New World screwworm confirmed in United States, USDA announces

The US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) confirmed on June 3 the detection of a New World screwworm (NWS) in a bovine in Zavala County, Texas.

The affected animal is a 3-week-old calf, with larvae identified in its umbilical area. To date, APHIS has reported no further detections.

For veterinarians, the detection marks the moment NWS shifts from an approaching threat to a domestic reality, especially for those practicing in or near South Texas. It arrives after nearly a year of preparation that has reshaped the clinical landscape: a slate of FDA emergency use authorizations now covers dogs, cats, cattle, horses, and other species, so authorized treatment options exist that did not a year ago. As a reportable foreign animal disease, NWS also carries surveillance and notification obligations that now move from theoretical to operational for practitioners in affected regions.

"If screwworms establish themselves in the US again, we will be facing a severe and urgent problem in veterinary and public health,” said Christopher Lee, DVM, MPH, DACVPM, a dvm360 editorial advisory board member, during a 2025 interview.

NWS is a foreign animal disease reportable to state animal health authorities and USDA-APHIS. The larvae burrow into the living flesh of warm-blooded animals, most often entering through an open wound, and can be fatal to livestock if untreated. The pest affects livestock, pets, and wildlife, and less commonly, people and birds.

USDA is urging veterinarians and producers in the area to examine livestock and pets for signs of infestation, including draining or enlarging wounds and signs of discomfort. Clinicians should inspect body openings such as the nose, ears, and genitalia, as well as the navel of newborn animals, for larvae and eggs. Laboratory diagnosis is typically made by identifying the parasites under a microscope, and the Zavala County specimen was sent to USDA's National Veterinary Services Laboratories in Ames, Iowa, for confirmatory testing.

In a statement posted to social media, US Secretary of Agriculture Brooke Rollins called on producers to remain watchful.

"I now call on our producers to be vigilant and watchful," Rollins said. "The most effective thing to do right now is to put eyes on all livestock. If you see anything suspicious, contact your state animal health official."

Any veterinarian who suspects infestation should report it immediately. While infestation in people is uncommon, anyone who notices a suspicious lesion or suspects infection should seek medical attention.

USDA and Texas officials have activated a unified Incident Command Team with the Texas Animal Health Commission, following the NWS Response Playbook. The response includes a 20-km infested zone around the detection with quarantines, movement controls, and surveillance; expedited release of sterile NWS flies via ground release chambers, in addition to the 4 million sterile flies per week already being released aerially in the area; expanded trapping along the border and outside the dispersal area; wildlife surveillance and management; and targeted local outreach. USDA's National Veterinary Stockpile is ready to provide treatments, equipment, and logistics support as needed.

The agency said it will work with state departments of agriculture, animal health officials, industry, and producers to mitigate the economic impacts of movement restrictions, including negotiating with trading partners to regionalize trade restrictions on live animals. USDA also noted the US food supply is not at risk, as screwworms do not infest meat or other food sources.

A year of preparation

The Zavala County detection comes after nearly a year of regulatory and logistical preparation.

In August 2025, federal health authorities declared NWS a significant potential public health threat, giving the FDA authority to issue emergency use authorizations (EUAs) for animal drugs — temporarily clearing products approved for other uses, or in other countries, for use against NWS.

A cascade of authorizations followed: lotilaner (Credelio; Elanco) for dogs in October 2025 — the first-ever animal-drug EUA for NWS — and Credelio Cat for cats that November; ivermectin injectable (Ivomec 1%; Boehringer Ingelheim) for cattle in February 2026, alongside NexGard and NexGard COMBO (Boehringer) for dogs and cats; F10 antiseptic spray and ointment (Health and Hygiene Pty Ltd) for multiple species, including horses and birds, that spring; Negasunt Powder (Elanco) for livestock in April 2026; and an expanded EUA for doramectin injection (Dectomax; Zoetis) covering dairy cattle, swine, horses, sheep, and deer in May 2026.

As a result, veterinarians now have authorized treatment options that did not exist a year ago.

On the eradication side, USDA has leaned on the sterile insect technique, opening a sterile-fly dispersal facility in Mexico and another in South Texas, and breaking ground in April 2026 on a larger production facility at Moore Air Base near Edinburg, Texas, expected to produce roughly 300 million sterile flies per week once operational in 2027.

Prior to the current detection, about 4 million sterile flies per week were already being released aerially in the affected area, with nearly 8000 traps jointly monitored along the border.

The history of New World screwworm

NWS (Cochliomyia hominivorax) is not a new adversary for US livestock. The parasite was endemic across the southern United States for much of the 20th century before a decades-long eradication campaign — pioneered by the USDA's use of the sterile insect technique — cleared it from the country by the 1960s and from Central America in the years that followed. The approach works by releasing mass-reared, radiation-sterilized male flies that mate with wild females, producing no offspring and collapsing the population over successive generations. That program stands as one of the most successful pest-eradication efforts in agricultural history, and a protective barrier was ultimately pushed south to Panama.

The pest has resurfaced periodically. A 2016–2017 outbreak in the Florida Keys, centered on endangered Key deer, marked the first local US detection in more than 30 years before it was again eradicated. The current threat traces back to 2022, when NWS began advancing northward through Central America and into Mexico, with case numbers climbing steadily toward the southern border — the progression that culminated in the Zavala County calf.

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