New World Screwworm Is Back in the U.S.: How to Protect Your Horses

For the first time in almost 60 years, the New World screwworm (NWS) has been confirmed in the United States. On June 3, 2026, the USDA confirmed a case in a young calf in Zavala County, Texas, and additional detections have since followed in nearby Texas counties and southeastern New Mexico. While cattle have dominated the headlines, horse owners need to pay just as much attention: screwworm readily infests horses, and a single overlooked wound can turn into a life-threatening emergency in a matter of days.

This guide explains what the screwworm is, why it is so dangerous, the signs to watch for in your horses, and the practical steps you can take right now to keep them safe.

What Is the New World Screwworm?

The New World screwworm (Cochliomyia hominivorax) is a parasitic blowfly. Unlike common flies that feed on dead tissue or manure, screwworm larvae feed on the living flesh of warm-blooded animals — which is exactly what makes them so destructive.

Here is how an infestation unfolds:

• A female fly is attracted to a wound — even a small one. She is also drawn to natural openings like the nostrils, eyes, ears, and the sheath or genitals.
• She lays her eggs at the edge of the wound. A single female can lay up to 3,000 eggs over her lifetime.
• The eggs hatch into larvae (maggots) that burrow downward into healthy tissue using hook-like mouthparts — the "screwing" motion that gives the pest its name.
• The wound rapidly enlarges. The feeding larvae destroy tissue, the wound attracts even more flies, and without treatment the infestation can become fatal.

After about a week of feeding, the larvae drop to the soil to pupate, and the cycle repeats.

Why Horse Owners Should Care

Screwworm affects cattle, horses, wildlife, pets, and — rarely — people. In horses, flies most often target:

• Open wounds of any size, including scrapes, bites, and rubs
• Surgical or procedure sites — castration, branding, or recent injuries
• The navel of newborn foals
• The sheath, genitals, and the nostrils
• Tick-bite sores and any moist, irritated skin

Because horses are turned out, trailered to events, and frequently pick up minor cuts, they offer plenty of opportunities for an infestation to take hold. The good news: screwworm is also highly preventable with consistent wound care and daily observation.

The Current Situation (June 2026)

The screwworm was officially eradicated from the U.S. decades ago, but an outbreak that began in Panama and Costa Rica in 2023 spread north through Central America and Mexico, affecting more than 185,000 animals before reaching the U.S. border.

• First U.S. case: June 3, 2026 — a calf in Zavala County, Texas
• Additional detections: confirmed in Zavala, La Salle, and Gillespie Counties (TX) and Lea County, New Mexico
• Federal response: quarantine zones and animal-movement restrictions, expanded surveillance, and the release of sterile male flies near the border to collapse the wild population
• Human risk: as of early June 2026, no locally acquired human cases had been reported in the U.S.

Because the situation is evolving, conditions and movement rules can change quickly. Always check your state animal health official's website before traveling with horses.

Warning Signs to Look For

Early detection is everything. Inspect your horses daily and watch for:

• A wound that is enlarging or not healing as expected
• Foul or unusual odor coming from a wound
• Bloody or brown discharge that drains from the site
• Visible maggots in a wound, or in the nostrils, ears, eyes, sheath, or genitals
• Signs of pain, head-shaking, or irritation around a natural body opening
• Restlessness, going off feed, isolating from the herd, or other behavior changes

A screwworm wound often looks and smells distinctly worse than an ordinary cut, and it tends to deepen rather than scab over.

What to Do If You Suspect Screwworm

Screwworm is a reportable foreign animal disease. If you see anything suspicious, act immediately:

• Call your veterinarian right away so the horse can be examined and treated.
• Report it to your state veterinarian or USDA APHIS — your vet can help initiate this, and official guidance is available at screwworm.gov.
• Do not move the animal unless your veterinarian or animal health official directs you to. Moving an infested horse can spread the pest.
• If possible, collect any larvae in a sealed container of rubbing alcohol so they can be identified. Your vet will advise.

When in doubt, treat it as an emergency. Rapid response protects your horse and your community.

Prevention: Keeping Your Horses Safe

Screwworm thrives on neglected wounds, so most prevention comes down to attentive, everyday horsemanship.

1. Make Daily Wound Checks a Habit

Run your hands over each horse every day. Check the legs, belly, face, ears, sheath or udder, and under the tail. Catching a small cut early — before a fly finds it — is your single best defense.

2. Clean and Cover Every Wound

Treat even minor cuts and scrapes promptly. Clean them, apply an appropriate wound dressing or fly-repellent ointment, and keep them protected until they heal. Don't dismiss "just a scratch."

3. Time Procedures Carefully

Castration, branding, and other procedures that create open wounds are high-risk during an outbreak. Talk to your veterinarian about timing, and provide extra protection and monitoring for any surgical site.

4. Watch Newborn Foals

The drying navel of a newborn foal is a classic screwworm target. Keep navels clean and dry, and check foaling mares closely too.

5. Run a Strong Fly-Control Program

Fewer flies means less risk. Use EPA-approved repellents and fly sprays, manage manure and standing water, and consider fly sheets, masks, and barn screening to reduce fly pressure around your horses.

6. Be Smart About Travel

If you compete or trail ride, check your state veterinarian's requirements before you haul, especially near affected areas. Inspect your horse for new wounds before and after every trip, and avoid unnecessary movement in and out of quarantine zones.

7. Stay Informed

Outbreak boundaries and movement rules can shift week to week. Follow updates from USDA APHIS, your state animal health office, and disease-tracking resources so you know what is happening in your area.

Quick Reference

The Bottom Line

The screwworm's return is serious, but it is not a reason to panic — it is a reason to be vigilant. This pest depends on wounds and natural openings to gain a foothold, which means consistent wound care, daily inspections, and good fly control go a long way. Know the warning signs, keep your veterinarian's number handy, and report anything suspicious without delay.

Early detection and quick action are your best defenses. Stay informed about disease activity in your region with Equine Alert, and when something doesn't look right, isolate the situation and call your vet.

This article is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian regarding the health of your horses.