On February 18, 2026, the FDA issued an Emergency Use Authorization for NexGard (afoxolaner) chewables to treat New World screwworm larvae in dogs. If you hike in southern border states (Texas, New Mexico, Arizona, California), read this before your first spring trip.

This isn’t a tick article. The threat is different, the mechanism is different, and the geographic risk zone is more specific. Here’s the quick version, and then we’ll break it down.

Quick Overview: NexGard + New World Screwworm

Detail	Info
FDA Authorization Date	February 18, 2026
Authorization Type	Emergency Use Authorization
Active Ingredient	Afoxolaner (NexGard chewable)
What It Treats	New World screwworm larvae (Cochliomyia hominivorax)
Primary Risk Area	Southern U.S. border states
Standard Dosing Still Applies	Monthly, weight-dosed (same as tick/flea use)
Dual Purpose Now	Flea/tick prevention + screwworm treatment

Bottom line: If your dog is already on monthly NexGard, you have some screwworm protection built in. If they’re not, and you’re hiking in southern border states this spring, it’s worth a vet conversation before your trip.

What New World Screwworm Actually Is

Most dog owners have heard of ticks, foxtails, heartworm. Screwworm is less familiar, and for good reason. Cochliomyia hominivorax was eradicated from the United States in 1966 through one of the most successful wildlife management campaigns in U.S. history. It’s been functionally absent for decades.

That’s changed.

The screwworm fly lays eggs in open wounds: a cut, a scratch, a small abrasion that a dog picks up on trail. The larvae burrow into living tissue and feed on it. That’s not a misread. These are flesh-eating larvae that grow inside a wound, often deepening and widening it rapidly. Left untreated, a screwworm infestation can be fatal within days.

This is categorically different from tick-borne disease risk. Ticks transmit pathogens. Screwworm flies deposit larvae directly into tissue. The damage is mechanical and fast.

The re-emergence in southern border regions is driving the FDA’s emergency response. Outbreaks in Mexico and Central America, combined with cross-border movement, have made screwworm a real and current concern in the southern U.S. Not a historical one.

Why Spring Hiking Season Is the Risk Window

Screwworm flies are heat-loving. They’re most active in warm months. Spring, when most of us are excited to get back on trail after winter, is exactly when fly activity increases.

A dog with any open wound in fly habitat is vulnerable. Even small injuries that wouldn’t warrant a vet visit under normal circumstances become entry points.

Think about what a 10-mile desert trail hike looks like for a dog: brush contact, rocky terrain, cactus encounters, creek crossings with sharp rocks on the bank. Rocky comes home with minor scrapes regularly. In traditional tick country, those scrapes get antiseptic and a quick check. In active screwworm territory, they need a different level of attention.

The southern border states see screwworm risk earlier in the spring season and for a longer window than northern regions. If you’re planning hiking trips in Texas hill country, the Chihuahuan Desert, Arizona’s Sonoran Desert, or near the California-Mexico border, the screwworm window is open.

What the FDA Authorization Means for Your Dog

The Emergency Use Authorization covers NexGard chewables for treatment of Cochliomyia hominivorax infestations in dogs. This is a labeled use. Your vet can now prescribe or recommend NexGard specifically for screwworm treatment, not just flea and tick prevention.

Here’s what the authorization actually covers:

Standard monthly dosing. The FDA authorization doesn’t change how you dose NexGard. It’s still weight-based, still monthly. The dosing guidance that’s existed for years for flea and tick prevention applies here too.

Treatment, not just prevention. The authorization covers treatment of active screwworm infestations. This matters because afoxolaner works systemically. Once it’s in your dog’s bloodstream, larvae feeding on tissue are exposed to it.

Dual-purpose coverage. Dogs on monthly NexGard for flea and tick prevention now have screwworm protection as part of the same dosing schedule. If you’re hiking in southern border regions, this is a meaningful benefit of staying current on their monthly dose.

NexGard is still prescription-only in the U.S. You’ll need a vet visit or an existing relationship with your vet to get it. That’s also the right place to have the conversation about whether screwworm risk applies to your specific hiking plans.

For the full FDA announcement, see the FDA’s Emergency Use Authorization notice for afoxolaner on fda.gov. The USDA APHIS New World Screwworm page has ongoing updates on the geographic spread of the outbreak.

Recognizing a Screwworm Infestation

Speed matters here. Screwworm infestations escalate fast once established.

Signs to look for in a dog who’s been in southern border terrain:

• Wounds that smell unusually foul (a rotten, distinct odor out of proportion to the size of the injury)
• A wound that appears to be getting larger or deeper after an injury
• Visible movement in wound tissue (larvae are visible if you look)
• Your dog excessively licking or pawing at a wound site
• Fever, lethargy, or loss of appetite combined with a wound

If you’re seeing any of these signs within days of a hike in a risk area, call your vet the same day. Don’t wait. Screwworm doesn’t follow a slow disease progression timeline like tick-borne illness. It moves quickly.

Practical Trail Protocol for Southern Border Hikers

The risk is manageable, but it takes some attention to your field practice.

Keep monthly NexGard current. If you’re hiking in Texas, New Mexico, Arizona, or SoCal border areas, staying current on NexGard gives you baseline screwworm protection alongside flea and tick coverage. Let it lapse, and you’re not covered.

Wound inspection matters more here. Do a full body check after every hike in southern border terrain, same as you would for ticks, but include a close look at every scrape, cut, or puncture. Anything that broke the skin gets cleaned, antiseptic applied, and covered if possible.

Don’t let wounds sit open in fly habitat. Screwworm flies need an open wound to oviposit (lay eggs). A clean, covered wound is significantly less attractive than an open one. For managing trail injuries, our dog first aid kit guide covers what to carry and how to clean and bandage wounds in the field. The wound-covering protocol that’s standard trail first aid is exactly the right move in screwworm territory.

A close-fitting protective layer helps. In heavy brush terrain where scrapes and contact wounds are likely, a dog shirt or protective layer reduces exposed surface area for flies. Rocky wears a lightweight base layer through dense desert scrub for the same reason we talked about tick protection: less exposed skin, fewer opportunities for parasites. Our dog performance apparel guide covers options that work in desert heat.

Know the nearest emergency vet. This is standard for backcountry trips, but it’s worth a specific search before trips in remote southern border areas. Distance to emergency care matters when you’re dealing with something that progresses as fast as screwworm.

How This Differs From Tick Prevention

The tick prevention post on this site covers spring parasite risks in detail. The screwworm situation is related but distinct enough to handle separately. Our spring tick prevention guide covers the nymphal tick peak, the preventatives that work, and the body-check protocol. All of that still applies in spring.

Screwworm adds a second layer. The geographic overlap is partial: tick risk is present across a much wider area than screwworm risk. If you’re hiking in the Southeast, Northeast, or Midwest, focus on the tick prevention protocol. If you’re in southern border states, you’re managing both simultaneously.

The two threats also require different responses to an injury: a tick bite gets the standard tick-removal protocol; a wound in screwworm territory gets covered, monitored, and your vet knows you were in a risk area if your dog develops symptoms.

What Rocky’s Spring Protocol Looks Like

Rocky’s spring 2026 prep includes NexGard current before the March window opens. That’s been the standard for flea and tick coverage anyway. The screwworm authorization just adds another reason to not let it lapse heading into spring.

For southern border trips specifically (we’ve done a couple of Texas hill country routes in past springs), the additional steps are wound management focus and a same-day vet call protocol if anything looks wrong.

We’re not skipping southern border hiking. But we’re treating wound care more seriously than we would on a Colorado trail.

Keep Your Gear Current Too

Spring is a good time to audit the whole system. Fresh NexGard dosing is one piece. An updated first aid kit is another. Vet wrap, antiseptic wipes, and proper wound-covering supplies matter more in screwworm territory than they’d be on a trail where a small scrape is just a small scrape.

For spring adventures generally, the spring hiking mud season guide covers the full pre-season checklist. The wound-care habit it builds is directly applicable here.

And if you’re doing longer southern routes where you and your dog are covering serious miles in remote terrain, a GPS collar gives you visibility on where your dog has been moving, including whether they pushed through heavy brush that could mean contact wounds you didn’t see. See our best GPS collars for hiking dogs for options that hold up to desert conditions.

Bottom Line

The FDA’s Emergency Use Authorization for NexGard against screwworm is a direct response to a real and re-emerging threat. The parasite was gone for decades. It’s not gone now.

If you’re hiking in southern U.S. border states this spring (Texas, New Mexico, Arizona, the California-Mexico border zone), keep NexGard current and take wound management seriously in the field. That combination covers most of the risk.

Your action item: If you’re planning spring trips in southern border states, call your vet this week and confirm NexGard timing. Make sure your first aid kit includes vet wrap and antiseptic, the basic tools to clean and cover a trail wound before flies have a chance to use it.

The threat is specific and manageable. But March doesn’t wait around.

---

FDA Emergency Use Authorization issued February 18, 2026, for afoxolaner (NexGard) chewables for treatment of New World screwworm (Cochliomyia hominivorax) in dogs. Consult your veterinarian before changing your dog’s parasite prevention protocol. Field experience with Rocky (50 lb Australian Shepherd mix) on desert and mountain trails, 2024–2026. Screwworm risk information informed by FDA announcements and USDA APHIS New World Screwworm response program.