New World screwworm has re-emerged in Texas after decades of eradication, raising concerns for livestock, agriculture and public health. Learn what New World screwworm is, why it matters, how it spreads and what scientists are doing to help stop the outbreak.
What is New World screwworm?
New World screwworm is one of the most destructive livestock parasites ever to affect North America.
Unlike most flies, whose larvae feed on dead or decaying tissue, the larvae (maggots) of the New World screwworm feed on the living flesh of warm-blooded animals. Female flies lay their eggs in open wounds, surgical sites, or even the navels of newborn animals. Once the eggs hatch, the larvae burrow into healthy tissue, causing painful wounds that continue to enlarge as they feed.
Without treatment, these infestations can become life-threatening.
Although cattle are among the most commonly affected animals, screwworm can also infest horses, sheep, goats, pigs, deer, wildlife, pets and, in rare cases, people.
Why is screwworm back in the news?
For decades, New World screwworm was considered one of agriculture’s greatest success stories.
After an intensive international eradication campaign using sterile male flies, the parasite was eliminated from the United States in the 1960s. Since then, a continuous surveillance and sterile fly program has helped keep the pest confined farther south in Central America.
However, beginning in late 2024, screwworm was detected again in southern Mexico. Since then, it has continued moving north despite aggressive containment efforts. In 2026, the first U.S. cases in decades were confirmed in South Texas, prompting an extensive response by federal and state agencies.
How is screwworm affecting Texas?
Texas has one of the nation’s largest livestock industries, making the state especially vulnerable.
The return of screwworm poses several challenges:
- Increased risk to cattle, sheep, goats, horses and wildlife
- Higher veterinary costs for ranchers
- Potential livestock losses if infestations are not detected quickly
- Increased surveillance and movement restrictions for livestock
- Economic impacts on ranching and agricultural trade
The U.S. Department of Agriculture has warned that a widespread outbreak could cost billions of dollars if the parasite became re-established.
Can humans get screwworm?
Human cases are rare, but they can occur.
People typically become infected only when flies lay eggs in untreated wounds or other openings in the skin. Most cases occur in regions where screwworm is well established and access to medical care may be limited.
The overall risk to the general public in Texas remains low. However, anyone with livestock, outdoor pets or frequent exposure to animals should promptly clean and cover wounds and seek medical or veterinary care if they notice unusual symptoms.
How is the outbreak being controlled?
One of the most remarkable tools in modern pest control is the Sterile Insect Technique.
Scientists raise millions of male screwworm flies in specialized facilities and sterilize them before releasing them into affected areas. When sterile males mate with wild females, no offspring are produced. Over time, the wild population collapses.
This approach successfully eliminated screwworm from the United States decades ago and remains the cornerstone of today’s response.
Federal and international partners are now expanding sterile fly production while increasing surveillance, veterinary inspections and rapid response efforts across Texas and Mexico.
Why research matters
The return of screwworm highlights how animal health, human health and environmental health are closely connected—a concept known as One Health.
Protecting livestock protects food security, rural economies and public health.
Scientists continue to develop better tools to detect, monitor and control emerging biological threats before they become widespread. Research institutions, government agencies and industry partners all play important roles in strengthening preparedness and protecting both animals and people.
At Texas Biomed, researchers collaborate with partners to advance innovative approaches for understanding infectious diseases and emerging biological threats that affect both animals and humans. Through scientific research and strategic partnerships, the institute helps build the knowledge needed to respond more quickly to future outbreaks.
“For Texas, the threat of screwworm extends beyond agriculture. In addition to disrupting ranching operations and harming local, state, and national economies, an outbreak can have a serious clinical consequences for wildlife, pets, and even people. Investing in research and preparedness today will help ensure we have the tools needed to respond quickly and protect human and animal health, as well as economic stability.” – Victoria Baxter, DVM, PhD, DACLAM, Assistant Professor, Texas Biomed.
Frequently Asked Questions
Is screwworm a worm?
No. Screwworm is the larval stage of the New World screwworm fly (Cochliomyia hominivorax).
Can pets get screwworm?
Yes. Dogs and cats can become infected if flies lay eggs in untreated wounds, although livestock are most commonly affected.
Is Texas currently seeing screwworm cases?
Yes. The first confirmed U.S. cases in decades were detected in South Texas during 2026, prompting increased surveillance and response efforts.
Is meat safe to eat?
Yes. New World screwworm does not pose a food safety risk. Meat entering the food supply is inspected under established food safety regulations.
Looking ahead
The re-emergence of New World screwworm is a reminder that diseases and agricultural pests do not recognize borders. Continued investment in surveillance, research and international collaboration will be essential to protecting livestock, safeguarding food supplies and preparing for future biological threats.