Top questions on New World screwworm: What every veterinarian should know

New World screwworm (NWS; Cochliomyia hominivorax) is once again an active threat in the Americas. Cases in Mexico have increased, and detections are approaching the US border. Meanwhile, federal agencies and veterinary groups have released guidance for clinicians on identification, laboratory submission, reporting, and therapeutic options.

Below are the most common and practical questions clinicians are asking, with concise answers and links to supporting resources and the US Department of Agriculture (USDA)/Centers for Disease Control and Prevention (CDC) guidance where appropriate.

What is NWS, and how does it present clinically?

NWS larvae cause primary myiasis by burrowing into and feeding on living tissue. Affected animals often present with foul-smelling, expanding wounds with visible, active maggots. Mucosal infestations of the nose, mouth, or ears can also occur and are especially destructive. For clinicians, larvae are typically oriented head-down (posterior spiracles exposed), and wounds can expand quickly if not treated.¹

Read more about identification and clinical signs: Screwworms: When the larvae don't care about boundaries

Is NWS in the US now? How close is it?

According to recent federal and dvm360 reports, NWS detections in Mexico have surged, with cases now detected within approximately 70 miles of the US border. The USDA reported that the parasite is actively moving northward and that border-proximate detections have increased concern. That proximity prompted US trade and surveillance actions earlier this year.^2,3

Read more about the current status and risk: New World screwworm detected less than 70 miles from the US-Mexico border

Watch now to hear an expert break down the latest on NWS: As screwworm nears the border, an expert explains how it spreads and what it means for the US

Who do I notify, and how do I submit specimens if I suspect NWS?

USDA guidance is to immediately notify your state animal health official and your regional Animal and Plant Health Inspection Service (APHIS) office. APHIS has a dedicated NWS response resource with standard operating procedures (SOPs), reporting flows, and a Stop Screwworm information hub for clinicians. The CDC also provides clinical guidance for human cases. Follow APHIS SOPs for quarantine, movement restrictions, and premises for disinfestation if a confirmed case occurs.⁴

As for submissions: According to APHIS, preserve larvae in leak-proof containers with 70% ethanol (or ≥ 70% isopropanol or 5-10% formalin if necessary). Aim to submit multiple larvae in various stages. Follow your state animal health office and APHIS SOP for sample submission and chain of custody. If possible, photograph the lesion and preserve live larvae separately (if the lab requests live specimens).⁴

Read more about reporting and get specimen guidance: New World Screwworm Emergency Response

What are the current treatment options for infested animals?

Treatment is primarily the mechanical removal of larvae coupled with wound debridement, topical care, and systemic support. In 2025, the FDA issued emergency use authorizations (EUAs) allowing lotilaner (Credelio) for dogs and lotilaner chewable tablets (Credelio Cat) for cats as therapeutic options under EUA conditions; follow FDA EUA and local regulatory guidance when using these products for NWS. Also use standard wound care, antibiotics if indicated, and reexamine lesions within 24 to 48 hours to ensure no live larvae remain.^5-7

Read more about treatment and EUAs: FDA grants emergency use authorization for lotilaner to treat New World screwworm in dogs and Antiparasitic receives FDA emergency authorization to treat New World screwworm in cats

What should I tell clients about prevention and protection of pets and livestock?

Advise owners to promptly treat and protect open wounds, practice routine wound checks, and keep companion animals on year-round effective parasiticides when risk is high. Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology), has recommended isoxazoline class products as one protective measure to kill larvae acquired by pets. For livestock, emphasize wound management, good husbandry, minimizing fly attraction, and rapid reporting of suspicious cases.⁸

Watch now to learn more about client education and prevention: Vet Watch update: Do you have normalcy bias?

What should clinics do to prepare?

According to APHIS, update your clinic’s wound management protocols and specimen submission checklists, brief staff on identification and personal protective equipment for handling larvae, and have contact info for your state animal health official and APHIS accessible. Consider stockpiling recommended topical/systemic wound care products and have an isolation/quarantine plan for affected patients. Additionally, review recorded USDA/CDC webinars for staff training.⁴

Are there One Health or human health concerns I should be aware of?

According to the CDC, human infestations are uncommon but possible. Clinicians should counsel clients that NWS can rarely infest humans when there are exposed wounds or mucosal lesions. If there is potential human exposure, notify public health authorities and follow CDC clinical guidance. Collaboration between veterinary and public health agencies is a core part of the federal response.⁹

Read more about NWS and CDC guidance: Clinical Overview of New World Screwworm

References

1. New World screwworm myiasis. CDC. September 10, 2025. Accessed December 1, 2025. https://www.cdc.gov/dpdx/newworldscrewwormmyiasis/index.html
2. Mexico confirms case of New World screwworm in Nuevo Leon. News release. USDA. September 21, 2025. Accessed September 22, 2025. https://www.usda.gov/about-usda/news/press-releases/2025/09/21/mexico-confirms-case-new-world-screwworm-nuevo-leon
3. Bautista-Alejandre A. New World screwworm detected less than 70 miles from the US-Mexico border. dvm360. September 22, 2025. Accessed December 1, 2025. https://www.dvm360.com/view/new-world-screwworm-detected-less-than-70-miles-from-the-us-mexico-border
4. Standard operating procedure for possible detections of New World screwworm in animals. USDA. Accessed December 1, 2025. https://www.aphis.usda.gov/sites/default/files/aphis-sop-detection-nws-in-animals.pdf
5. Bautista-Alejandre A. FDA grants emergency use authorization for lotilaner to treat New World screwworm in dogs. dvm360. October 24, 2025. Accessed December 1, 2025. https://www.dvm360.com/view/fda-grants-emergency-use-authorization-for-lotilaner-to-treat-new-world-screwworm-in-dogs
6. FDA issues emergency use authorization for New World screwworm drug for dogs. News release. FDA. October 24, 2025. Accessed December 1, 2025. https://www.fda.gov/news-events/press-announcements/fda-issues-emergency-use-authorization-new-world-screwworm-drug-dogs
7. Bautista-Alejandre A. Antiparasitic receives FDA emergency authorization to treat New World screwworm in cats. dvm360. November 21, 2025. Accessed December 1, 2025. https://www.dvm360.com/view/antiparasitic-receives-fda-emergency-authorization-to-treat-new-world-screwworm-in-cats
8. Lee C, McCafferty C. Vet Watch update: do you have normalcy bias? dvm360. August 6, 2025. Accessed December 1, 2025. https://www.dvm360.com/view/vet-watch-update-do-you-have-normalcy-bias-
9. New World screwworm outbreak. CDC. Updated December 9, 2025. Accessed December 9, 2025. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html