New World Screwworm in Texas: What Grass-Fed Beef Buyers Need to Know
Expert reviewed • Updated June 2026 • 8 min read
The recent detection of New World screwworm in South Texas has brought renewed attention to a parasite that once caused major losses for American ranchers. Here's what it is, how it affects cattle, and what grass-fed beef buyers should know.
New World screwworm larvae feed on living tissue and can cause severe health problems in cattle if infestations are not detected and treated quickly.
Why Is the New World Screwworm Suddenly in the News?
For decades, most American ranchers never had to worry about screwworm infestations because the parasite had been successfully eradicated from the United States.
That changed when recent detections in Mexico prompted increased surveillance efforts along the southern border. The discovery of a screwworm case in South Texas has renewed concerns about preventing the parasite from becoming reestablished in the United States.
For ranchers, the concern isn't food safety. The concern is animal health and the potential economic impact on the cattle industry.
What Is the New World Screwworm?
The New World screwworm is the larval stage of a fly called Cochliomyia hominivorax. Unlike ordinary fly maggots that feed on dead tissue, screwworm larvae feed on living flesh.
Adult female flies are attracted to wounds on warm-blooded animals. They lay eggs in cuts, scratches, branding wounds, tick bites, navels of newborn calves, or other openings in the skin.
When the eggs hatch, the larvae burrow into the living tissue surrounding the wound and begin feeding. As more larvae hatch and feed, the wound grows larger and more severe.
Without treatment, infestations can become life-threatening. Understanding how a screwworm infestation develops helps explain why ranchers take even small wounds on cattle seriously. The process can progress rapidly if an infestation is not detected and treated.
Why Is It Called a Screwworm?
The name comes from the appearance of the larvae. The maggots have rows of spines that cause them to burrow into tissue in a corkscrew-like fashion. As they feed, they literally screw themselves deeper into the wound, creating extensive tissue damage.
How Does Screwworm Affect Cattle?
Screwworm infestations can occur in cattle of any age, but newborn calves are particularly vulnerable because the healing navel provides an attractive site for egg-laying flies.
Common signs of infestation include:
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A foul-smelling wound
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Swelling around the affected area
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Drainage or bloody discharge
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Irritation and discomfort
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Weight loss
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Reduced performance
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Visible larvae within the wound
If left untreated, infestations can become severe enough to kill the animal. Because cattle often spend their time spread across large pastures, early detection is extremely important.
Are Grass-Fed Cattle More Vulnerable?
Not necessarily.
Grass-fed cattle are exposed to many of the same environmental conditions as conventionally raised cattle. Screwworm infestations occur because flies gain access to a wound, not because cattle are grass-fed.
A grass-fed steer grazing on pasture is not automatically at greater risk than a conventionally raised animal.
What matters most is management.
Ranchers who regularly monitor their herds, inspect calves, and promptly treat wounds can greatly reduce the risk of severe infestations.
Can Humans Get Screwworm?
Yes. Although uncommon, New World screwworm can infest humans.
Cases typically occur when flies deposit eggs in an open wound or other susceptible area. The larvae then begin feeding on living tissue just as they would in an animal host.
Human cases are rare in the United States but are one reason health officials take screwworm detections seriously.
Is Beef Safe to Eat?
Yes. The presence of New World screwworm does not make beef unsafe to eat. Screwworm is an animal health issue, not a food safety issue.
Animals with severe health problems would not enter the food supply through normal inspection processes. The parasite itself does not infect meat products that consumers purchase from grocery stores, butcher shops, or ranches.
For consumers, there is no evidence that buying beef carries a risk of contracting screwworm.
Why Ranchers Take Screwworm So Seriously
To understand why ranchers are concerned, it's helpful to look at history. Before eradication efforts began, screwworm was one of the most destructive livestock pests in North America.
Infestations affected cattle, sheep, goats, horses, wildlife, pets, and occasionally humans. The economic losses were enormous.
Ranchers spent significant time and money treating animals, monitoring wounds, and dealing with livestock deaths caused by infestations.
The parasite represented a constant threat to animal health.
One of Agriculture's Greatest Success Stories
The United States ultimately eliminated New World screwworm through an innovative program known as the Sterile Insect Technique.
Scientists discovered that if enough sterile male flies were released into the environment, wild females would mate but produce no offspring. Over time, the screwworm population collapsed.
Millions of sterile flies were produced and released over many years, leading to the eradication of screwworm from the United States.
The effort is still considered one of the most successful pest eradication programs in agricultural history.
Could Screwworm Increase Beef Prices?
Potentially.
The U.S. cattle herd is already near historic lows following years of drought, liquidation, and reduced herd expansion.
Any widespread animal health problem that affects calf survival or increases production costs could place additional pressure on cattle supplies.
At this time, there is no reason for consumers to panic.
However, ranchers and government agencies are working aggressively to prevent the parasite from becoming established again because the long-term economic consequences could be significant.
How Ranchers Protect Their Herds
Modern ranchers use several practices to reduce the risk of screwworm infestations and other wound-related problems.
These include:
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Frequent observation of livestock
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Monitoring newborn calves
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Prompt treatment of wounds
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Fly control programs
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Veterinary consultation when needed
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Reporting suspected infestations quickly
Early detection remains one of the most effective tools available.
What Grass-Fed Beef Buyers Should Know
For consumers, the recent news about New World screwworm is primarily a reminder of the challenges ranchers face in producing healthy cattle.
Every day, cattle producers manage weather, predators, parasites, disease, nutrition, and countless other factors that affect animal welfare and beef production.
The reappearance of screwworm in the news does not mean beef is unsafe.
Instead, it highlights the importance of strong animal health programs, ongoing surveillance, and the dedication of ranchers who work every day to care for their livestock.
Whether you buy beef from a local ranch, a butcher shop, or a grocery store, the beef supply remains safe.
The real concern is protecting the nation's cattle herd from a parasite that previous generations of ranchers worked hard to eliminate.
Frequently Asked Questions
What is the New World screwworm?
The New World screwworm is the larval stage of a parasitic fly whose larvae feed on living tissue in warm-blooded animals.
Can humans get screwworm?
Yes. Human infestations are rare but can occur when flies deposit eggs in open wounds.
Does screwworm affect beef safety?
No. Screwworm is an animal health issue and does not make beef unsafe to eat.
Are grass-fed cattle more likely to get screwworm?
Not necessarily. The primary risk factor is the presence of wounds where flies can lay eggs.
Why was screwworm eradication such a big deal?
Before eradication, screwworm caused massive economic losses and animal suffering across the livestock industry. Eliminating it is considered one of the greatest achievements in agricultural pest control history.
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Sources:
Centers for Disease Control and Prevention (CDC) — New World Screwworm Information USDA Animal and Plant Health Inspection Service (APHIS) — New World Screwworm Program and Eradication Efforts USDA APHIS – New World Screwworm Information