Four confirmed cases in Texas in less than one week. Here’s an accurate, calm, and actionable guide for Texas horse owners.

Updated June 8, 2026. USDA has now confirmed four cases of New World Screwworm in Texas: two calves in Zavala County (June 3 & 5), a calf in La Salle County, and a dog in Andrews County (both June 8). Epidemiological investigations are ongoing. Guidance is subject to change as additional cases are identified and new products receive authorization.

Don’t Panic — But Pay Attention. For most horse owners outside the affected area, the appropriate response today is increased awareness, careful wound monitoring, and staying informed.

The USDA, Texas Animal Health Commission (TAHC), American Association of Equine Practitioners (AAEP), American Veterinary Medical Association (AVMA), Texas Veterinary Medical Association (TVMA), and the veterinary community have been preparing for this possibility for more than a year. Authorized tools are available, and the official response framework is active.

Current Status

Zavala, La Salle & Andrews Counties — Confirmed cases — quarantine zone expanding, check TAHC for current boundaries
Rest of Texas — No restrictions, but heightened vigilance advised
Interstate Travel — Entry permit required before leaving TX — CVI may be 72-hr window — verify destination state now
Canada — TX horses banned if in Texas within prior 21 days (as of June 5)
Dectomax EUA — Active — authorized for prevention in horses ≥1 yr

Quick Actions

Inspect all horses daily, especially wounds, foal navels, and surgical sites
Do not move an animal with suspected infestation — call your vet first
Obtain destination state entry permit before leaving Texas — permits may not be available on weekends or immediately
Verify CVI timing requirements — may be as short as 72 hrs; coordinate exam date accordingly
Continue your normal parasite control program — it remains important

What Is New World Screwworm?

Despite the name, New World Screwworm is not an intestinal parasite. It is a fly — Cochliomyia hominivorax — whose larvae develop in the living tissue of warm-blooded animals. Horses, cattle, sheep, goats, dogs, wildlife, and in rare cases people can all be affected.

Unlike common fly strike, which typically targets dead or decaying tissue, New World Screwworm larvae feed on healthy living tissue — which is what makes it so destructive. Female flies are attracted to wounds and may lay hundreds of eggs at a time. Even seemingly minor wounds are vulnerable:

Wire cuts and abrasions
Tick bites
Castration and surgical incision sites
Fresh dental extraction sites
Umbilical areas in newborn foals
Any open or draining wound

Without prompt treatment, significant tissue destruction can occur within days.

How Does It Spread?

Adult flies locate wounds and deposit eggs near the wound margins. The eggs hatch rapidly, and larvae burrow into surrounding tissue where they continue feeding and developing. New World Screwworm is spread by the fly itself — not by direct horse-to-horse contact. Because adult flies can travel significant distances, animal health officials focus heavily on surveillance, movement controls, and rapid case identification.

Temperature, Weather, and Fly Activity

New World Screwworm is a tropical fly with real temperature limitations — a fact worth understanding clearly:

Temperature Threshold	Effect on Screwworm	Impact
Below 58°F	Larval and pupal development stops	Limiting
Below 59°F	Adult fly activity drops off sharply	Limiting
Freezing temperatures	Lethal to fly and larvae	Eradicating
~81°F, humid	Optimal survival and reproduction	Peak Risk
Above 95°F (surface soil)	Larval mortality increases; adult activity reduced	Partially Limiting

Cold weather was historically the natural northern barrier that prevented the fly from overwintering in Central and North Texas. Unfortunately, we are now in peak fly season. Warm, humid summer conditions across South and Central Texas are ideal for screwworm activity. Horse owners throughout the state should remain vigilant through the warmer months.

Good News for Fall / Winter: As temperatures drop in autumn, fly activity and larval development will be naturally suppressed. Central and North Texas horse owners face meaningfully lower risk during the cooler months — but the situation in South Texas may persist year-round.

Can Horses Get New World Screwworm?

Yes. Although the confirmed Texas cases have primarily involved cattle, the June 8 Andrews County case involved a dog — a reminder that screwworm affects all warm-blooded animals, including horses and pets. Horses are susceptible to infestation and have historically been affected during previous outbreaks. Fortunately, horse owners are generally accustomed to observing their animals closely and monitoring wounds — which is the single most important tool for early detection.

What Does a Suspected Infestation Look Like?

Contact your veterinarian immediately if you observe:

Maggots within a wound
A wound that appears larger than expected or is rapidly worsening
Excessive drainage or a foul odor from a wound
Significant swelling around a wound or delayed wound healing
Unusual irritation, rubbing, or attention directed toward a wound

Important: Not All Wound Maggots Are Screwworm. Many wound infestations involve common fly larvae, not New World Screwworm. Laboratory identification is required to determine the species involved. Do not treat before collecting samples and consulting your veterinarian — and do not move the animal.

What Is the USDA Doing?

A coordinated federal response was underway well before the first Texas detection. The cornerstone is the Sterile Insect Technique (SIT): male flies are sterilized by irradiation and released by aircraft and ground dispersal chambers. Because female screwworm flies mate only once in their lifetime, mating with a sterile male produces eggs that never hatch — collapsing the wild population over successive generations.

Currently 8 million sterile flies per week are being deployed in and around the Texas quarantine zone. Two facilities at Moore Air Base in Edinburg, TX are central to the response — and it is important to distinguish between them:

Sterile Fly Dispersal Facility — Completed February 9, 2026 and currently operational. Receives sterile flies produced at the COPEG facility in Panama and disperses them by aircraft along the border and into South Texas.
Sterile Fly Production Facility — A separate, larger facility. Broke ground April 17, 2026 and is under construction. Initial production targeted for November 2027, scaling to 300 million sterile flies per week.

With all planned facilities at full capacity, USDA projects 500 million sterile flies per week — the same scale used to eradicate NWS from the United States in 1966.

We Beat It Before: New World Screwworm was successfully eradicated from the United States in 1966 using the sterile insect technique. The same proven strategy is being deployed today at larger scale, with modern tools, federal infrastructure, and a veterinary community that has been preparing for this moment for more than a year.

Should You Treat Your Horses Right Now?

Current USDA / TAHC Guidance: For most horses outside the current quarantine zone, the answer is: not yet. Current official guidance emphasizes vigilance, wound monitoring, and reporting suspicious cases — not widespread preventive treatment.

Routine preventive treatment of horses outside affected areas is not currently recommended unless directed by state or federal animal health officials. Unnecessary use of antiparasitic products creates resistance pressure and gets ahead of the official response framework.

Horse owners traveling to or through affected regions of South Texas should consult their veterinarian before travel.

What About Dewormers?

One of the most common misconceptions is that screwworm is simply another internal parasite. It is not — and the treatment landscape reflects that difference:

Fenbendazole and pyrantel — your standard paste dewormer rotation — have no effect on screwworm infestations whatsoever
Ivermectin (a macrocyclic lactone) has demonstrated activity against screwworm in some livestock species and has been used historically in other countries for prevention. However, according to the AAEP’s Infectious Disease Committee, there are currently no published studies documenting ivermectin’s efficacy specifically in horses for screwworm prevention or treatment. Its use for this purpose in horses would be extra-label, and indiscriminate avermectin use raises legitimate concerns about resistance in the internal parasites we manage routinely — including small strongyles. Critical safety note: do NOT administer ivermectin by injection (IM or SubQ) in horses. Injectable ivermectin formulations are not labeled for horses and carry significant risk of severe tissue reaction and toxicity. Oral paste is the only appropriate form of ivermectin for horses.

Currently Authorized Products for Horses

Follow current USDA, FDA, AAEP, and TAHC recommendations. Two product categories have received formal authorization:

Dectomax® (Doramectin Injectable · Zoetis) — FDA EUA, May 19, 2026
Macrocyclic lactone in the same drug class as ivermectin, but with demonstrated superior tissue persistence in head-to-head NWS cattle studies. Authorized for prevention of screwworm myiasis. Over-the-counter injectable.
Not authorized for horses under 1 year of age. Prevention only — not treatment of active infestation.

F10® Antiseptic Products (Wound Spray & Barrier Ointment) — FDA EUA, March & April 2026
Topical products authorized for wound-level prevention and treatment of New World Screwworm myiasis in horses, small ruminants, exotic mammals, and birds.

Key Takeaways on Prevention:

Continue your veterinarian-directed parasite control program.
Do not assume ivermectin paste provides adequate screwworm prevention in horses.
Consult your veterinarian regarding Dectomax if traveling to or through affected regions.
Monitor wounds carefully throughout summer — this remains the most important action for most horse owners.

Because recommendations may change as additional cases are identified and new products become available, always consult your veterinarian before implementing any prevention or treatment protocol.

Travel Restrictions and Summer Show Season

As of June 8, 2026, USDA and TAHC have confirmed four cases across three counties: Zavala County (two calves), La Salle County (one calf), and Andrews County (one dog). Quarantine and movement control areas are expanding as investigations continue. Animals in affected areas may not be moved without official authorization, inspection, and movement documentation. For horse owners outside the quarantine zone, there are currently no broad intrastate movement restrictions. However, the geographic spread of cases — now extending to West Texas — makes it essential to stay current with TAHC guidance.

Entry Permits, CVI Timing, and the Reality of Interstate Travel

Under normal circumstances, a Certificate of Veterinary Inspection (health certificate) is valid for 30 days for interstate travel. That assumption no longer holds during an active disease event.

With screwworm now confirmed in Texas, destination states have broad authority to impose additional requirements for horses originating from Texas. Depending on the state, you may now face:

A CVI valid for only 72 hours from the time of examination
Documentation of antiparasitic treatment on the CVI
An entry permit issued by the destination state — which must be secured before you leave Texas, not at the state line
Mandatory inspection upon arrival at a location designated by the destination state

Entry Permits: What You Must Understand Before You Load
Entry permits are issued by state animal health officials — not by your veterinarian and not by BVEH. They must be obtained before you leave Texas. Arriving at a state line without a required permit means you are at the mercy of that state’s officials and timeline.

Permits are not always issued immediately, and they may not be available on weekends. If your show or event starts Saturday and the destination state requires a permit, you may need to apply Thursday or earlier.

Requirements are changing rapidly. BVEH is actively monitoring interstate entry requirements and will share updates as we receive them — but you should be tracking your destination state’s requirements directly as well. This is a dynamic situation and we can only move as fast as the destination state allows.

A previously issued health certificate — even one that was valid last week — may no longer meet current requirements. Do not assume. Verify.

Canada has already acted: as of June 5, 2026, the Canadian Food Inspection Agency has temporarily banned horses physically present in Texas within the prior 21 days from entering Canada.

Before any out-of-state travel this summer, take these steps in order:

Contact the destination state’s animal health authority and confirm current entry requirements for horses from Texas
Determine whether an entry permit is required and apply immediately — do not wait until the week of travel
Call your veterinarian to ensure your CVI will meet the destination state’s current requirements, including any treatment documentation
Plan your health certificate examination timing based on the destination state’s CVI validity window — which may be as short as 72 hours
Confirm whether a mandatory arrival inspection is required and where it must occur

What to Do If You Suspect a Case

If you discover maggots in a wound or observe rapid deterioration of a wound:

Contact your veterinarian immediately
Do not move the animal
If instructed by your veterinarian, collect samples before initiating treatment
Follow guidance from your veterinarian and state animal health officials

Rapid identification is critical for disease surveillance and containment. Reporting is not just required — it directly supports the national response effort.

What Should You Do Right Now?

For most horse owners, the action plan is straightforward:

Inspect horses daily — wounds, skin folds, umbilicus on newborn foals, post-surgical sites, fresh dental work
Monitor wounds closely — watch for unexpected enlargement, foul odor, or maggots
Pay special attention to surgical sites, dental extraction sites, and newborn foals
Maintain your normal, veterinarian-directed parasite control program
Stay informed through TAHC and USDA official updates
If traveling to affected regions — consult your veterinarian about Dectomax and confirm CVI requirements
Report suspicious wounds promptly — do not treat before collecting samples

Final Thoughts

The confirmation of New World Screwworm in Texas is significant — but it is not a reason for panic. Horse owners who routinely monitor their animals, manage wounds appropriately, and maintain a strong relationship with their veterinarian are already doing the most important things to protect their horses.

Brazos Valley Equine Hospital is actively monitoring updates from USDA, TAHC, AAEP, and FDA. We will continue providing timely information as recommendations evolve. If you have questions about wound management, travel requirements, or potential exposure risks, please contact our veterinary team.

Official Resources

This page is provided for client education purposes. Guidance reflects information available as of June 2026 and is subject to change as the situation evolves. Always consult your veterinarian before implementing any prevention or treatment protocol.

Sources: USDA APHIS · Texas Animal Health Commission · American Association of Equine Practitioners · FDA

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New World Screwworm Confirmed in Texas: What Horse Owners Need to Know