After decades of successful control and a localized six-month outbreak in the Florida Keys in 2016, New World screwworm is once again a U.S. concern. In June 2026, the United States Department of Agriculture Animal and Plant Inspection Services (USDA APHIS) confirmed New World screwworm (NWS) in a 3-week-old calf in Zavala County, Texas. Larvae were found in the calf’s umbilical area. Since then, cases have been seen in livestock and pets in Texas and New Mexico.
For practicing veterinarians, this outbreak makes recognizing and reporting this zoonotic parasite newly relevant. For veterinary students preparing for the NAVLE®, it makes an already testable parasite especially worth reviewing.
Here is what you need to know about Cochliomyia hominivorax.
NWS Clinical Presentation
The most distinctive clinical feature of NWS is straightforward: New World screwworm larvae invade and feed on living tissue (obligatory myiasis). That distinction separates NWS from many other fly larvae encountered in wounds, which primarily feed on decaying organic matter and opportunistically infect necrotic tissue in living hosts (facultative myiasis).
NWS is zoonotic and reportable. Cochliomyia hominivorax is a dipteran fly whose larvae require living tissue to develop. Any warm-blooded animal can potentially be affected, including:
- Cattle and other livestock
- Horses
- Dogs and other pets
- Wildlife
- Birds
- Humans
The classic presentation is a painful, foul-smelling, rapidly enlarging wound containing maggots. There may also be serosanguineous discharge or bleeding. Unlike larvae simply scavenging dead tissue, NWS larvae burrow head-first deep into viable tissue. As the larvae destroy tissue, the wound progressively enlarges. The expanding wound can then attract additional female flies, resulting in further egg deposition and infestation. Without intervention, severe infestations cause extensive tissue destruction, secondary bacterial infection, debilitation, and potentially death.
Infestations often begin with a preexisting wound, even a very small one. Potential sites include:
- Mucous membranes and body openings are preferred, including the nose, mouth, eyes, ears, and genitalia
- Tick or insect bites
- Traumatic wounds
- Surgical wounds
- Dehorning sites
- Castration sites
- The umbilicus of neonates
Here’s the key association for NAVLE® candidates to remember: Painful, rapidly enlarging wound + foul odor + bleeding or discharge + maggots invading living tissue = suspect New World screwworm.
Just as importantly, not every maggot found in a wound is NWS. Other fly larvae in North America may colonize wounds but consume necrotic tissue. However, if an exam question emphasizes larvae actively burrowing into viable tissue, that distinction should point you toward Cochliomyia hominivorax.
C. Hominivorax Life Cycle and Pathogenesis
Female C. hominivorax flies are attracted to wounds, where they deposit masses of eggs along the wound edges. The eggs hatch rapidly, and the larvae burrow head-first into living tissue to feed. This tissue destruction enlarges the wound and may attract additional flies. Once mature, the larvae leave the host and fall to the ground, where they pupate in the soil. Adults emerge, and females seek another warm-blooded host.
Under favorable tropical conditions, the entire life cycle can be completed in less than three weeks, allowing populations to increase rapidly. Treating individual animals is essential but does not eliminate the reproducing fly population, so successful control and eradication require a population-level approach.
Diagnosis and Reporting
Finding maggots in a wound does not automatically mean an animal has NWS. The key clinical concern is whether the larvae are invading living tissue rather than feeding primarily on necrotic tissue. If you suspect NWS, immediately report the case to the State Animal Health Official and USDA Veterinary Official.
Definitive diagnosis requires identification of the larvae. Characteristic features include pigmented tracheal tubes and distinct segmental spines. Government laboratories may also use single nucleotide polymorphism (SNP) panels and genetic analysis for identification and surveillance. For veterinary diagnostic submission, place larvae in 70% ethanol, not formalin, and submit to the appropriate state animal health agency.
Image courtesy of John Kucharski, U.S. Department of Agriculture.
Image courtesy of Mark Fox, U.S Centers for Disease Control and Prevention.
For both clinical practice and NAVLE® preparation, remember the sequence: Suspect larvae invading living tissue → report immediately → submit for definitive larval identification.
Treatment and Clinical Management
Treatment focuses on eliminating the infestation and administering appropriate wound care. Remove all larvae, followed by thorough cleaning and debridement of the wound as appropriate. Additional therapy varies by species and clinical circumstances. Treatment options may include:
- Pyrethroid spray in livestock
- Oral nitenpyram or lotilaner in dogs
- Oral spinosad/milbemycin in dogs
- Oral afoxolaner/milbemycin in dogs
Address secondary bacterial infections when indicated. Consider analgesia and other supportive care based on the patient’s clinical condition.
Because NWS is reportable, treatment of the individual animal is only one component of the response. Suspect cases also require involvement of animal health authorities for surveillance and population-level control.
Prevention and NWS Population Control
Prevention focuses heavily on identifying and managing wounds that could attract female flies. Regularly inspect animals living in at-risk areas for wounds and evidence of myiasis, and treat wounds promptly.
Carefully manage husbandry practices that create wounds when NWS flies are active in an area. Procedures such as dehorning and castration, for example, can provide potential sites for egg deposition.
Image courtesy of Denise Bonilla, U.S. Department of Agriculture.
At the population level, one of the most important control tools is the sterile insect technique (SIT). With SIT, government officials release large numbers of laboratory-bred sterile male flies into affected areas. Mating between sterile males and wild females does not produce viable offspring. Females only mate once per lifetime, so this suppresses the wild fly population over time.
SIT played a central role in the historical eradication of NWS from the United States and remains a critical population-level control strategy.
Historical Context and Species Distinction
New World screwworm was historically eradicated from the United States in 1966, largely through the SIT. A localized outbreak occurred in the Florida Keys in 2016. More recently, NWS reemerged and began spreading northward through Central America and Mexico beginning in 2023. In 2026, NWS was again detected in animals in the United States, with the first confirmed case identified in Texas.
This history is particularly relevant for today’s veterinarians and veterinary students. NWS is no longer simply a disease to recognize from textbooks. Its return makes surveillance, rapid recognition, and reporting clinically relevant again.
It is also important to distinguish New World screwworm from Old World screwworm, which is found in Africa and southeast Asia. Both names may appear in parasitology material, so be careful not to confuse the two organisms:
- New World screwworm: Cochliomyia hominivorax
- Old World screwworm: Chrysomya bezziana
Staying Prepared for New World Screwworm
For NAVLE® candidates, focus on the features that distinguish NWS from other causes of myiasis. Cochliomyia hominivorax causes obligatory myiasis, and its larvae burrow into and consume living tissue.
For practicing veterinarians, the same recognition now has immediate clinical and public health significance. A rapidly enlarging, painful, foul-smelling wound containing larvae that are invading viable tissue should raise concern for NWS and prompt immediate reporting.
Knowing how to recognize New World screwworm is once again more than an exam-day skill. It is part of being prepared for the realities of clinical practice.
Stay Up To Date on NWS:
- Commission for the Eradication and Prevention of Screwworm (COPEG): https://www.copeg.org/en/
- US Department of Agriculture: https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm
- Center for Disease Control and Prevention: https://www.cdc.gov/new-world-screwworm/situation-summary/index.html
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