New World Screwworm and Your Horses: What to Watch For and What to Do

KEY TAKEAWAYS

• New World Screwworm has been confirmed in Texas for the first time in 60 years, with cases in Zavala and La Salle Counties as of June 7, 2026
• The pest affects all warm-blooded animals, including horses. Any open wound, even a tick bite, can serve as an entry point
• NWS larvae burrow into living tissue and can be fatal within one week if left untreated
• Early detection is everything. Knowing what to look for is your first line of defense
• Quarantine zones are active in Zavala, La Salle, Uvalde, and Webb Counties. Movement of animals out of these zones requires prior TAHC authorization
• Keeping horses healthy, stress-low, and their gut and immune systems supported is part of a sound management response

QUICK ANSWER

New World Screwworm is a parasitic fly whose larvae burrow into the living flesh of warm-blooded animals, causing severe, rapidly worsening wounds. It was eradicated from the U.S. decades ago but has moved northward through Central America and Mexico since 2023. On June 3, 2026, USDA confirmed its first domestic detection in 60 years in a calf in Zavala County, Texas. A second case was confirmed in La Salle County on June 7. Horse owners, especially in the South Texas region, need to know what this pest is, what to look for, and what to do right now.

What Is New World Screwworm?

New World Screwworm (Cochliomyia hominivorax) is not your typical pest. Most flies are attracted to dead tissue. This one targets the living.

A female screwworm fly lays her eggs in open wounds on warm-blooded animals. That wound doesn't have to be significant. Cuts as small as a tick bite may attract a female fly to lay her eggs, and a single female can lay 200 to 300 eggs at a time.¹ Those eggs hatch into larvae that burrow into the flesh and feed on living tissue, creating wounds that worsen rapidly from the inside out.

If not treated early, infestations can lead to extensive tissue damage and potentially death.² The timeline is blunt: animals may die within one week of being infested if left untreated.

This is not a slow-moving threat.

Why This Is Being Taken Seriously

NWS was eradicated from the United States in the 1960s through a sustained sterile fly release program. It has not been seen here in 60 years. Since 2023, an outbreak has been moving northward through Central America and Mexico, primarily infesting livestock, pets, and wildlife.² The working horses and donkeys in those communities have been living with this threat for years. It's a region FullBucket knows well. Our One-4-One giving program funds year-round veterinary care for working equids in Central America, and the animals our vet team cares for in Guatemala have faced this risk long before it reached Texas.

On June 3, 2026, USDA APHIS confirmed a detection in a three-week-old calf in Zavala County, Texas, with larvae identified in the umbilical area.³ A second case was confirmed June 7 in La Salle County in a one-week-old calf with an umbilical lesion.⁴

Governor Abbott issued a disaster proclamation, and quarantine zones with movement controls are now active in parts of Zavala, La Salle, Uvalde, and Webb Counties.⁴ Canada has also temporarily restricted imports of livestock, including horses, from affected areas of the U.S.⁵

The response is significant because the stakes are. A screwworm outbreak is not just an animal welfare issue. It's an agricultural and economic one.

What to Watch For in Your Horses

Horses are warm-blooded. Every wound, scrape, bite, and body opening is a potential entry point.

Inspect your horses daily and pay close attention to:

• Any wound that appears to be draining, enlarging, or worsening without explanation
• Wounds that smell unusually foul
• Visible larvae or eggs in or around wounds
• Body openings: nose, ears, eyes, mouth, genitalia, and the navel in foals
• Behavioral changes. Screwworm infestations are painful. Horses may become depressed, stop eating, or isolate themselves

Foals are especially vulnerable. Both confirmed U.S. cases so far have involved umbilical lesions in very young animals. If you have a foal on the ground right now, the navel area warrants close daily attention.

What to Do If You Suspect an Infestation

Do not wait.

Contact your veterinarian immediately if you notice any suspicious wound. If you are in or near a quarantine zone, also contact the Texas Animal Health Commission directly. Time matters here in a way it doesn't with most conditions.

Animals in the infested zone may not move out of the zone without prior authorization from TAHC. During inspection, an animal health official will fill out a movement certificate listing the treatment used and the animal's official ID.⁴

If you are outside of Texas, this is still the moment to tighten your wound monitoring habits. This pest is active, it is moving, and early detection is the only thing that changes the outcome.

Protecting Your Horses Right Now: A Practical Checklist

You cannot prevent a fly from existing. You can make your horses a harder target and catch problems early.

✓ Inspect every horse daily. Wounds, body openings, navel on foals

✓ Clean and treat any wound promptly, no matter how small

✓ Reduce sources of new wounds where possible. Check fencing, manage fly pressure, watch for skin irritations

✓ Minimize hauling into or through affected counties until the situation is clearer

✓ Know your vet's emergency contact and have it accessible

✓ If you're in South Texas, contact TAHC before moving any animals: 1-800-550-8242

✓ Keep horses eating well, stress low, and gut health stable

Supporting Your Horses Through an Uncertain Time

This is a moment that asks something of horse owners: attention, consistency, and a management program that keeps horses in good health so that if something does go wrong, they're in the best position to handle it.

Gut health is part of that foundation. A lot of horse owners don't think about gut health until something goes wrong. But the gut is where a significant portion of the immune system lives. Immune cells line the gut wall, interact with beneficial microbes, and help the body respond normally to stress. When that system is stable, everything downstream works better.

Daily probiotic support is part of keeping that foundation solid. FullBucket's Probiotic Pellets provide Saccharomyces boulardii, prebiotics, and L-glutamine to help maintain microbial balance and support normal immune function at the gut level — every day, not just when something disrupts the routine.

For horses that are hauling or needing easy to administer probiotic support, the Equine Probiotic Paste gives a concentrated dose of the same core ingredients to help support digestive stability during travel and transitions. It's the go-to for stress in the barn and on the road.

Neither product is a response to screwworm. They're part of keeping horses in good baseline health so that if something does go wrong, you're working from a position of strength, not trying to catch up.

When to Call Your Veterinarian

Any wound that is worsening, draining unexpectedly, or showing visible larvae warrants an immediate call. Do not attempt to treat a suspected screwworm infestation on your own. Your vet needs to see it, and report it. Early detection and reporting is how outbreaks get contained.

The Takeaway

New World Screwworm is in Texas. The response is active. And horse owners anywhere in the southern U.S. need to be paying attention right now.

The best thing you can do today is know what to look for, inspect your horses daily, and have your veterinarian's number ready. Keep your horses healthy, their wounds clean, and their stress managed.

Stay vigilant. Report anything suspicious. Take care of your horses.

For up-to-date information and quarantine zone maps, visit aphis.usda.gov or tahc.texas.gov.

FREQUENTLY ASKED QUESTIONS

What is New World Screwworm? New World Screwworm is a parasitic fly (Cochliomyia hominivorax) whose larvae infest the living tissue of warm-blooded animals through open wounds. Unlike most flies, it targets living flesh, not dead. Infestations worsen rapidly and can be fatal within one week if untreated.

Is this the first time NWS has been in the U.S.? No. NWS was present in the U.S. historically but was eradicated in the 1960s through a sterile fly program. The current detections in Texas mark its return after 60 years.

Can horses get screwworm? Yes. All warm-blooded animals are susceptible, including horses. Any wound, scrape, or body opening is a potential entry point.

What does a screwworm wound look like? Wounds are typically draining, enlarging, and foul-smelling. Larvae may be visible inside the wound. The horse may show signs of pain, depression, or loss of appetite.

What should I do if I think my horse has screwworm? Call your veterinarian immediately. If you are in or near a quarantine zone in Texas, also contact the Texas Animal Health Commission at 1-800-550-8242. Do not attempt to treat it yourself.

Is the food supply affected? No. USDA's Food Safety and Inspection Service inspects the commercial meat supply and any affected product would not enter commerce.³

REFERENCES

1. Texas Tribune. Screwworm is in Texas. Here is what to know. June 4, 2026.
2. CDC. New World Screwworm Outbreak Situation Summary. June 2026.
3. USDA APHIS. USDA Confirms Presence of New World Screwworm in United States. June 3, 2026.
4. Texas Animal Health Commission. New World Screwworms Situation Update. June 7, 2026.
5. CNBC. U.S. confirms second Texas screwworm case, Canada restricts livestock imports. June 6, 2026.