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

New World screwworm (Cochliomyia hominivorax) is once again an active threat in the Americas. Cases in Mexico have risen and detections are approaching the US border, while 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 with links to supporting resources and USDA/CDC guidance where appropriate.

What is New World screwworm and how does it present clinically?

New World screwworm (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 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 screwworm in the US now? How close is it?

As of recent federal and dvm360 reporting, screwworm detections in Mexico have surged and cases have been 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 and Vet Watch updates

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 New World screwworm?

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 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 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 chewables (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 re-examine lesions within 24–48 hours to ensure no live larvae remain.^5-7

Read more about treatment and EUAs: FDA grants EUA for lotilaner in dogs and Antiparasitic receives FDA emergency authorization to treat NWS in cats

What should I tell clients for 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); Cert: FFC, CFV, RECOVER-ALS, 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: 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 (PPE) 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. Also 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. Centers for Disease Control and Prevention. DPDx – New World Screwworm (myiasis). 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. US Department of Agriculture Animal and Plant Health Inspection Service. News release. September 21, 2025. Accessed September 22, 2025.
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. U.S. Department of Agriculture, APHIS. Standard Operating Procedure for Possible Detections of New World Screwworm in Animals. (PDF). 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. U.S. Food and Drug Administration. FDA Issues Emergency Use Authorization for New World Screwworm Drug for Dogs. 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. 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. Centers for Disease Control and Prevention. New World Screwworm: Situation Summary. Updated 2025. Accessed December 1, 2025. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html
10. Reuters. US plans sterile fly facility in Texas to combat screwworm. June 18, 2025. Accessed December 1, 2025. https://www.reuters.com/world/us/usda-plans-sterile-fly-facility-texas-combat-screwworm-2025-06-18/