Screwworm Detected for First Time in Arizona as Outbreak Spreads

New World screwworm was found in Arizona for the first time, detected in a dog, as cases of the deadly parasite accelerate.

(Bloomberg) -- New World screwworm was found in Arizona for the first time, detected in a dog, as cases of the deadly parasite accelerate.

The US Department of Agriculture confirmed the infection late Wednesday in a dog in Cochise County, Arizona. 

The spread of the parasitic fly, whose larvae burrow into the wounds of animals, had remained relatively contained since the pest was first detected in the US in June. Most of the early cases were limited to Texas. However, the pace of infections has increased this month, particularly in New Mexico. 

There have been a total of 71 cases of screwworm in the US since the outbreak began, according to the USDA. Eighteen are still active, with 10 located in New Mexico. 

The increase in October comes after screwworm, which can be deadly to livestock if left untreated, was discovered for the first time in wildlife last week. Spread among wildlife has been a key concern for cattle ranchers since the animals travel through the same terrain that their herds graze on. 

The spread also comes as the USDA has reopened ports in Arizona and New Mexico to shipments of cattle from Mexico. The US had halted livestock shipments from the country, and largely maintained that ban until late August when it began a phased reopening of the ports. 

The US had previously eradicated screwworm, pushing the pest south of Panama. But the fly began spreading north again after the pandemic, reaching Mexico in late 2024. 

While screwworm is a major threat to the US cattle herd, the parasite has also infected animals from goats to horses. The most recent five cases involved four dogs and one cat.