What veterinarians need to know about New World screwworm
By MWI Animal Health
This article is intended to communicate MWI Animal Health's capabilities which are backed by the author’s expertise. However, MWI Animal Health and its parent, Cencora, strongly encourage readers to review the references provided with this article and all available information related to the topics mentioned herein and to rely on their own experience and expertise in making decisions related thereto as the article may contain certain marketing statements and does not constitute legal advice.
For most veterinarians in the United States, New World screwworm (NWS) is something they learned about in school or heard referenced as a historical success story in eradication—not something they expected to encounter in practice since it was eradicated from the United States in 1966.
That changed in early June 2026, when the USDA confirmed the first United States case in cattle after the parasite’s steady movement north through Central America and Mexico. Since that initial case—the calf has been treated by veterinarians and is recovering— there have been continual occurrences across multiple species, including cattle, goats, and at least one dog. This reinforces a new and urgent truth: NWS is not just a livestock problem, and it is no longer theoretical. It requires teams to recognize when a wound is not typical, respond with urgency, and know when to escalate a suspected case.
A broader host range
NWS has historically been associated with livestock, where its herd-level and economic consequences can be significant. But any warm-blooded animal can serve as a host, including cattle, sheep, goats, horses, swine, dogs, cats, wildlife, and, rarely, humans.
“Horses, dogs and cats — and humans as well, though rare — are also susceptible to having the screwworm in their wounds,” said Lori Teller, DVM, Executive Director of the Texas Veterinary Medical Association in an interview with KVUE in Austin.
For a companion animal clinic, the first concern may be a single dog or cat with a surgical incision, bite wound, tick-bite lesion, ear margin wound, moist dermatitis, or other wound that is not healing as expected.
Veterinary teams should be especially alert when a wound shows one or more of the following signs:
- Larvae are present in a wound that appears to be worsening rather than stabilizing.
- The wound is enlarging quickly in depth or diameter.
- Tissue damage appears more severe than expected for the original injury.
- Larvae appear embedded deeper in tissue or may be hidden in wound pockets.
- The wound has a strong odor, hemorrhagic or foul-smelling discharge, or pain that seems disproportionate.
- The patient shows increased irritation, restlessness, attention to the wound, decreased appetite, agitation, or separation from the herd.
These signs do not confirm NWS on their own, but they should change the clinical mindset from routine wound care to a more systematic response.
Teller continued in the interview: “Check them for wounds, and it can be wounds as small as bug bites. It doesn't have to be a big gaping wound."
1. Type of myiasis:
- New World screwworm: Obligate myiasis, requiring living tissue to develop.
- Typical myiasis: Facultative myiasis, colonizing necrotic tissue.
2. Wound impact:
- NWS: Rapidly enlarges wounds, causing deep tissue destruction.
- Typical myiasis: Limited to pre-existing necrosis, with slower progression.
3. Larval location:
- NWS: Larvae burrow deep into living tissue.
- Typical myiasis: Larvae remain superficial on necrotic tissue.
4. Progression speed:
- NWS: Rapid and aggressive.
- Typical myiasis: Slower and localized.
Source: https://www.aphis.usda.gov/sites/default/files/nws-ows-case-definition.pdf
Moving from suspicion to action
When a wound raises concern for NWS, the response should shift from routine wound care to controlled escalation. AVMA guidance emphasizes starting with a thorough exam, documenting wound location and appearance, and collecting a clear patient history, especially when wounds are rapidly enlarging, unusually painful, foul-smelling, or accompanied by visible larvae in living tissue.
Visible larvae should be removed, but removal alone is not enough. USDA APHIS case guidance notes that larvae may be hidden deep in wound pockets, so the wound should be explored completely, flushed, and reassessed to help ensure eggs or larvae do not remain. If specimens are needed, AVMA advises reporting the case first and collecting and submitting samples only as directed by state or federal animal health officials rather than sending them independently to a laboratory.
Treatment still matters, but it should happen within that larger response. The FDA has shared resources for veterinarians on conditionally approved and emergency-authorized products for NWS myiasis. Those options can support prevention and treatment in multiple species, but they do not replace careful wound exploration, larval removal, pain control, management of secondary infection, and close follow-up. Most recently, the agency announced the emergency use authorization of the first generic medication to treat NWS in dogs and cats.
Reporting is also part of the clinical response. NWSA is a reportable foreign animal disease in the United States. AVMA and USDA APHIS both stress that suspected cases should be reported immediately to the State Animal Health Official and the USDA Area Veterinarian in Charge. That matters because the goal is not just treatment of the individual animal, but rapid containment, surveillance, and coordinated next steps.
Confidence comes from readiness
The difference between a delayed response and a confident one is rarely clinical knowledge alone. It is preparation. The AVMA frames veterinarians as a key line of defense in early recognition, treatment, reporting, and client education. Readiness starts before a case appears, with clear escalation pathways, reporting contacts, and team understanding of how a suspicious wound will move through the clinic.
For companion animal practices, readiness may mean stronger wound recheck protocols, more targeted travel and transport questions, and making sure staff know that dogs and cats can be part of the detection picture. It also means therapeutic awareness: the FDA’s recent guidance gives veterinarians more clarity on products that may be available for prevention or treatment in certain species, helping teams act with more confidence.
Most U.S.-based veterinarians may never have seen New World screwworm firsthand. But they can still prepare their teams to recognize the difference between routine myiasis and a wound that deserves urgent escalation. Confidence comes from knowing how to diagnose, who to call, how to document, when to report, and how to quickly pair wound management with the most current treatment guidance. That kind of readiness helps veterinarians protect individual patients while supporting the broader animal health.
New World screwworm is different from typical myiasis.
Its larvae require living tissue and can burrow deeply, causing rapid tissue destruction.
Do not dismiss larval wounds as routine.
A wound with larvae that is worsening, expanding, painful, foul-smelling, or deeper than expected should raise suspicion.
Any warm-blooded animal may be at risk.
Livestock, horses, dogs, cats, wildlife, and rarely humans can be affected.
Common wounds can become risk sites.
Surgical incisions, bite wounds, abscesses, branding and castration sites, umbilical cords, tick bites, moist dermatitis, and mucous membranes may all be vulnerable.
Preparedness is a team workflow.
Triage staff, technicians, and veterinarians should all know how suspicious wounds are flagged, escalated, managed, and reported.
Reporting supports containment.
New World screwworm is a reportable foreign animal disease in the United States, and suspected cases should be communicated promptly to the appropriate animal health authorities.
Disclaimer: Information about NWS is constantly changing. For the most up-to-date information, please visit https://www.aphis.usda.gov/livestock-poultry-disease/stop-screwworm
