New World screwworm reaches Arizona for the first time, confirmed in a Cochise County dog

Arizona has recorded its first confirmed case of New World screwworm, a parasitic fly whose larvae burrow into the living flesh of warm-blooded animals, after a dog on a ranch in Cochise County turned up with a maggot-infested wound, state and federal agriculture officials said this week. The U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) said samples from the animal were tested and confirmed on Wednesday at its National Veterinary Services Laboratories in Ames, Iowa, after a local veterinarian examined the dog on Oct. 26 and found a heavily infested wound on its back.

It is the first time the pest has been detected in Arizona and makes the state the third, after Texas and New Mexico, to record an animal case since the parasite reestablished itself in the United States this year. The Arizona Department of Agriculture said the dog is under veterinary care and that officials are working with the animal's owner and attending veterinarian to gather more information, including evaluating other livestock on the same property. The precise location within the border county has not been disclosed.

A pest the US eradicated decades ago

New World screwworm (Cochliomyia hominivorax) was wiped out in the United States through a decades-long sterile-fly release program and declared eradicated in 1966. The current outbreak traces back to 2023, when cases began reappearing in Panama and Costa Rica after the pest had been pushed back for years. It has since spread north through Central America and into Mexico, prompting repeated, and at times contentious, halts to Mexican cattle imports.

The USDA recorded the first U.S. animal case of the renewed outbreak on June 3, in Texas. Since then, cases have turned up in sheep and cattle in Texas and New Mexico, and on Oct. 21 officials confirmed the first U.S. wildlife case: a female elk on a game ranch in Pecos County, Texas, found by a hunter who reported it. Wildlife officials there are now advising hunters to inspect the eyes, ears, nose, mouth and other soft tissue of game animals before field-dressing a carcass. The Arizona dog is the first confirmed U.S. case in a companion animal.

• 71 screwworm cases have been recorded in the U.S. since the outbreak's first detection on June 3, with 18 currently classified as active, according to USDA figures reported Wednesday by wire services.
• Arizona is the third state, after Texas and New Mexico, to confirm an animal case.
• Federal officials are establishing a 20-kilometer (about 12.4-mile) "infested zone" with animal movement restrictions around the detection site.
• The dog's wound was discovered Oct. 26; the case was laboratory-confirmed Oct. 28.

Ranchers and pet owners on alert near the border

The parasite poses little direct risk to people — officials say human infestation remains very low and no U.S. human cases have been reported this year — but it is a serious threat to livestock, pets and wildlife. Left untreated, infested wounds can kill an animal within one to two weeks as larvae consume living tissue. For Arizona's cattle industry, already squeezed by near-record beef prices and periodic halts to Mexican cattle imports tied to the outbreak, a confirmed domestic case adds pressure on an industry watching the border closely. Hunters in affected areas are also being told to check game animals before moving or dressing a carcass, following the Texas elk case.

"We have been preparing for this possibility for a long while and are acting swiftly," said Arizona State Veterinarian Dr. Ryan Wolker.

Wolker, who also noted the pest had not been seen in the U.S. since its eradication nearly 50 years ago, described the infestation as "very treatable" when caught early. Arizona Department of Agriculture Director Paul E. Brierley said at a virtual press briefing Wednesday that the state had spent more than a year preparing for the pest's likely arrival, including pushing for a dedicated sterile-fly production and dispersal facility in Douglas, Arizona, that has not yet opened.

What officials are doing now

APHIS and Arizona animal health officials are running a joint response under the agency's standard protocol for new detections: establishing the infested zone around the ranch, increasing live-animal surveillance and insect trapping, releasing sterile flies to suppress reproduction, and conducting outreach to local producers and veterinarians. The agency is also expanding sterile-fly production nationally, with a goal of approaching roughly 500 million flies a week, and coordinating border surveillance and cattle-import protocols with Mexican counterparts. Arizona officials are separately renewing calls for the long-sought Douglas facility to come online.

Preparation for the pest's spread predates this week's case. In 2025, the USDA awarded the University of Arizona's College of Veterinary Medicine a federal grant to make its Veterinary Diagnostic Laboratory one of three U.S. sites screening suspect flies for the species, forwarding confirmed specimens to the Ames lab for final verification. "Our lab conducts surveillance of important animal diseases that can cause significant damage to the economy," said lab director Dr. Yan Zhang.

Animal owners in the area are being told to check livestock and pets daily for draining or enlarging wounds, visible maggots or egg masses, signs of discomfort, and lesions around the ears, nose, genital area or navel, and to contact a veterinarian or state animal health official immediately if they see any of those signs. The USDA maintains a tracking dashboard of confirmed U.S. detections, which the agency says is updated daily. Officials have not said when the Cochise County infested zone might be lifted, and the investigation into how the dog was exposed, including whether it had recently traveled, remains open.