Screwworm Myiasis: Prevention, Natural Deterrents, Treatment & Recovery

A Parasite That Was Gone Has Returned

For decades, screwworm was something veterinarians in the United States mostly learned about in textbooks. It was considered a historical success story, one of veterinary medicine’s greatest eradication achievements. But very recently, that story has changed.

New World screwworm has once again been confirmed in the United States. As of June 26, 2026, there have been 27 cases of infection identified in animals, including cattle, goats, sheep, and one dog, across Texas and New Mexico¹ .

For pet parents, livestock owners, rescue groups, and anyone living in warm climates, this is not something to ignore. It also isn’t something to panic about; knowledge and preparation matter. These recommendations are for pet parents living in these high-risk areas.

Screwworm is highly preventable when wounds are recognized early and protected appropriately. Many people have never seen screwworm before, so infestations may initially be mistaken for an ordinary skin infection, a hot spot, or delayed wound healing.

That delay can become dangerous. Unlike common fly larvae that feed on dead tissue, screwworm larvae consume living tissue. That distinction is a game-changer.

What Exactly Is Screwworm Myiasis?

Screwworm myiasis is a parasitic infestation caused by the larvae (maggots) of specific fly species that feed on living tissue ². That may sound dramatic, but unfortunately, it’s true. Unlike ordinary blowflies that strictly feed on dead tissue or material, once hatched, screwworm larvae actively burrow into healthy flesh, causing progressive and potentially fatal tissue destruction that can accelerate surprisingly fast if left untreated.³

There are two species: the New World screwworm (Cochliomyia hominivorax), found in the Americas and Caribbean, and the Old World screwworm (Chrysomya bezziana), found in Southeast Asia, Africa, and parts of the Middle East.

Female screwworm flies are extraordinarily efficient. They locate wounds using odor molecules released by damaged tissues and body secretions⁴. A single female can lay 200–400 eggs at the wound edge. Within 12–24 hours, the eggs hatch into tiny larvae that immediately begin burrowing into living tissue, feeding and growing for 5–7 days before dropping to the ground to continue their development into adult flies.

In warm conditions, the entire cycle from egg to adult takes roughly 3 weeks, meaning a single untreated wound can support an entire local population.

How Did Screwworm Come Back?

Screwworm was previously eradicated from the continental United States in the 1960s. That successful elimination required a massive government program called the sterile insect technique (SIT)⁵. Millions of sterile male flies were released repeatedly. Female flies mated but produced no offspring. Over time, populations collapsed.

For decades, a biological barrier zone helped limit movement and was maintained at the Panama-Colombia border to prevent northward spread.⁶ However, the threat has now become reality.

As of June 27, 2026, twenty-seven cases of the New World screwworm have been confirmed in the United States⁷. All but one of the infestations occurred in Texas. One case was recorded in a dog in New Mexico. Thankfully, the dog appears to be recovering, according to Searchlight New Mexico, an Independent Investigative Journal⁸:

“The affected dog, a small-breed adult male that lives in Eunice, is being treated and recovering from the infestation, Dr. Samantha Holeck, state veterinarian with the New Mexico Livestock Board, said at a news conference Monday.

Which Pets Are Most at Risk?

Female screwworm flies don’t randomly choose animals. They locate specific chemical signals released by wounds and inflamed tissue. Even tiny skin injuries or healing wounds require attention during active outbreaks.

Any warm-blooded animal with an open wound in locations where screwworm has been identified is a potential target, but certain factors increase risk significantly:

• Unneutered male dogs and cats: Intact males are statistically more likely to sustain fight wounds, which are the most common entry point for screwworm infestation. In a study of 76 dogs and cats with screwworm myiasis, intact males with suspected fight-related wounds were the most commonly affected group.⁹

• Dogs with ear infections, skin folds, or chronic skin conditions: The most common infestation sites in dogs are the external ear canals, followed by the perineum and medial canthus (inner corner of the eye). In cats, paws and tail are most commonly affected.

• Pets with recent surgical sites, lacerations, or hot spots: Any open wound, no matter how small, can attract egg-laying females.

• Outdoor pets in warm, humid climates: Screwworm thrives in tropical and subtropical environments. With the current outbreak in south-central Texas and southeastern New Mexico, pets in these areas are at immediate risk.

• Pets traveling to or imported from endemic regions: Dogs transported from Central America, South America, the Caribbean, or Southeast Asia may carry active infestations.

Prevention is the most powerful tool against screwworm. It centers on three pillars: wound management, fly deterrence, and environmental awareness. If you live in a high-risk area, this is critical information. The strongest prevention plan combines wound management with environmental management.

Prevention Pillar #1: Wound Management

Prompt wound care is the single most important preventive measure. Screwworm flies are specifically attracted to wound odors; research has identified the precise chemical signals from wounds and animal secretions that draw female flies to lay eggs.

• Clean and cover all wounds immediately. Flush with saline or dilute chlorhexidine, apply a wound dressing, and bandage when possible.
• Monitor surgical sites closely, especially in warm months. Keep incisions covered and check twice daily.
• Treat ear infections, hot spots, and skin lesions promptly; these are common entry points.
• Check your pet's entire body daily during warm months if you live in or are traveling through at-risk areas, paying special attention to ears, eyes, perineum, paws, tail, and any skin folds.

Given the ongoing outbreak, daily wound checks are critical for pets in Texas, New Mexico, and neighboring states for the remainder of summer 2026.

By the time larvae become visible, they may already be deeply established. The good news is that unlike many infectious diseases, screwworm requires access to vulnerable tissue. That means reducing every opportunity can dramatically reduce your pet’s risk.

I think about prevention in three layers:

1. Protect the skin.
2. Reduce fly attraction.
3. Lower environmental exposure.

When these three strategies work together, risk drops substantially.

Your Pet’s First and Best Defense

Prompt wound care remains the single most important prevention tool. Female screwworm flies don’t randomly choose animals. They locate specific chemical signals released by wounds and inflamed tissue.

That means even tiny skin injuries deserve attention during outbreak periods. I recommend becoming more intentional about daily skin checks during warm weather.

That includes places pet parents commonly overlook.

Daily High-Risk Body Check Areas

Screwworm Prevention Checklist

My Daily Wound Protection Routine

If you discover a wound:

1. Flush gently using sterile saline or dilute betadine.
2. Pat dry and avoid aggressive rubbing.
3. Protect with breathable coverage when appropriate.
4. Prevent licking.
5. Recheck twice daily.

Small wounds become large problems when ignored.

Prevention Pillar #2: Natural Fly Deterrents

Several plant-based compounds have documented fly-repellent and larvicidal activity that can be used as part of a prevention strategy. If you live in or plan to travel through an area with active screwworm risk, prevention becomes your most powerful tool. One of the reasons screwworm is so concerning is that prevention opportunities often happen before you ever realize your pet was exposed.

Layered Protection Using Evidence-Informed Botanical Support

Natural deterrents should never replace wound care. But they may add another layer of protection. When I discuss natural approaches, I prioritize ingredients with documented effects against fly behavior or larval development, not folklore.

These recipes are intended as preventive support, not treatment for active screwworm.

Neem Oil Fly Spray (Dogs & Cats)

Why Neem?

Neem is the most extensively studied natural fly deterrent. Its active compound, azadirachtin ¹⁰, acts as a feeding deterrent, growth disruptor, egg-laying repellent, and a deterrent against blowflies and screwworm species at concentrations as low as 10 ppm. At this concentration, azadirachtin deterred blowfly larvae from feeding on treated tissue.

This spray creates a 3% neem oil emulsion, a water-based formula that sprays easily and distributes evenly over the coat.

Cat Considerations

This neem-only preparation may be better tolerated and safer than formulas containing essential oils. Because cats groom extensively, use lightly and monitor closely.

Recipe for DOGS ONLY – Thyme & Oregano Synergistic Fly Spray

DO NOT USE ON CATS. Thymol and carvacrol (the active compounds in thyme and oregano oils) are phenolic compounds that can be toxic in cats, which lack the liver enzyme (glucuronyl transferase) needed to metabolize them safely.

Laboratory studies identified thyme oil as the most potent essential oil tested against screwworm larvae, achieving 100% larval mortality. Oregano oil also achieved 100% mortality¹¹. When the active compounds thymol and carvacrol are combined, they produce a synergistic effect, achieving 83–99.5% larval mortality at concentrations below what either compound achieves alone. This recipe creates a water-based emulsion spray with a 2% essential oil concentration, strong enough to deter flies yet safe for canine skin.

Ingredients:

• 1 teaspoon (5mL) Thyme essential oil (Thymus vulgaris, look for "thymol chemotype" or "red thyme")
• 1 teaspoon (5 mL) Oregano essential oil (Origanum vulgare)
• 2 teaspoons (10 mL) Liquid castile soap (emulsifier)
• Warm water, enough to fill a 16 oz (500 mL) spray bottle

Directions:

1. In a small bowl, combine 1 teaspoon thyme oil and 1 teaspoon oregano oil with 2 teaspoons liquid castile soap
2. Stir thoroughly for 30 seconds until the oil and soap are fully blended into a uniform mixture. The soap emulsifies the essential oils so they can disperse in water
3. Pour this mixture into the spray bottle
4. Fill the rest of the bottle with warm water
5. Cap and shake vigorously for 30 seconds
6. The result is a slightly cloudy, lightweight emulsion that sprays easily

Shake well before every use.

For a Stronger 5% Spray (high-risk situations during active outbreaks):

• Increase thyme oil to 2½ teaspoons (12 mL) and oregano oil to 2½ teaspoons (12 mL)
• Increase castile soap to 1 tablespoon (15 mL)
• Use the same 16-oz water bottle.
• Start with the 2% formula. If well tolerated after 48 hours without skin irritation, the 5% version may be used during active outbreaks.

How to Apply:

• Shake well before each use.
• Mist onto ears, perineum, skin folds, belly, legs, and around (not directly into) any wounds
• Avoid the eyes, nose, mouth, and genital mucous membranes.
• Reapply every 12–24 hours, or after swimming or heavy rain.
• Do not spray directly into open wounds. Use the Neem & St. John's Wort wound dressing (Recipe 3) for direct wound protection.
• Shelf life: Make a fresh batch every 7–10 days. Store in a cool, dark place.

Neem & St. John’s Wort Wound Dressing (DOGS ONLY)

This is not a spray; it is an oil-based wound dressing for direct application to wound margins. In a study of 44 domestic animals with active myiasis wounds¹² this formulation achieved complete wound healing in 10–32 days with zero reinfestation and zero bacterial complications without antibiotics or chemical insecticides. This is the strongest clinical evidence for any natural wound management protocol against myiasis.

Use this dressing for:

• Protecting healing wounds from fly strike during the current outbreak
• Post-veterinary-treatment wound care after screwworm larvae have been removed
• Any open wound on a pet in an at-risk area during warm months
• Cold-pressed neem oil (unrefined, organic preferred)
• St. John's Wort infused oil (Hypericum perforatum oil, the herb infused in a carrier oil, NOT the essential oil or oral supplement; it has a distinctive deep red color)
• A small, clean glass bowl for mixing
• Cotton gauze pads or clean cotton swabs for application
• A small dark glass bottle for storage (1–2 oz amber dropper bottle)

Recipe:

1. Measure equal parts cold-pressed neem oil and St. John's Wort infused oil (e.g., 1 tablespoon of each)
2. Combine in a clean glass bowl and stir gently until well blended
3. Transfer to a dark glass storage bottle

No water, no emulsifier, no additional ingredients needed. This is an oil-based dressing applied directly to wound margins — not a spray.

How to Apply:

1. Clean the wound first. Gently flush with sterile saline or lukewarm clean water. Remove any visible debris. If larvae are present, do not attempt removal; see your veterinarian immediately.
2. Apply the oil blend generously to the wound margins and surrounding skin (approximately 1–2 inches around the wound edge) using a cotton gauze pad or clean cotton swab.
3. Do not bandage tightly. A loose, non-adherent dressing may be placed over the area to prevent licking, but avoid airtight wrapping.
4. Reapply every 12–24 hours. Gently clean the wound with saline before each reapplication.
5. Continue daily application until the wound is fully closed and healed.

Prevention Pillar #3: Environmental Awareness

Know your risk zone. If you live in or are traveling through Texas, New Mexico, or any southern border state, the risk is currently elevated. Monitor USDA APHIS and CDC updates for the latest outbreak information.

Limit outdoor wound exposure. If your pet has any wounds, keep them indoors as much as possible during peak fly activity (warm daylight hours).

Report suspicious findings. If you find maggots in a wound on any animal, contact your veterinarian and local animal health authorities immediately. Early reporting is critical to containing the outbreak.

Travel precautions. If traveling with pets to Mexico, Central America, South America, or the Caribbean, inspect your pet thoroughly before and after travel. Quarantine periods may apply.

What to Do If Your Pet Shows Signs of Infestation

If you suspect screwworm, treat the situation as a veterinary emergency. This is not a “watch and wait” condition. Unlike many skin infections that progress over days or weeks, screwworm larvae actively destroy living tissue while they feed. Every hour matters because larvae continue burrowing deeper, enlarging the wound and increasing inflammation, pain, and the risk of secondary infection.

The earlier treatment begins, the easier recovery tends to be.

Step 1: Get to a Veterinarian Immediately

Screwworm myiasis is a veterinary emergency. Do not delay. The larvae are actively destroying tissue, and the infestation will worsen rapidly without treatment.

Step 2: Do Not Try to Remove Larvae Yourself

Screwworm larvae have backward-facing spines that anchor them deep in tissue. Pulling them out can leave mouthparts behind, worsen tissue damage, and increase the risk of infection. Your veterinarian has the tools and training to remove them safely.

Step 3: Veterinary Treatment — Modern Pharmaceutical Approach

Modern veterinary treatment has moved beyond manual extraction alone (what vets did during the last outbreak was debride large amounts of tissue; we don’t do that anymore). The current standard of care uses systemic oral medications to kill larvae inside the wound first, followed by removal of dead larvae and wound care. This approach is faster, less painful, and more effective than manually removing live larvae.¹³

How It Works

Your veterinarian will give your pet an oral medication that enters the bloodstream. When larvae feed on the treated tissue, they are killed within hours. Dead larvae either fall out on their own or are easily removed with gentle flushing (without the need for deep sedation or aggressive surgical debridement, as previously required¹⁴). Isoxazolines are the standard treatment, but there are other options that integrative vets, including me, prefer:

Safest Treatment: Nitenpyram (Capstar)

• Dose: 1–4.4 mg/kg orally (one tablet by mouth)
• Kills 100% of screwworm larvae in just 6 hours faster than any other option
• Available over-the-counter at most pet stores and pharmacies
• Excellent safety profile with no MDR1 interaction
• Excellent safety profile with no MDR1 interaction
• Short-acting (lasts ~24 hours), so it does not provide residual reinfestation protection — wound care and fly deterrents are essential during recovery

A comparative study of 40 naturally infested dogs tested five different oral medications head-to-head¹⁵. Here is how they performed:

Standard Treatment Protocol:

The standard veterinary treatment follows these steps:¹⁶

1. Systemic larvicide: Your vet gives your pet an oral medication (see table above) to kill the larvae from the inside
2. Wait for larval death: 6–24 hours, depending on the drug chosen
3. Manual removal: Dead and dying larvae are gently removed with forceps and flushed out with saline
4. Wound debridement: Any dead or damaged tissue is removed
5. Wound flushing: The wound is thoroughly cleaned with antiseptic solution
6. Antibiotics: Prescribed if there is secondary bacterial infection. Use a probiotic if your pet takes an antibiotic.

Ivermectin as an Alternative

Some owners don’t want to give Capstar or isoxazoline medications (NexGard, Credelio, Simparica, Bravecto). For these dogs, ivermectin is an option for treating screwworm myiasis, but it must be used with caution in certain breeds due to the required dose.

Dose: 200 µg/kg (0.2 mg/kg) given as a single subcutaneous injection or orally. This is the standard extra-label dose extrapolated from the bovine formulation. It provides 100% larval kill and 16–22 days of residual protection against reinfestation.

Check Breed First

Ivermectin at this dose is dangerous for dogs carrying the MDR1 (ABCB1-1Δ) gene mutation. The 200 µg/kg treatment dose is more than 33 times the heartworm prevention dose and can cause severe neurological toxicity (tremors, blindness, seizures, coma, and death) in MDR1-mutant dogs. See the MDR1 Breed Sensitivity section of this guide for the full list of at-risk breeds. MDR1 genetic testing should be performed before using ivermectin at treatment doses in any herding breed or mixed-breed dog of unknown heritage.

• Dog cannot take isoxazolines, NOT an MDR1-risk breed → Ivermectin 200 µg/kg SC or PO (100% kill + 16–22 days residual protection)
• Dog cannot take isoxazolines AND IS an MDR1-risk breed. MDR1 mutations have been detected in various dog breeds such as rough- and smooth-coated Collie, Shetland Sheepdog, Australian Shepherd, McNab, Longhaired Whippet, Silken Windhound, Old English Sheepdog, English Shepherd, Border Collie and Wäller¹⁷. (or unknown breed) → Nitenpyram (Capstar) 1–4.4 mg/kg PO (100% kill in 6 hours, no residual)

Step 5: Recovery and Monitoring

Keep the wound clean and covered during healing.

Apply neem oil + St. John's Wort oil dressing to wound margins to prevent reinfestation during recovery.¹⁸

Keep your pet indoors or in a screened area during recovery

Follow up with your veterinarian as directed

Sources and References:

• ¹ USDA Confirmed Detections of New World Screwworm
• ² The New World Screwworm in the United States:2025 Travel- Associated Human Case
• ³ The British Journal of Nutrition, 2011 Oct
• ⁴ Scientific Reports, 27 November 2020
• ⁵ USDA Stop Screwworm Using SIT
• ⁶ Cureus, 2025 Oct 7
• ⁷ USDA Screwworm Current Confirmed Cases
• ⁸ Searchlight New Mexico
• ⁹ Vet Dermatol. 2018 Oct;29
• ¹⁰ Parasitol Res. 2015 Jan
• ¹¹ Larvicidal activity in vitro of essential oils against Cochliomyia hominivorax
• ¹² Parasitology Research, Aug 2019
• ¹³ Vet Dermatol. 2018 May 6
• ¹⁴ Vet Dermatol. 2017 Apr 28
• ¹⁵ Vet Dermatol. 2018 May 6
• ¹⁶ Vet Dermatol. 2017 Apr
• ¹⁷  Parasit Vectors, 2014 Mar 6
• ¹⁸ Vet Parasitol. 2010 Oct 11

About Karen Shaw Becker, DVM, CVH, CVA, CCRT

Veterinarian Dr. Karen Shaw Becker believes biologically appropriate food and an animal's immediate environment are essential in determining health, vitality, and lifespan. She has spent her career as a wildlife and exotic animal veterinarian and small animal clinician, empowering animal guardians to make intentional lifestyle decisions to enhance the well-being of their animals.