🚨  Active Threat Advisory — 2025 / 2026

Protect Your Animals from
New World Screwworm

Cochliomyia hominivorax — one of the most destructive livestock parasites in the Western Hemisphere — is now approaching U.S. borders. Know the signs, understand your treatment options, and keep your herd safe.

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⚡ Quick Dosage Estimator NEW
Volume to Administer —
Dose (mcg/kg) 200 mcg/kg
Route —
Withdrawal —

⚠ Reference only. Always confirm with a licensed veterinarian. NWS is a reportable disease — contact USDA APHIS.

$3.6B
Annual livestock losses in
South America from NWS
10–21 hrs
For eggs to hatch after
being laid on a wound
5–7 Days
Larvae actively consume
living tissue before pupating

Understanding the Threat

What Is the
New World Screwworm?

The New World Screwworm (Cochliomyia hominivorax) is a parasitic fly native to tropical and subtropical regions of the Americas. Unlike most flies that feed on dead or necrotic tissue, screwworm larvae burrow into and consume the living tissue of warm-blooded animals — including livestock, pets, wildlife, and in rare cases, people.

Adult females lay 200–400 eggs on the edges of open wounds, ticks bites, navel of newborns, or natural body openings. Eggs hatch in as little as 10–21 hours. The larvae then spiral downward (hence "screwworm"), feeding and enlarging the wound for 5–7 days before dropping to the soil to pupate.

The U.S. successfully eradicated NWS from North America by the 1960s through the Sterile Insect Technique — but the parasite has been spreading northward since 2022 and was detected in Texas in 2025, posing a renewed threat to America's $130+ billion livestock industry.

  • Adult fly is green-to-blue with orange eyes
  • 200–400 eggs per egg mass
  • Eggs hatch in 10–21 hours
  • Detected in TX (2025–2026)
  • Thrives in warm, humid climates
  • Reportable disease in the U.S.
Cochliomyia hominivorax New World Screwworm Fly
Animals at Risk: 🐄 Cattle 🐎 Horses 🐑 Sheep 🐐 Goats 🐷 Swine 🦌 Deer & Wildlife 🐕 Dogs 🐈 Cats ⚠️ Humans (rare)

Clinical Signs

Recognize Screwworm
Before It's Too Late

Screwworm progresses rapidly — an untreated infestation can become life-threatening within days. Know these warning signs and act within hours of suspicion.

Immediate Action
🪱

Visible Larvae in Wound

Small cream-to-white larvae (0.5 in. long) with ring-like spines spiral deeply into tissue. They cluster at the base of wounds, often hidden from surface view. Do not attempt field diagnosis — contact your vet and USDA immediately.

Immediate Action
🤢

Foul-Smelling, Enlarging Wound

A wound that grows in size rather than heals, with a distinctive fetid, dark discharge. The odor is strong enough to attract more flies, rapidly worsening the infestation. Wounds may appear "hollowed out" or cavernous.

Immediate Action
🌡️

Fever & Systemic Illness

Elevated body temperature, rapid heart rate, and signs of systemic infection indicate the infestation has progressed. In cattle, normal temperature is 101–102.5°F. Values above 104°F with wound involvement warrant emergency veterinary care.

😟

Restlessness & Agitation

Animals become increasingly restless, switching their tail frequently, rubbing against objects, or isolating from the herd. They may stamp their feet or exhibit unusual head movements directed toward the affected area.

🍽️

Loss of Appetite & Lethargy

Reduced feed intake, weight loss, and general depression are common as the infestation progresses. Animals may separate from the group and stand listlessly. Cattle may stop milking normally in early cases.

👁️

Self-Directed Attention to Wounds

Animals lick, bite, or rub at specific areas of their body — particularly the navel (in newborns), tick bite sites, castration wounds, dehorning wounds, or other recent injuries. This is often the earliest behavioral sign.

🚨
If you suspect screwworm, do NOT wait.

New World Screwworm is a federally reportable foreign animal disease in the United States. Suspected cases must be reported immediately to your State Animal Health Official and USDA APHIS. Diagnostic samples are confirmed at the National Veterinary Services Laboratories (NVSL).

Biology of the Parasite

Understanding the
Screwworm Life Cycle

The screwworm's rapid, short life cycle is what makes it so dangerous — and why early detection and treatment within the first 24–48 hours is critical to outcome.

1
Egg-Laying
Adult female lays 200–400 eggs in rows on wound edges or skin openings. Females only mate once and may lay multiple egg masses over their 2–3 week lifespan.
2
Larval Hatching (10–21 Hours)
Larvae hatch rapidly and immediately burrow into living tissue. They secrete enzymes that liquefy tissue, spiraling deeper with each molt over 5–7 days.
3
Pupation in Soil (7–21 Days)
Mature larvae drop from the wound into the soil and form a hardened pupal case. They undergo complete metamorphosis. Duration varies with soil temperature.
4
Adult Emergence & Mating
Adult flies emerge, feed on plant nectar, and mate within days. Females seek warm-blooded hosts to lay eggs, completing the cycle. Adults can fly 20+ miles in search of hosts.
STAGE 1 Egg-Laying 200–400 eggs on wound STAGE 2 Larval Feeding Burrows into living tissue 5–7 days STAGE 3 Pupation in Soil 7–21 days underground STAGE 4 Adult Fly Mating & new egg-laying begins LIFE CYCLE ~3–4 weeks total (warm climate)

Reference Tool — Always Consult Your Veterinarian

Screwworm Dosage Calculator

Calculate reference doses for FDA-authorized New World Screwworm treatments based on animal species and body weight. All NWS cases are reportable — contact USDA APHIS immediately.

📊 Dosage Reference — —
Animal Weight —
Product & Concentration —
Dose Rate 200 mcg/kg (0.02 mL/kg)
Volume to Administer
Calculated volume for this animal —
Injection Route —
Injection Site —
Meat Withdrawal (cattle) —
Milk Withdrawal —
Reinfestation Prevention —
Regulatory Status —
⚠ Important — Veterinary Supervision Required: These doses are reference guidelines based on FDA conditional approval and EUA labeling. New World Screwworm is a federally reportable disease — all suspected cases must be reported to USDA APHIS and your State Animal Health Official before treatment. Extralabel use is NOT permitted for products operating under EUA or conditional approval. Always treat under veterinary supervision. Withdrawal periods must be strictly observed for food-producing animals. This calculator is for informational purposes only and does not replace professional veterinary advice.

Find doramectin and ivermectin antiparasitic products at:

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Treatment Products

Products Used in
Screwworm Management

The FDA has issued Conditional Approvals and Emergency Use Authorizations for specific antiparasitic drugs for NWS use. All NWS cases are reportable and treatment requires veterinary oversight. Consult your veterinarian to confirm which products and labeling apply to your situation. Vetr.com carries doramectin and ivermectin injectables and offers telehealth vet consultations.

✓ Conditionally Approved (CA)

Dectomax-CA1

Doramectin 1% (10 mg/mL) Injectable

The first FDA conditionally approved product for New World Screwworm. Provides treatment of active infestations AND prevention of reinfestation for 21 days. Available over-the-counter in 250 mL and 500 mL multi-dose vials.

Dose: 1 mL per 110 lbs (200 mcg/kg), SC or IM
Species: Cattle (dairy ≥20 mo), Horses ≥1yr, Swine, Sheep (non-lactating), Deer
Withdrawal: 35 days (cattle meat) | Not for veal calves
OTC or Rx: Over-the-counter
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⚡ Emergency Use Authorization

Ivomec 1% Injectable

Ivermectin 1% (10 mg/mL) Injectable — EUA

Authorized under Emergency Use Authorization specifically for prevention of NWS larvae in cattle when administered within 24 hours of birth, castration, or wound appearance. One of the most widely available antiparasitic injectables.

Dose: 1 mL per 110 lbs (200 mcg/kg), SC
Species: Cattle (except dairy producing milk & veal calves)
Withdrawal: 35 days (meat) | Not for milk-producing dairy
OTC or Rx: OTC (Rx for EUA use — vet required)
🛒 Shop Ivermectin →
Rx — Veterinarian Required

Dog & Cat Treatments

Nitenpyram, Lotilaner, Spinosad/Milbemycin — Rx Required

Several oral treatments have been authorized for dogs and cats. A single oral dose of nitenpyram achieves rapid larval expulsion within 6 hours. Lotilaner achieves 100% efficacy within 24 hours. Requires a valid veterinarian-client-patient relationship and veterinary prescription.

Species: Dogs and Cats
Nitenpyram dose: 1.43–4.42 mg/kg orally
Lotilaner: Minimum 20.5 mg/kg
OTC or Rx: Prescription required
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Vetr.com is your trusted source for veterinary-grade antiparasitic products, telehealth vet consultations, and prescription support.

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Best Practices

Screwworm Prevention
Protocol

You can't treat your way out of a screwworm outbreak. Prevention through reduced wound exposure, vigilant monitoring, and a vetted management plan is your most powerful defense.

1

Minimize Open Wounds

Schedule calving, dehorning, castration, branding, and ear-tagging for cooler months (winter/early spring) when fly populations are lowest. Tighten calving windows and use sanitary techniques. Every wound is a potential egg-laying site.

2

Daily Animal Inspection

Inspect every animal daily — especially newborns, recently wounded, and castrated animals. Focus on the navel, any skin wounds, ears, and natural body openings. Early detection (within 24–48 hours) can be the difference between full recovery and death.

3

Develop a Vet-Guided Treatment Plan

Establish a veterinarian-client relationship now. Discuss which treatment products are appropriate for your operation, when to use them, withdrawal times, and how to submit diagnostic samples if needed. Don't wait for an outbreak to find a vet partner. Use Vetr.com for telehealth support.

4

Integrated Fly Control Program

Reduce fly pressure with integrated pest management: sanitation to eliminate breeding sites, targeted use of fly traps and insecticide ear tags, pour-on insecticides for tick and fly control, and management of manure and standing water around facilities.

5

Wound Hygiene & Monitoring

Keep all wounds clean, dry, and protected. Apply topical wound sprays (pyrethroid-based) only when larvae are visibly present — not as a preventative. Check wounds at least twice daily until fully healed. Use wound coverings where practical to reduce fly access.

6

Know Reporting Requirements

Familiarize yourself with the reporting chain before an emergency: State Animal Health Official → USDA APHIS Veterinary Services → National Veterinary Services Laboratories (NVSL). Do NOT discard larvae — preserve them in isopropyl alcohol for lab submission to confirm NWS identification.

Mandatory Reporting

Suspect Screwworm?
Report Immediately.

New World Screwworm is a federally reportable foreign animal disease in the United States. Do not wait for laboratory confirmation — report suspected cases immediately. Every hour matters in containing an outbreak.

USDA APHIS
1-866-536-7593
Veterinary Services — National Call Center
Texas Animal Health Commission
1-800-550-8242
For TX livestock — border area priority line
NVSL — Lab Submissions
1-515-337-7266
Preserve larvae in isopropyl alcohol before shipping
State Vet — Find Yours
aphis.usda.gov →
Directory of all state animal health officials
Visit USDA APHIS NWS Resource Page →

Common Questions

Frequently Asked
Questions

Everything producers, pet owners, and veterinary professionals need to know about New World Screwworm management in 2025–2026.

The New World Screwworm (Cochliomyia hominivorax) is a flesh-consuming fly larva that targets any warm-blooded animal with an open wound. The U.S. eradicated it from North America in the 1960s, but it has been spreading northward from Central and South America since 2022. In 2025, it was confirmed in Texas near the U.S.–Mexico border, triggering a federal response. Bioclimatic modeling suggests that rising global temperatures will continue to expand its habitat range.
Most fly species that cause myiasis (wound fly infestations) target dead, necrotic, or decaying tissue — they are secondary invaders. New World Screwworm is uniquely aggressive: it targets living, healthy tissue and actively burrows deeper to consume it. This makes NWS significantly more dangerous and destructive than common blowfly infestations. An untreated screwworm infestation can kill a large animal within 7–14 days.
Dectomax-CA1 is dosed at 1 mL per 110 lbs of body weight (200 mcg/kg, or 0.02 mL/kg), administered by subcutaneous (SC) or intramuscular (IM) injection in the neck region. The meat withdrawal period for cattle is 35 days — the same as standard Dectomax. It is NOT for use in veal calves or dairy cattle under 20 months of age. Use our Dosage Calculator above to calculate the exact volume for your animal. Always confirm dosing with your veterinarian.
While some products (like Dectomax-CA1) are classified as over-the-counter, treating NWS without veterinary involvement is not recommended. First, New World Screwworm is a reportable disease — you are legally required to notify animal health authorities. Second, proper treatment involves larval removal, wound debridement, and ongoing wound management that requires training. Third, veterinarians must inform you of EUA obligations, withdrawal times, and alternative treatments. Contact Vetr.com's telehealth service for accessible vet guidance.
Do not panic, but act immediately: (1) Collect larvae without crushing them — place in a sealed container with isopropyl alcohol for lab submission. (2) Call your vet right away — you will need a valid vet-client relationship for prescription treatments. (3) Report to authorities — contact USDA APHIS (1-866-536-7593) and your State Animal Health Official. (4) Isolate the animal if possible to prevent attracting more flies. Do not attempt to self-diagnose NWS in the field — many other fly species cause similar-looking infestations, and official lab confirmation is required.
Yes. For horses (1 year and older), Dectomax-CA1 (doramectin injection) is authorized for both prevention and treatment at the same dose (1 mL/110 lbs). For dogs and cats, several oral treatments have been authorized including nitenpyram (rapid action — larval expulsion in 6 hours), lotilaner (100% efficacy in 24 hours), and oral spinosad/milbemycin combinations. These companion animal products are prescription-only and require a veterinarian-client relationship. Contact Vetr.com for telehealth consultations.
Vetr.com carries veterinary-grade doramectin and ivermectin injectable products and offers telehealth consultations with licensed veterinarians who can guide you on product selection, NWS-specific dosing, and documentation requirements. Visit vetr.com/shop to browse available products, or download the Vetr App to connect with a vet directly. Note: always confirm with your veterinarian that the specific product you purchase carries the appropriate NWS Conditional Approval or EUA labeling. For Texas producers, the USDA National Veterinary Stockpile has also placed NWS treatment supplies with the Texas Animal Health Commission (TAHC).
Producers in border areas should take these immediate steps: minimize and schedule all surgical procedures for cooler months; inspect animals twice daily, especially newborns and wounded animals; maintain a written emergency treatment protocol with your veterinarian; review and tighten your integrated pest management to reduce fly populations; ensure all working pens, calving areas, and housing are sanitary; and make sure all workers know the signs of screwworm and the reporting chain. Contact your county extension office or state veterinarian for region-specific guidance.
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