The New World butcher worm (Cochliomyia hominivorax) is spreading further across the United States and finding new types of hosts to burrow into.
Several new cases of infestations caused by the parasitic fly were reported last week, including the first ever documented in Arizona since the butcher worm reappeared in the country earlier this year. Officials in New Mexico also recently reported the first U.S. case found in a pet cat. The month of October saw a significant increase in reports of dung worms affecting both domestic and wild animals.
“We have been preparing for this eventuality for a long time and acting quickly,” Ryan Wolker, state veterinarian with the Arizona Department of Agriculture, said in a statement released Wednesday.
Infesting cats and dogs
Although it is a fly, the butcher worm gets its name from its horrible larval form. Unlike many species, dung worms prefer to feed on living flesh. Flies lay eggs on a host via open wounds or mucous membranes, then the maggots use their screw-shaped bodies and sharp mouth hooks to burrow deep into the animal. This then causes extensive, potentially fatal, tissue damage in the host and increases the risk of serious secondary infections.
For decades, eradication programs in the United States and other countries in the Americas have pushed the cork caterpillar back, creating a barrier all the way to Panama. However, this barrier has fallen in recent years and the parasite has gradually reconquered its former territories throughout North America. In June, the screwworm officially returned to the United States
After a lull during the summer, the butcher caterpillar has reappeared. As of Oct. 29, a total of 74 cases in the United States have been reported this year, including 22 cases documented this month, according to the U.S. Department of Agriculture. Last week, Texas health officials reported the first case found in wildlife, a female elk captured by hunters. This week, eight cases were reported, including a few more firsts.
Until now, Texas and New Mexico were the only two states where butcher worm was documented. On Wednesday, the Arizona Department of Agriculture confirmed the state’s first case, found in a pet dog whose back had become “heavily infested with maggots.” The case was reported in Conchise County, which borders New Mexico and Mexico. New Mexico also reported the first case of dung worms in a cat, as well as three additional cases in dogs.
What to worry about
The New World butcher beetle poses a particularly serious threat to livestock, but it can also infest humans and their pets. In areas of Mexico where the parasite has reappeared, more than 200 human cases have been reported since April 2025, with at least one death attributed to the infestation.
No human cases have been reported in the United States during the current outbreaks. These latest cases in Arizona in non-livestock animals are still of great concern, as they indicate that the fly is expanding its range and potential host reservoirs. And the more it spreads, the more difficult it will be to prevent the parasite from regaining a foothold in the country.
Federal and local health officials say they are taking strict measures to prevent this from happening.
“Because of the extensive awareness, coordination and surveillance already in place, Arizona producers, veterinarians and pet owners know what to look for and how to report suspected cases. This preparation allows us to act quickly to protect animal health in the state,” Wolker said.
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