A veterinarian's guide to New World screwworm cases, treatment, reporting, response in the US & more

New World screwworm (Cochliomyia hominivorax), coined ‘the flesh-eating parasite,’ returned to the United States on June 3, 2026, when USDA confirmed the first domestic case in roughly 60 years in a calf in Zavala County, Texas.¹ Since then, as of July 23, 2026, USDA's Animal and Plant Health Inspection Service (APHIS) has confirmed 42 US animal cases: 21 cattle, 13 sheep, 5 goats, and 3 dogs, across 16 Texas counties and 1 New Mexico county.¹

Over the past year, the FDA has cleared 12 drug products for the parasite through 3 regulatory pathways, USDA has built out quarantine protocols and sterile fly production capacity, and CDC has stood up a human surveillance system for a disease that was not nationally notifiable until this year. In a session at the 2026 American Veterinary Medical Association (AVMA) Convention in Anaheim, California, officials from all 3 agencies walked veterinarians through what has been done and what practitioners are expected to do.²

Where New World screwworm cases have been confirmed in the US

Confirmed cases have been identified in several counties across south and west Texas, along with 1 case in Lea County, New Mexico.1 Crockett County has reported the most cases (11), followed by Edwards County (6) and Terrell County (4).¹

Brianna Schur, DVM, MPH, associate deputy administrator, Veterinary Services, Strategy & Policy for USDA APHIS, said during the AVMA session that not one of those cases came from a government trap or a highway checkpoint, but rather stemmed from education, outreach, accredited practitioners, and ranchers bringing in samples.

Schur said one of the most common questions USDA receives is why no New World screwworm flies have been detected in traps. She said fertile female flies are far more likely to seek out live animal hosts than the attractants used in surveillance traps, and although traps baited with rotting liver perform better, they are too labor-intensive for widespread deployment.²

"That is supported by the data that we've seen in Mexico, even in states that we have seen very heavy infestations. It is exponentially more that is identified in animals than it's identified in traps," Schur said.

Schur said USDA is encouraged by the low number of trap detections and believes the wild-type fly population is likely small, while acknowledging that trap counts have limitations.

"We don't know if it'll stay like that," Schur added.

The explanation leaves an open question. If traps routinely detect few flies, even during heavy infestations, how can low trap counts be used to infer that the wild-type population is small? USDA did not explain what evidence would indicate otherwise.

Figure. The 42 confirmed New World screwworm cases in US animals, June 3 to July 21, 2026. Each dot represents one county’s cases of a single species, sized by case count.

New World screwworm in humans: US cases, risk factors, and surveillance

During the session, Barbara Williams, senior veterinary scientist in CDC's One Health office, reported that the outbreak across Central America and Mexico has produced more than 2200 human cases since 2023, including 11 deaths associated with screwworm infestation.² CDC data states that more than 189,000 animal cases in that region have been reported.³

According to the CDC, there have been no reports of locally acquired human cases inside the US.³ The one US case, confirmed in August 2025, was travel-associated in a patient returned from El Salvador with a scalp lesion linked to a preexisting skin condition.^2,4

Risk to people is driven by exposure to flies, not contact with infested animals. Williams emphasized that direct contact with an animal with screwworm myiasis is not considered a risk factor.² Instead, risk increases with prolonged exposure to flies, such as spending extended time outdoors near livestock or sleeping outside during the day. According to the CDC, agricultural workers, people experiencing homelessness, and older adults who may have difficulty caring for their own wounds are among those at highest risk.²

Screwworm did not become a nationally notifiable condition in the US until this year. Williams said the CDC has since adopted a national case definition, developed standardized case report forms, and begun onboarding states into the national reporting system.²

Which animals are at risk for New World screwworm?

Cattle dominate the conversation because of the economic consequences of an outbreak. Experts estimate screwworm could cost the Texas cattle economy close to $2 billion a year.

“It's important to note that these flies are not picky, and as long as it is a living, warm-blooded host, it is susceptible,” Kathryn E. Reif, PhD, MSPH, said in a recent dvm360 Vet Watch episode.⁵

According to the USDA, screwworm is most often detected in cattle, swine, horses, small ruminants, and dogs, with wildlife, birds, and people also susceptible.⁶ The pattern is reflected in confirmed US cases so far, which have involved cattle, sheep, goats, and 3 dogs.

The wounds that attract screwworm flies can be surprisingly small. USDA notes that even a tick bite can provide an entry point. Animals that have recently given birth, been injured, or undergone procedures such as dehorning or branding are among the most vulnerable. For dogs and cats, joint guidance from USDA, CDC, FDA, and US Customs and Border Protection identifies pets that spend significant time outdoors, have skin folds, long hair, allergies or other medical conditions, are recovering from surgery or recent birth, or have mites, fleas, or ticks as being at increased risk.⁷

New World screwworm wildlife surveillance: Drones, cameras, and coordination challenges

“Wildlife is a really unique challenge, and it's a really important piece of this puzzle,” Schur noted.

APHIS Wildlife Services leads wildlife coordination, partnering with the Bureau of Land Management, tribal nations, US Fish and Wildlife Service, state wildlife agencies, and the National Park Service.² Schur said drones and trail cameras have generated more than 20,000 wildlife images for biologists to review.² A camera cannot confirm a diagnosis, but it can flag an animal that appears to be failing, which lets biologists follow up in the field and collect samples.

The harder problem is a legal one, as authority over wildlife is divided differently in every state, and one of the session's federal One Health coordination officials said those jurisdictional splits leave gaps that the animals themselves ignore.

"We recognize that there's complexities around the authorities over wildlife in all of our states, and that wildlife don't see borders, so they're going to cross wherever they want to cross," she said.

A federal joint wildlife working group exists to close those gaps, meeting to identify missing information and figure out how to bridge resources and surveillance across the National Park Service, US Fish and Wildlife Service, industry partners including the Association of Fish and Wildlife Agencies, and state leads, so that surveillance and outreach reach the places no single agency covers.²

Sterile fly program: How USDA plans to eliminate New World screwworm

Sterile insect technique eliminated screwworm from the US the first time, and it’s the centerpiece of the current response. USDA estimates it needs 500 million sterile flies to push the pest back, against current production of about 100 million per week at the COPEG facility in Panama, which is at capacity.

Three build-outs are meant to close that gap:

• Metapa, Mexico. USDA invested $21 million to renovate an existing fruit fly facility, with a ribbon cutting at the end of June 2026. Production is ramping up rather than running at capacity.
• Edinburg, Texas dispersal facility. Completed at Moore Air Base in February 2026, expanding the range over which sterile flies can be released along the border and into the United States.
• Edinburg, Texas production facility. Ground broken April 2026. Schur said the original target was November 2027, and USDA is now aiming for April of next year.

Each new production facility adds up to another 100 million flies per week, which would bring the network to the 500 million target.² Until then, the limited supply is dispersed strategically, with a team adjusting drop polygons daily as detections change.

FDA New World screwworm drug approvals: Full approval, conditional approval, and EUA explained

Amber McCoig, DVM, MPH, senior veterinary medical officer with FDA’s Center for Veterinary Medicine, outlined 3 regulatory pathways currently used for screwworm drugs. The pathway a product follows matters in practice because it affects the level of scientific data available to support its use and the conversations veterinarians need to have with clients.

Full approval

McCoig called this the gold standard. FDA determines the product is safe and effective, confirms it is manufactured properly, and reviews the labeling.2 That takes several years and requires substantial evidence of effectiveness. Approval does not expire as long as the sponsor keeps marketing the product, and extralabel use is permitted aside from the limited list of drugs prohibited under 21 CFR 530.41.²

No product currently holds full FDA approval for New World screwworm.

Conditional approval

Conditional approval runs through FDA's minor use, minor species program, which exists where the treatable population is too small for a sponsor to justify a full approval program. Sponsors demonstrate safety and run minimal effectiveness studies, and once they have a reasonable expectation of effectiveness, they may market the product for 5 years while collecting the data needed for full approval. Three products currently hold conditional approval with a screwworm label indication.

Asked whether a sponsor picks its own pathway, McCoig said the situation generally determines it. The point of the 5-year window is for those products to become fully approved within it.

Emergency use authorization

The New World screwworm response marked the first time FDA’s Center for Veterinary Medicine has used its emergency use authorization (EUA) authority. Under an EUA, FDA can authorize a product when available evidence indicates it may be effective in addressing an emergency, rather than requiring evidence of effectiveness as would be needed for full approval.

EUA products are temporary and remain available only while the emergency declaration is in effect. They may also carry restrictions on distribution or administration that do not apply to conditionally approved or fully approved products. In its June 5, 2026, Dear Veterinarian letter, FDA outlined veterinarians’ responsibilities when using EUA products: inform clients that the product is authorized for emergency use but not FDA approved; discuss risks, benefits, and alternatives; provide applicable withdrawal times for food animals; and maintain records including client and patient information, dose, lot number, and EUA-specific requirements.⁸

EUAs and conditional approvals to date

FDA has issued 11 EUAs and 3 conditional approvals for New World screwworm products, most recently a July 15, 2026, EUA for an ivermectin oral solution for short-term prevention of NWS in horses. The products span multiple species and drug classes, and some carry both a conditional approval and an EUA, meaning a single product may support more than one use. McCoig cautioned that authorization does not necessarily mean immediate availability, as products may take time to reach distributors.

New World screwworm treatment options: How to interpret product indications & more

Indications for these products are not interchangeable, and reading the label closely determines whether the product matches the situation in front of the veterinarian.

• Treatment of myiasis means an infestation is already present and the product is intended to kill NWS larvae.
• Prevention of myiasis applies before an infestation develops, including situations where wounds or procedures may create an opportunity for fly infestation.
• Prevention of reinfestation refers to continued protection after treatment. For example, the conditional approval for Dectomax-CA1 in cattle includes a 21-day prevention of reinfestation claim.

Wound management runs alongside drug therapy. USDA advises cleaning the affected area thoroughly, removing all visible larvae, using antiseptics to prevent secondary infection, and administering approved topical or systemic antiparasitic treatment to eliminate remaining larvae.⁶

Why New World screwworm products with the same active ingredient may differ

Two products can share the same active ingredient and still have different performance against New World screwworm. McCoig urged veterinarians to start with FDA’s authorized product list rather than assume a familiar product will be effective against this parasite.

“One product, as you guys know, could have the same active ingredient [but] could work very differently. So only the products that have label indications for emergency use authorization or conditional approval for prevention or treatment for New World screwworm have been evaluated for this. So we really want you to reach for those first,” said McCoig.

Extralabel use remains an option under the Animal Medicinal Drug Use Clarification Act, but veterinarians must have scientific information to support any extended withdrawal period. A product that has not been evaluated against New World screwworm does not have evidence supporting its effectiveness against this parasite, even if it is effective against other flies.

Why veterinarians should use New World screwworm products with approved indications

Using products specifically evaluated for New World screwworm is important not only because efficacy can differ among products, but also because unnecessary use could contribute to resistance. McCoig emphasized that the goal is to preserve these treatment options for future cases.

“One of the things we want to make sure that we don't do is cause resistance issues. We all want to be able to have these tools for the future, and not all of these products work the same, and we want to use them as judiciously as possible,” McCoig said.

Why anthelmintics in medicated feed are not approved for New World screwworm

Asked about anthelmintics in medicated feed, McCoig said no products are approved for that use against screwworm, here or in countries fighting the pest longer. Feed intake varies animal to animal, several studies have shown limited ability to reach necessary blood levels through feed, and FDA is still evaluating whether the route can work at all.

In the last 10 months, we've gotten these...products out there to make sure that veterinarians know what we have seen to have a level of effectiveness. So, if it's not out there, then at this point, it hasn't demonstrated that level of effectiveness yet. — Amber McCoig, DVM, FDA Center for Veterinary Medicine

Traveling with pets and New World screwworm: What veterinarians should discuss with clients

For clients traveling to or from affected countries, federal guidance asks veterinarians to discuss risk factors, recommend pre-travel and post-travel exams, teach owners the signs of infestation, and consider prophylactic medication before return.⁷ Dogs arriving from affected countries need a health certificate from a full-time government veterinarian confirming inspection for screwworm within 5 days of travel. On exam after travel, investigate any irritated or broken skin no matter how small, especially skin folds around the face and neck, ears including internal canals, perianal and genital areas, nail beds, the navel, areas of excessive licking, and recent surgical sites.⁷

What veterinarians should do if they suspect a New World screwworm case

For Schur, one of the simplest steps practices can take is having response information and sample collection materials ready before a suspected case appears. When myiasis, maggots, or an animal movement history raises concern, the scramble to find the state veterinarian or USDA Area Veterinarian in Charge (AVIC) can slow the process.

“Now go home, please. Have that information handy for yourself, for your technicians in your practice,” Schur said during the session. “Having that information available, having what we'll call kind of a collection kit, which is pretty simple … and then having those forms handy is something that you guys can have available and ready.”

USDA recommends placing collected larvae in alcohol for submission and keeping the necessary forms available. Schur said having these materials ready allows USDA to turn around confirmed and negative results quickly, often faster than samples can be shipped.

What happens after a New World screwworm case is confirmed

Once a sample is confirmed positive, USDA follows the response framework outlined in its New World Screwworm Playbook. Schur described the sequence:

• USDA confirms the positive myiasis sample, and the affected state immediately implements its animal health order. In Texas, that is a hold order; many other states use quarantine orders.
• A 20-kilometer infested zone is established around the detection. Either a confirmed animal case or a positive fly trap detection can trigger it.
• A 40-kilometer buffer zone and a fly surveillance zone extending up to 90 kilometers are added, with all zones shifting as new detections occur.
• About 175 state and federal personnel deploy to the affected area to inspect animals and oversee permitting, inspection, and treatment before animals can move out of the zone. Checkpoints operate on roads outside the regulated areas.

Rather than relying on simple radius maps, Texas translates those zones into polygons bounded by recognizable features such as major highways, rivers, and county lines, making it easier for producers to determine whether they fall within a regulated area.

Human detections feed into the same response system. Williams said confirmed human cases are shared with USDA through interagency partners so animal health officials can respond as they would to any other detection.

Why veterinarians are central to New World screwworm detection and response

Every one of the 42 confirmed US cases reached USDA the same way: A producer, rancher, or veterinarian recognized something was wrong and submitted a sample. Traps did not detect them. Highway checkpoints did not detect them. While sterile fly production continues to ramp up, early detection still depends on people recognizing suspicious cases.

Williams said screwworm awareness remains limited in human medicine, making veterinary teams a key source of information for clients before a case occurs.

Schur described a similar pattern in South Texas, where roughly 200 deployed state and federal personnel have gone door to door to talk with producers and residents. She credited a year of education and outreach with the largely positive reception. Some people are hearing about screwworm for the first time; others remember the pest from the 1960s and 1970s.

Veterinarians also see animals that fall outside USDA's direct regulatory role. USDA does not regulate companion animals directly, and Schur credited AVMA with helping close much of the information gap for small animal practitioners. With rescue organizations transporting dogs long distances across the country, veterinarians who ask about travel history may be the first to recognize an imported case.

References

1. New World screwworm confirmed detections in the United States. USDA Animal and Plant Health Inspection Service. Accessed July 23, 2026. https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/current-status/us-confirmed-cases-new-world
2. Headen A, Basler CA, Williams B, McCoig A, et al. Taking the Fight to New World Screwworm: The Role of Veterinary Professionals. Presented at: 2026 American Veterinary Medical Association Convention; July 10-14, 2026; Anaheim, CA.
3. New World screwworm outbreak: situation summary. Centers for Disease Control and Prevention. Accessed July 23, 2026. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html
4. HHS and USDA confirm singular traveler-associated New World screwworm case; precautionary and proactive surveillance ongoing. US Department of Health and Human Services. August 25, 2025. Accessed July 23, 2026. https://www.hhs.gov/press-room/hhs-cdc-usda-traveler-associated-screwworm-case-surveillance.html
5. Lee C, Reif KE. New World screwworm: What veterinary professionals need to know. dvm360. July 9, 2026. Accessed July 23, 2026. https://www.dvm360.com/view/new-world-screwworm-what-veterinary-professionals-need-to-know
6. New World screwworm: be aware and prepare. Information for veterinarians. USDA Animal and Plant Health Inspection Service. APHIS-25-028. April 2025.
7. New World screwworm: veterinary considerations for dogs and cats that travel internationally. USDA Animal and Plant Health Inspection Service, Centers for Disease Control and Prevention, US Food and Drug Administration, US Customs and Border Protection. APHIS-25-038. December 2025.
8. Dear veterinarian letter: use of FDA-authorized animal drugs against New World screwworm myiasis. US Food and Drug Administration. June 5, 2026. Accessed July 23, 2026. https://www.fda.gov/animal-veterinary/product-safety-information/dear-veterinarian-letter-use-fda-authorized-animal-drugs-against-new-world-screwworm-myiasis
9. USDA announces completion of sterile fly dispersal facility in Texas. US Department of Agriculture. February 9, 2026. Accessed July 23, 2026. https://www.usda.gov/about-usda/news/press-releases/2026/02/09/usda-announces-completion-sterile-fly-dispersal-facility-texas