New World screwworm expands across the US Southwest, reaching Arizona, pets and wildlife

The US New World screwworm outbreak is continuing to expand geographically and across animal species. After spreading through Texas and New Mexico, Cochliomyia hominivorax has now been confirmed in Arizona, first in a dog and then in a horse with no known travel history. A domestic cat in New Mexico has also become the first feline case reported in the current US outbreak, while infections in wildlife add a new challenge to eradication efforts.

New World screwworm (NWS) has continued its northward spread across the southwestern United States since reappearing in the country in June 2026. By late October, at least 71 animal cases had been confirmed in the United States, mainly in Texas and New Mexico, affecting cattle, sheep, horses and dogs. The parasite has since reached Arizona and expanded into additional host species.

Arizona confirms local transmission concerns

Arizona confirmed its first NWS case on October 28, in a dog from Cochise County. A second case was confirmed on November 2 in a horse near Hereford, around 45 miles from the first case. Importantly, the horse had no known travel history, raising concern that local transmission may now be occurring within Arizona rather than cases being limited to animals introduced from affected areas.

Authorities have established infested zones around confirmed cases, with movement restrictions, intensified surveillance and sterile-fly releases.

First feline case in the current US outbreak

A domestic cat in New Mexico has become the first feline case reported in the current US outbreak. The finding is important because NWS is not limited to livestock.

Female screwworm flies lay their eggs on wounds or natural body openings of living warm-blooded animals. The larvae then feed on viable tissue, causing rapidly expanding lesions that can become severe or fatal if untreated. Dogs and cats therefore need to be included in surveillance, particularly in affected rural areas.

New Mexico expands its infested zones

New Mexico remains one of the main centres of the outbreak. By early November, authorities had expanded from individual circular control zones to a broader regional system covering all or part of several southern counties. Affected areas now include all of Doña Ana, Grant, Hidalgo, Luna and Otero counties, together with parts of Catron, Chaves and Sierra counties.

Recent detections have involved cattle, sheep and horses.

The decision to move to larger regional zones reflects the increasing geographical spread of the parasite and the difficulty of managing the outbreak as a series of isolated cases.

Wildlife infections complicate eradication

The outbreak has also reached wildlife. USDA confirmed the first US wildlife infection on October 21.

This development is important because wildlife cannot be inspected, treated or moved under controlled conditions in the same way as livestock or pets. Infected wild animals could therefore maintain local fly populations and make eradication more difficult.

Hunters, wildlife professionals and the public are being asked to report animals with suspicious wounds or visible maggots.

Sterile flies remain the main control strategy

The principal control tool remains the sterile insect technique. Large numbers of male screwworm flies are bred, sterilised and released over affected areas.

Because female C. hominivorax usually mate only once, mating with a sterile male results in eggs that do not hatch. This method successfully eradicated New World screwworm from the United States in the twentieth century and remains central to the current response. Sterile-fly releases are now being intensified in Arizona, New Mexico and other high-risk areas, while a new dispersal facility is being developed in Douglas, Arizona.

Movement controls are expanding

Confirmed cases trigger animal movement restrictions within infested zones. Livestock and other warm-blooded animals may require veterinary inspection before leaving affected areas to reduce the risk of transporting larvae into previously unaffected regions. Some states are also tightening import rules for animals originating from Arizona, New Mexico and Texas.

These measures show how the outbreak is beginning to affect livestock trade and companion-animal movement beyond the immediate outbreak zone.

Mexico remains the main source of regional pressure

The US outbreak remains closely linked to continued transmission in northern Mexico. Hundreds of recent Mexican detections have been reported within 100 miles of the US border, involving cattle, goats and dogs.

This creates continuing pressure on southern US states despite control measures. At the same time, livestock trade through border ports is being progressively restored under enhanced surveillance and biosecurity protocols.

Pets and livestock require close wound inspection

Authorities are advising owners in affected areas to inspect animals daily for wounds that enlarge or fail to heal, visible larvae or egg masses, unusual irritation or discomfort, lesions around the eyes, ears or nose, genital wounds, or umbilical lesions in newborn animals.

Early treatment is usually effective, but untreated infestations can destroy large amounts of living tissue.

Human infection remains rare

Humans can also become infected, although cases are much less common than in animals. Flies do not bite people. Infection occurs when eggs are deposited in untreated wounds or, less commonly, around natural body openings. No locally acquired human case has yet been reported in the current US outbreak.

People working outdoors or in close contact with livestock and wildlife are advised to keep wounds clean and covered and seek medical attention for suspicious lesions.

The Arizona horse case is particularly important because the animal had not travelled, strengthening concern over local circulation. For now, US control efforts remain based on rapid diagnosis, wound surveillance, animal-movement controls, fly trapping and large-scale sterile-insect releases.

But the widening range of hosts and locations shows that preventing permanent establishment of New World screwworm in the US Southwest is becoming an increasingly urgent animal-health priority.