New World Screwworm: Vigilance Without Panic

How veterinary teams and animal health reps can prepare for New World screwworm this fall.

In 2026, New World screwworm has returned to the animal health conversation with an urgency few U.S. veterinarians or producers have experienced firsthand. While the parasite can cause devastating injury, experts say the appropriate response is preparation, not panic.

“The re-emergence of New World screwworm is a big concern,” said Kathryn E. Reif, MSPH, PhD, Bailey-Goodwin Endowed Associate Professor in Parasitology at Auburn University. “But despite it being a big concern, I don’t think we’re at a moment where we need to be panicking about it.”

Animal health representatives can help by giving veterinary teams and producers accurate information, practical guidance and timely resources. Guilherme G. Verocai, DVM, MSc, Ph.D., DACVM (Parasitology), associate professor and director of the Parasitology Diagnostic Laboratory at Texas A&M University, described the U.S. risk heading into fall 2026 as “low but real and evolving.”

“The key for representatives is to pair calm, factual risk framing with strong messages about vigilance, wound care and rapid reporting,” Dr. Verocai said. “They can stay current through USDA APHIS, state animal health authorities and FDA updates.”

A parasite that changes the meaning of a wound

New World screwworm differs from the fly larvae practitioners more commonly associated with dead or decaying tissue.

“The New World screwworm is completely different from other wound-infesting flies because its larvae feed only on living tissue,” said Nancy C. Hinkle, Ph.D., professor in the University of Georgia Department of Entomology. “Other maggots may consume necrotic material, but screwworm larvae enlarge the wound as they feed.”

Dr. Reif noted nearly any break in the skin can become an entry point and an infestation may begin in a tick bite, abrasion or open umbilical area of a newborn animal. A female can deposit hundreds of eggs at a time, which may hatch within about 24 hours. As the larvae feed, a small wound can expand quickly and attract more flies.

The concern is not limited to cattle. Any warm-blooded animal with a wound or exposed mucous membrane can potentially become a host. Domestic cases have involved multiple livestock species as well as dogs, Dr. Reif said, making awareness important to both livestock and companion animal practices.

What fall weather means

Cooler weather should reduce fly activity across much of the country this fall, but the risk will not disappear everywhere. Cassandra Olds, Ph.D., assistant professor in Kansas State University’s Department of Entomology, said the fly thrives in warm temperatures and cannot maintain normal development once temperatures fall below roughly 46 degrees Fahrenheit.

“In cold-winter regions, that interrupts pupation,” Dr. Olds said. “Parts of the South, however, may remain warm enough for populations to persist at reduced levels.”

The subtropical fly historically could spread north during warm months but not survive prolonged cold. Movement of infested animals can carry it farther, making pre-movement inspection important for livestock and pets traveling from higher-risk areas.

“New World screwworm will continue to be a threat to animal agriculture in the United States so long as breeding flies are present in North America,” said Isabelle Louge, DVM, assistant clinical professor at Cummings School of Veterinary Medicine at Tufts University and a veterinarian with Tufts Veterinary Field Service.

Recognize, report and confirm

Representatives can reinforce a straightforward message to customers: Suspicious wounds require prompt veterinary attention, and maggots in a living animal’s wound should never be casually dismissed.

Verocai noted warning signs include fly eggs near wound margins, visible larvae, foul odor, rapidly worsening tissue damage and wounds that look much more severe than the original injury would suggest. Animals may lick the area, withdraw, become less active or show marked pain.

Appearance alone cannot confirm the parasite. Most practitioners cannot distinguish screwworm larvae from other fly larvae without specialized training and microscopy. Any myiasis in a living or recently dead animal should be reported through state animal health channels rather than diagnosed visually.

USDA recommends that veterinarians who encounter a nonhealing, enlarging, draining or foul-smelling wound, or one containing maggots or white egg masses, contact the APHIS Veterinary Services Area Veterinarian in Charge and the state animal health official. Those officials can initiate a foreign animal disease investigation and direct sample submission.

Proper specimen handling matters. Dr. Verocai recommended preserving larvae in a screw-cap vial containing 70% ethanol or isopropyl alcohol while retaining material for confirmation. Dr. Hinkle cautioned against formalin because it can interfere with PCR identification. Practitioners should follow state and federal instructions rather than improvise.

Prevention starts with wound reduction

Prevention begins with reducing opportunities for the fly to find living tissue. That means controlling ectoparasites, preventing avoidable injuries and finding small wounds before they become larger problems.

“One of the best strategies is providing good ectoparasite management,” Dr. Reif said. “Whether that’s keeping flies, common biting flies or ticks off of our animals — whether they be pets or livestock — doing that ectoparasite management reduces the opportunity for those ectoparasites to bite and cause wounds on the animal.”

Furthermore, in or near an affected region, veterinarians and producers should consider the timing of elective procedures that create wounds. Castration, dehorning, branding and similar management practices may need to be postponed until cooler weather when possible. When a procedure cannot wait, the wound should be monitored closely for signs of infestation.

The animal’s environment also deserves attention. Protruding bolts, damaged fencing and unsafe equipment can cause punctures, cuts and abrasions that become potential egg-laying sites.

“General facility maintenance can also go a long way,” Dr. Reif said.

Young animals require particular attention. According to Dr. Longe, all youngstock should be monitored closely, especially around the navel, which can be a common site of infection.

“Any farm where animals undergo procedures that cause a break in the skin, such as castration or disbudding, should make a plan with their veterinarian for preventing fly infestation in those animals,” she said.

Dr. Olds noted many useful precautions are familiar animal-management practices.

“It comes down to a management situation,” she said. “Making sure that umbilicus areas are kept dry and dipped with iodine, and making sure that wounds are prevented as much as possible. And then when we do have a wound, we make sure that it cleans and heals as fast as possible.”

Dr. Verocai recommended frequent, systematic inspections, daily in higher-risk areas. Particular attention should be paid to umbilical stumps, perineal areas after calving or whelping, surgical sites and recent management wounds.

“These measures are not new, but NWS risk raises the stakes for getting them right,” he said.

The rep’s role in product conversations

Animal health representatives can help practices understand available tools, but a product should not be presented as a substitute for surveillance, wound care or regulatory reporting.

That includes resisting the impulse to recommend widespread prophylactic treatment where infestations have not been confirmed. Dr. Olds said treating too early could mean drug concentrations have declined if the threat later reaches the region.

“We have great tools, but we want to make sure that we use them at the right time and where appropriate,” she said.

Product authorizations and recommendations are changing as regulators and manufacturers respond to the evolving threat. Dr. Hinkle urged representatives to rely on current federal guidance instead of outdated materials or assumptions.

“Because the North American screwworm situation is new and evolving, recommendations are changing almost daily,” Dr. Hinkle said. “Pharmaceutical firms are working to get new products registered with the FDA and to expand existing labels to include use against screwworms.”

She directed animal health professionals to screwworm.gov, particularly its veterinarian section, for current response procedures and FDA-authorized treatment information.

USDA APHIS also recommends that representatives reinforce several foundational points: New World screwworm is a parasite, not a virus; it is not contagious between animals or from animals to people; and animals receiving prompt treatment can recover.

“Reporting ensures rapid containment and prevents the spread of infested animals,” USDA stated. “It also unlocks immediate resources and assistance.”

Early intelligence drives containment

Prompt reporting does more than help the affected animal. It gives regulators the information needed to direct surveillance, quarantines and sterile-fly releases.

“Our most effective strategy at managing New World screwworm historically has been this sterile insect technique,” Dr. Reif said. “Female flies typically mate only once, so releasing sterile males reduces the likelihood that their eggs will produce viable larvae.”

The strategy works best when infestations are found early enough for sterile males to outnumber and outcompete wild flies.

“Detecting it early on in the infestation and making sure that we can deploy those flies where we really need them is going to be single-handedly the most impactful thing that we can do,” Dr. Olds said.

To be successful at combating this, it’s important to know the warning signs, direct customers to current official resources and keep product discussions tied to approved uses and veterinary oversight. The most valuable contribution may be ensuring that a practitioner or producer does not dismiss a suspicious wound or hesitate to make the call that activates the larger response.