Managing and Treating New World Screwworm in Dogs: A Texas-Based Veterinarian’s Perspective

Quick summary for veterinarians and pet owners: New World screwworm has re-entered the U.S. as of June 2026. Elanco’s Credelio Quattro-CA1 (lotilaner, moxidectin, praziquantel, and pyrantel chewable tablets) is the first and only FDA-conditionally approved treatment for dogs and puppies to treat New World screwworm larvae.  If you suspect an infestation, quarantine your pet and contact a veterinarian immediately.

As a Texas-based veterinarian, I’ll be the first to admit that when I see headlines about a flesh-eating parasite attacking live animals, it sounds like the stuff of horror movies; but it’s a real emerging threat bearing on the United States. New World screwworm (NWS), an emerging parasite once eradicated in the U.S. has now re-entered the United States. The first case was confirmed in U.S. livestock in Texas on June 3, 2026ⁱ; and just four days later, the first U.S. case in a dog was found in New Mexico.ⁱⁱ You can track confirmed cases via the USDA’s website here.

What is Elanco doing to help in the fight against New World screwworm for pets?

Elanco is working proactively with the U.S. Food & Drug Administration (FDA), the U.S. Environmental Protection Agency (EPA), and the U.S. Department of Agriculture (USDA) to ensure that veterinarians, livestock producers, and pet owners are prepared. As of the date of publication, we’ve worked to secure more Emergency Use Authorizations (EUA) and conditional approvals than any other animal health company, including Credelio Quattro-CA1 for treatment of NWS larvae in dogs and puppies. We’re committed to delivering innovation, scientific expertise, and available resources to help treat New World screwworm larvae and support the health and well-being of animals.

Available Elanco Pet Treatment Options  

What is New World screwworm? 

New World screwworm (NWS, Cochliomyia hominivorax) is a devastating parasitic fly that can affect wildlife, livestock, pets, and in rare cases, people. The larvae are what makes NWS so dangerous. Flies lay eggs in open wounds, even one as small as a tick bite. Unlike normal fly larvae, NWS larvae burrow into living tissue, feeding as they go. This “screwing” motion deepens wounds rapidly, causing serious damage to the animal. Untreated infections can lead to death in as little as a week or two. According to the Centers for Disease Control, one female can lay 200 – 300 eggs at a time and one wound can host more than one femaleⁱⁱⁱ, allowing infestations to escalate quickly.

How can we keep our pets covered from New World screwworm?  

I recommend pet owners keep their dogs on year-round parasite coverage like Credelio Quattro (lotilaner, moxidectin, praziquantel, and pyrantel chewable tablets) to help minimize self-inflicted scratching that could create vulnerable sites. Given New World screwworm’s aggressive and invasive nature, any minor wounds on pets from grooming, scratching, or outdoor activities can serve as an entry point for these parasites. It’s essential to treat or quickly address any wounds to help keep dogs thriving.

How can I treat New World screwworm in dogs?

If you suspect your dog has New World screwworm, immediately seek veterinary care.

Elanco offers the first and only FDA-conditionally approved treatment for NWS larvae in dogs and puppies with Credelio Quattro-CA1. Credelio Quattro/Credelio Quattro-CA1 offers the broadest parasite coverage of its kind^* targeting 7 types of parasites^§ in one flavored chewable tablet.

How do I know if my dog has New World screwworm?

Here is a common checklist of what to look for with an NWS infestation:

Checklist: Potential Signs of NWS Infestation

Can humans get New World screwworm from their dogs?

According to USDA, NWS is not contagious and does not spread directly from animals to people or from person to person.^iv

What should I do if I find a worm in my dog's wound?

Here are three immediate steps to take if you find a worm in your dog's wound:

Where in the U.S. is New World screwworm currently found?

As of August 20, New World screwworm has been found in:

USDA is constantly updating their surveillance map.

What is the difference between Credelio Quattro and Credelio Quattro-CA1?

Credelio Quattro/Credelio Quattro CA-1 is one product with two label claims.

Credelio Quattro protects dogs against ticks, fleas, heartworm disease, roundworms, hookworms, and tapeworms. Credelio Quattro-CA1 is conditionally approved for the treatment of New World screwworm larvae infestations in dogs pending a full demonstration of effectiveness.

To learn more about New World screwworm, the following resources are available:

PM-US-26-1329

^{* Based on label comparison of the number of parasite types covered in a chewable that targets ticks}

^{§ Credelio Quattro protects dogs against ticks, fleas, heartworm disease, roundworms, hookworms, and tapeworms. Credelio Quattro-CA1 is conditionally approved for the treatment of New World screwworm larvae infestations in dogs pending a full demonstration of effectiveness.}

Indications for Credelio Quattro/Credelio Quattro-CA1

Credelio Quattro is indicated for the prevention of heartworm disease and the treatment and control of roundworm, hookworm, and tapeworm infections. Credelio Quattro kills adult fleas and is indicated for the treatment and prevention of flea infestations and the treatment and control of tick infestations for 1 month in dogs and puppies 8 weeks of age and older and weighing 3.3 pounds or greater. Credelio Quattro is indicated for the prevention of Lyme disease infections as a direct result of killing black-legged ticks.

Credelio Quattro-CA1 is conditionally approved for the treatment of infestations caused by New World screwworm (NWS) larvae in dogs and puppies 8 weeks of age and older and weighing 3.3 pounds or greater.

Important Safety Information for Credelio Quattro/Credelio Quattro-CA1

Lotilaner, an ingredient in Credelio Quattro/Credelio Quattro-CA1, belongs to the isoxazoline class and has been associated with neurologic adverse reactions like tremors, incoordination, and seizures even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders. Dogs should be tested for existing heartworm infections before Credelio Quattro/Credelio Quattro-CA1 administration as it is not effective against adult heartworms. The safe use in breeding, pregnant, or lactating dogs has not been evaluated. The most frequently reported adverse reactions in clinical trials were vomiting and diarrhea.

Credelio Quattro-CA1 is conditionally approved by the FDA pending a full demonstration of effectiveness under application number 141-619. If you suspect that your dog is infested with NWS larvae, seek veterinary care immediately for treatment to include removal of larvae and appropriate wound care.

For complete safety information, please see the Credelio Quattro/Credelio Quattro-CA1 product label or ask your veterinarian.

Emergency Use Authorization of Credelio (lotilaner) Chewable Tablets for New World Screwworm (NWS)

The U.S. Food and Drug Administration (FDA) has issued an Emergency Use Authorization (EUA) for the emergency use of the approved product Credelio (lotilaner) chewable tablets for the treatment of infestations caused by NWS (Cochliomyia hominivorax) larvae (myiasis) in dogs and puppies. Credelio is not approved for this use.

Credelio (lotilaner) is approved for other uses in dogs and puppies.

For additional information on the EUA, please refer to the Credelio NWS Fact Sheet.

Limitations of Authorized Use

Credelio (lotilaner) chewable tablets is authorized for this use only for the duration of the declaration that circumstances exist justifying the authorization of the emergency use of Credelio (lotilaner) chewable tablets under section 564(b)(1) of the Federal Food, Drug, and Cosmetic Act (FD&C Act), 21 U.S.C. § 360bbb-3(b)(1), unless the authorization is terminated or revoked sooner.

Indications for Credelio

Credelio kills adult fleas and is indicated for the treatment and prevention of flea infestations and treatment and control of tick infestations (lone star tick, American dog tick, black-legged tick, brown dog tick, and longhorned tick) for one month in dogs and puppies 8 weeks and older and 4.4 pounds or greater. Credelio is indicated for the prevention of Lyme disease infections as a direct result of killing black-legged ticks.

Important Safety Information for Credelio

Lotilaner is a member of the isoxazoline class of drugs. This class has been associated with neurologic adverse reactions including tremors, incoordination, and seizures. Seizures have been reported in dogs receiving this class of drugs, even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders. The safe use of Credelio in breeding, pregnant or lactating dogs has not been evaluated. The most frequently reported adverse reactions are weight loss, elevated blood urea nitrogen, increased urination, and diarrhea. For complete safety information, please see Credelio product label or ask your veterinarian.

Emergency Use Authorization for Credelio CAT (lotilaner) for New World Screwworm (NWS)

The U.S. Food and Drug Administration (FDA) has issued an Emergency Use Authorization (EUA) for the emergency use of the approved product Credelio CAT (lotilaner) for the treatment of infestations caused by NWS (Cochliomyia hominivorax) larvae (myiasis) in cats and kittens. Credelio CAT is not approved for this use.

Credelio CAT is approved for other uses.

For additional information on the EUA, please refer to the Credelio Cat NWS Fact Sheet.

Limitations of Authorized Use

Credelio CAT (lotilaner) is authorized for this use only for the duration of the declaration that circumstances exist justifying the authorization of the emergency use of Credelio CAT (lotilaner) under section 564(b)(1) of the Federal Food, Drug, and Cosmetic Act (FD&C Act), 21 U.S.C. § 360bbb-3(b)(1), unless the authorization is terminated or revoked sooner.

Indications for Credelio CAT

Credelio CAT kills adult fleas and is indicated for the treatment and prevention of flea infestations for one month in cats and kittens 8 weeks of age and older and weighing 2 pounds or greater.

Credelio CAT is also indicated for treatment and control of black-legged tick infestations for one month in cats and kittens 6 months of age and older and weighing 2 pounds or greater.

Important Safety Information for Credelio CAT

Lotilaner is a member of the isoxazoline class of drugs. This class has been associated with neurologic adverse reactions including tremors, incoordination and seizures. Neurologic adverse reactions have been reported in cats receiving isoxazoline class drugs, even in cats without a history of neurologic disorders. Use with caution in cats with a history of neurologic disorders. The safety of Credelio CAT has not been established in breeding, pregnant and lactating cats. The effectiveness of Credelio CAT against black-legged ticks in kittens less than 6 months of age has not been evaluated. The most frequently reported adverse reactions are weight loss, rapid breathing and vomiting. For complete safety information, please see Credelio CAT product label or ask your veterinarian.

Credelio Quattro, Credelio, Elanco and the diagonal bar logo are trademarks of Elanco or its affiliates. © 2026 Elanco or its affiliates

^{i https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-presence-new-world-screwworm-united-states}

^{ii USDA Confirms First Case of New World Screwworm in a Dog in Lea County, New Mexico, Fourth Case in Texas | Animal and Plant Health Inspection Service}

^{iii About New World Screwworm Myiasis | Myiasis | CDC}

^{iv https://www.aphis.usda.gov/sites/default/files/nws-myth-busters.pdf}