prices in the domestic market.
Statements
Dudley Hoskins, under secretary for marketing and regulatory programs at the USDA, stated:
"USDA invested heavily in the tools needed to eliminate NWS ever since cases started increasing in Central America and Mexico. The United States has defeated this pest before, and we will do it again. "
Dr. Cheryl Whitehorn, a medical entomologist at the London School of Hygiene & Tropical Medicine, stated:
"This is regarded as a locally acquired infestation... The primary measure is vigilance, identification and isolation of cases, treatment and elimination of larvae and controlling movement of animals. A programme of sterile male release is the best long-term method of controlling this fly. "