7th case of New World screwworm identified in Arizona. This time, it's near Tucson

A horse near Tucson has tested positive for New World screwworm, bringing Arizona’s total number of confirmed cases to seven and placing part of Pima County under new animal movement restrictions. The finding means residents who own horses, dogs, livestock or other animals in the affected area will need to pay close attention to wounds and comply with state rules before moving animals. The Arizona Department of Agriculture reported the case Friday, November 13, after the parasite was identified in a Tucson area horse.

State officials established a mandatory infested zone extending 12.4 miles around the reported case. Animals cannot leave that area unless the restrictions are lifted or the state veterinarian authorizes their movement. The Pima County zone is one of four now designated in Arizona. The others are in Cochise, Greenlee and Graham counties. The state’s response is intended to limit the parasite’s ability to spread through infected animals, wildlife and the movement of pets or livestock across county and state lines.

The latest case is Arizona’s seventh in roughly three weeks. The confirmed infections include four horses, two dogs and one sheep. For Tucson area animal owners, the case adds a new concern to the practical challenges of transporting animals for veterinary appointments, boarding, shows, sales, work or travel. People planning to take pets out of Arizona may also encounter additional requirements as other states respond to the spread. The agriculture department is advising owners to check movement rules and to follow the direction of state animal health officials.

New World screwworm is a parasitic fly whose larvae consume the living tissue of warm blooded mammals. The adult fly lays eggs in open wounds, including small cuts and scratches that may be easy for owners to overlook. After the eggs hatch, the larvae feed in the wound, causing severe pain, tissue damage and infection. Untreated infestations can kill an animal within 10 days, although officials say cases are readily treatable and animals generally recover quickly once the infestation, myiasis and related infections receive treatment.

Daily checks are among the steps recommended for people caring for animals. Owners should look for wounds, changes in behavior or signs that an animal is sick or in pain, particularly after time outdoors. Monthly over the counter or prescription treatments are available to pet owners living inside infested zones, though the treatments are not required. The risk is not limited to animals kept on farms. Dogs and other pets can be affected, and wildlife may carry the parasite without being noticed until extensive myiasis and infection have developed.

The parasite’s life cycle makes animal movement a central concern. Once the larvae mature, they leave the wound, fall to the ground and pupate in soil. Flies eventually emerge and can reproduce again, creating opportunities for the infestation to spread. Arizona officials are asking livestock and pet owners, people who work with animals, hunters and others who spend time outdoors to report possible cases immediately. Suspected livestock or domestic animal cases can be reported to the Arizona Department of Agriculture at diseasereporting@azda.gov or 602-542-4293. Suspected wildlife cases should be reported to Arizona Game and Fish at 623-236-7201.

Containment also relies on releasing sterile flies, a method designed to reduce the parasite population by disrupting reproduction. The U.S. Department of Agriculture has released 3.3 million sterile flies in affected parts of Arizona, according to an agency spokesperson. The technique is considered the strongest available tool for reducing New World screwworm numbers. The fly has moved north of a containment area near Panama since 2023, and production capacity is currently based at facilities in Panama and southern Mexico. The USDA is working with the U.S. Army Corps of Engineers on a third facility at Moore Air Base in southern Texas.

Arizona agriculture officials and the University of Arizona’s Arizona Livestock Incident Response Team plan to hold a virtual public meeting Monday, November 16, from 5:30 to 8 p.m. The session will address the confirmed cases, containment work and questions from residents. Information about the parasite, case locations and the state’s infestation zone map is available through the Arizona Department of Agriculture. For Tucson area residents, the meeting offers a chance to learn how the Pima County restrictions apply, what symptoms to watch for and how to report a suspected infection before it becomes a larger threat.