New World Screwworm detected in Cochise County; Arizona launches containment plan

New World screwworm has been detected in Cochise County, putting animal owners, ranchers and veterinarians across Southern Arizona on alert. The infestation was reported near Hereford, and state and federal officials are creating a containment plan that could affect animal movement, ranch operations and livestock trade. While Tucson is outside the initial containment area, residents who own horses, dogs, cats or livestock could encounter new inspection and reporting requirements if the pest spreads or additional cases are identified.

The public details about the initial case have varied. KGUN reported that an infected horse near Hereford prompted the response, while a separate report citing the U.S. Department of Agriculture said the agency confirmed Arizona’s first New World screwworm case on Oct. 28 in a ranch dog in Cochise County. That dog is being treated and is expected to survive, according to the report. Officials are evaluating other livestock on the property, and the animal reportedly had no known history of travel to another county or state. The investigation into how it became infested remains underway.

New World screwworm is an invasive fly whose larvae feed on the living tissue of warm blooded animals. Adult flies are attracted to fresh wounds and other openings, where they lay eggs. The resulting larvae can rapidly enlarge an injury and cause severe tissue damage. Without treatment, a full grown animal may die in as little as 10 days. The pest can affect livestock and household animals alike, making daily checks important even for people who are not involved in ranching.

Arizona officials are establishing an infested zone extending 12 miles around affected animals, or approximately 20 kilometers under the federal description. Confirmed infected animals will be quarantined for 21 days, and animals in the surrounding area cannot be moved without state approval. The response includes expanded surveillance, trapping, inspections and outreach to animal owners and veterinarians. The USDA is asking owners to look for unusually large wounds, maggots and signs of pain or discomfort, and to contact a veterinarian or animal health official immediately if those symptoms appear.

Movement restrictions are intended to slow the pest’s arrival in places that have not yet been affected. That matters to Southwestern livestock commerce because animals regularly move through the region and across the border. The current outbreak has included detections in Texas and New Mexico, making Arizona the third state involved. A report citing Reuters said the USDA’s tally had reached 71 cases since June, including 18 active cases, as of Wednesday. Officials have not said that the Cochise County detection by itself represents a widespread infestation or established that it came through imported cattle.

The main eradication tool will be the sterile insect technique. State Assistant Veterinarian Devon Kartchner said sterile flies will be released to interfere with the pest’s reproduction, while movement controls help limit its geographic spread. Approximately 1 million sterile flies had been released in the Cochise County area after the detection, according to the separate report. Kartchner said the process is slow but has worked in the past, and the state expects removal efforts to continue into 2027.

The discovery is also creating uncertainty for ranchers and for livestock markets that depend on safe animal movement. Cochise County cattle and horse rescuers Steve Boyce and Theresea Warrell said they are monitoring their animals and preparing for possible spread. Warrell described the situation as a potential threat to their livelihood, while Boyce recalled the damage caused by screwworm in Arizona during the 1970s. Veterinarian Gary Thrasher said he has treated affected animals and seen wounds worsen quickly, leaving some horses with lasting scars.

Border trade is another concern, although the case has not resulted in a reported regionwide shutdown. Livestock imports have resumed at three ports in Arizona and New Mexico, and the Nogales port was scheduled to reopen Nov. 30 if conditions allowed safe trade. Douglas, Arizona, and Santa Teresa, New Mexico, were already accepting Mexican feeder cattle, with the three locations intended to give exporters and U.S. buyers more options. Further detections could delay additional reopenings, increase inspection and handling costs, or restrict cattle movement, but the single Arizona case does not establish a specific effect on beef prices or cattle supplies.

The next steps will include investigating the ranch, examining nearby animals, reviewing trapping results and determining whether additional cases emerge. For Tucson area animal owners, the immediate guidance is practical: inspect animals every day, pay close attention to wounds, and seek professional help quickly if maggots or unusual discomfort appear. Officials have emphasized that containment will require patience. Success will depend on finding infestations early, limiting animal movement when necessary and sustaining sterile fly releases until the pest is eliminated from the affected area.