What is New World Screwworm (NWS) and How Do You Treat It?

What is New World Screwworm or NWS?

According to CDC, New World screwworm (NWS), or Cochliomyia hominivorax, is species of parasitic fly that completes part of its lifecycle by feeding on the tissue or flesh of warm-blooded animals, with cattle, particularly calves, being reportedly the most commonly affected species. NWS flies are attracted to wounds and body openings like the nose, eyes, ears, and mouth, where they lay eggs. The eggs hatch into maggots that eat live tissue, causing a worsening, often painful and foul-smelling wound.

The term "screwworm" refers to the feeding behavior of maggots where they burrow into the wound, feeding as they go. They tear the host’s tissue with their sharp mouth hooks, leading the wound to become larger and deeper where they eventually hatch and feed. All this makes NWS a serious, often deadly pest to animals and livestock.

What are the signs and symptoms that animals have NWS?

• Irritated behavior
• Head shaking
• The smell of decay
• Presence of fly larvae or maggots in wounds
• If the animal’s wound continues to enlarge or deepen over time
• Restlessness
• Excessive licking or biting of wounds
• Uncharacteristic lethargy
• Egg masses found in or along wound margins

If you observe these from your pet, horses and/or livestock, contact your vet immediately.

What to do if you think your animal has NWS?

You can refer to this Standard Operating Procedure for Possible NWS Detections the Animal and Plant Health Inspection Service that lists the following actions:

1. Identification and Notification of Suspect Cases – Wounds and areas of infestation may be malodorous and suppurative with bloody or purulent discharge and should be examined.
2. Submission of Samples to the National Veterinary Services Laboratories (NVSL) - Take samples for official identification. Samples should not be collected nor submitted without first consulting with the APHIS VS AVIC or the SAHO. In most cases, a Foreign Animal Disease Diagnostician from the USDA Veterinary Services or State will be dispatched to collect samples.
3. Quarantine and Treatment of the Affected Animal - The affected animal(s) must be held in quarantine under the supervision of a USDA accredited veterinarian and/or an official (federal, state) veterinarian, until official parasite identification has been obtained and effective treatment implemented. Any animal confirmed to be affected with screwworm myiasis must undergo effective treatment along with treatment of the surrounding premises.
4. Cleaning and Disinfesting the Affected/Quarantine Premises - All expelled larvae in the environment must be collected into tightly sealed tubes with alcohol to kill the larvae, and Ziploc bagged before throwing in the trash. Holding areas (pens, stalls, kennels, etc.) and beddings should be sprayed with a larvicidal insecticide then wiped out to find potential larval or pupal flies.
5. Premises Monitoring - In the quarantine area, hang fly paper or other type of effective fly traps in several locations.

How is Cochliomyia hominivorax from NWS treated?

According to AVMA , standard procedures should be followed, under the supervision of the official veterinarian.

It is recommended to:

• Remove all visible eggs and larvae in and around the wound.
• Provide wound care, including analgesia, as appropriate.
• Use antiseptics to prevent secondary infections.
• Administer approved or otherwise authorized topical or systemic antiparasitics for the affected species. For prescription medication for Dogs, you can check Rx Credelio Quattro Chewables or Rx Credelio (Iotilaner) Chewables; and for cats, check out Rx Credelio Tablets.
• Reexamine the animal(s) 24 hours after treatment to confirm no live larvae remain in the wound. If live larvae do remain, remove them and retreat.

Animal Drugs Approved or Conditionally Approved by FDA for NWS

• Injectable Doramectin (Dectomax) - Prevention and treatment of infestations caused by larvae of Cochliomyia hominivorax (myiasis), and prevention of reinfestation for 21 days in cattle. Over the counter.
• Topical fluralaner (Exzolt Cattle-CA1) - Prevention and treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) and treatment and control of cattle fever tick (Rhipicephalus microplus) in beef cattle 2 months of age and older and replacement dairy heifers less than 20 months of age.
• Credelio Quattro-CA1 Chewable tablets - Treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) in dogs and puppies.
  
• Bimectin Injectable Cattle & Swine Wormer - Bimectin Injectable Cattle and Swine Dewormer is used for the control of internal and external parasites including roundworms, lungworms, grubs, sucking lice and mange mites in cattle.
• Blue Lotion Topical Spray Animal Antiseptic - This is a fast-drying, antiseptic wound dressing and gall lotion aids in the prevention of superficial infections.
• Jeffers Super Sulfur Healing Paste - Jeffers Super Sulfur natural wound paste for pets, horses & livestock is a sulfur-based skin care made with an animal-friendly blend of sulfur, calendula, bentonite clay, coconut oil and glycerin.
• Permethrins

Emergency Use Authorization of Animal Drugs for NWS

On August 18, 2025, the Department of Health and Human Services issued a declaration that enables FDA to issue Emergency Use Authorizations (EUAs) for animal drugs to treat or prevent infestations caused by NWS. For an overview of the EUA process, see: Summary of Process for EUA Issuance.

• Credelio (Iotilaner) Chewable Tablets - Treatment of infestations caused by New World screwworm larvae (myiasis) in dogs and puppies.

FOI Summary
Letter of Authorization
Fact Sheet for Veterinarians 
Granting Letter

• Credelio CAT (lotilaner) chewable tablets - Treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) in cats and kittens.

FOI Summary
Letter of Authorization
Fact Sheet for Veterinarians

• Ivomec (ivermectin) injectable solution - Prevention of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) when administered within 24 hours of birth, at the time of castration, or at the appearance of wound in cattle, except for female dairy cattle producing milk for human consumption and calves that will be processed for veal.

FOI Summary
Letter of Authorization
Fact Sheet

• NexGard (afoxolaner) chewable tablets - Treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) in dogs and puppies.

FOI Summary
Letter of Authorization
Fact Sheet for Veterinarians

• NexGard COMBO (esafoxolaner, eprinomectin, and praziquantel topical solution) - Treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) in cats and kittens.

FOI Summary
Letter of Authorization
Fact Sheet for Veterinarians

• F10 Antiseptic Wound Spray with Insecticide (benzalkonium chloride, polyhexanide and cypermethrin topical solution)* - Prevention and treatment of infestations caused by New World screwworm larvae (myiasis) in cattle, horses, minor species of hoof stock, raptors and other wild birds, pet birds, and captive wild, exotic, and zoo mammals.

FOI Summary
Letter of Authorization
Fact Sheet

• F10 Antiseptic Barrier Ointment with Insecticide (benzalkonium chloride, polyhexanide and cypermethrin topical ointment)*

FOI Summary
Letter of Authorization
Fact Sheet

• Negasunt Powder (coumaphos, propoxur, and sulfanilamide topical powder)* - Prevention and treatment of infestations caused by New World screwworm larvae in cattle, swine, goats, sheep, horses, donkeys, domestic hybrid equids, and captive wild, exotic, and zoo mammals.

FOI Summary
Letter of Authorization
Fact Sheet
Ficha TécnicaExternal Link Disclaimer

• Nitenpyram Tablets (nitenpyram) - Treatment of infestations caused by New World screwworm (Cochliomyia hominivorax) larvae (myiasis) in dogs, puppies, kittens, and cats that are 2 pounds of body weight or greater, and 4 weeks of age and older.

FOI Summary
Letter of Authorization
Fact Sheet

• Rx NexGard Flea and Tick Chewables for Dogs

• Rx NexGard Plus Chewable Flea & Tick Tablets for Dogs

How To Prevent New World Screwworms from Spreading

The best way to keep screwworms out of your farms and barns is to prevent their introduction.

In an article from USDA, here are some steps you can to do prevent infestations:

• Watch for signs of NWS in pets and livestock.
• Make sure pets traveling internationally are inspected for screwworm.
• Handle livestock carefully and inspect pens and equipment for sharp objects that can cause wounds.
• Treat the umbilical cords of newborn animals and all wounds immediately with an approved insecticide.
• Protect pets and livestock from other wound-causing parasites such as ticks.

Eradicating NWS is only possible through sterile insect technique, where sterile male flies are released into an area where a known population has become established. The sterile male flies mate with wild female NWS flies, which then lay unfertilized eggs. Because female NWS flies mate just once in their lifespan, the NWS population progressively reduces and ultimately dies out.

Other Facts of New World screwworm (Cochliomyia hominivorax)

• Adult New World screwworms, Cochliomyia hominivorax, are metallic blue blow flies with three distinct stripes that run down the top (thorax) of the fly just behind the head with large orange eyes.
• The Infestation is called Myiasis where eggs hatch into parasitic maggots in just a few hours. Maggots then burrow deeply into living flesh, feeding for 5 to 7 days.
• NWS thrives in warm, tropical and subtropical climates. It is endemic to parts of South America, the Caribbean, and Central America, with outbreaks occasionally reemerging further north.
• The U.S. eradicated the pest in the 1960s by continuously releasing hundreds of millions of sterile male flies into the wild, preventing the females from producing offspring.
• What’s the difference between Old World and New World Screwworms? New World screwworms are found in the Western Hemisphere, in the tropical and semitropical regions of South America and the Caribbean. Old World screwworms are found in parts of Asia, tropical and sub–Saharan Africa, and some countries in the Middle East. Both are parasitic flies that lay eggs in open wounds of living animals, with larvae feeding on living tissue.
• Adult female screwworm flies lay their eggs in open wounds or bodily openings of warm-blooded animals, including livestock and humans. When the eggs hatch, the larvae use sharp mouth hooks to burrow into and consume living flesh, causing a severe condition known as myiasis. Because screwworm larvae feed on living tissue and expand the wound, they create a perfect environment for bacterial colonization. These bacteria often form thick, persistent biofilms inside the wound bed.