The New World screwworm (Cochliomyia hominivorax) is spreading further across the United States and finding new kinds of hosts to burrow itself into.
Several new cases of infestations caused by the parasitic fly have been reported this past week, including the first ever documented in Arizona since the screwworm reemerged in the country earlier this year. New Mexico officials also recently reported the first U.S. case found in a pet cat. October has seen a sizeable increase in reports of the screwworm affecting both domestic and wild animals.
“We have been preparing for this possibility for a long while and are acting swiftly,” said Ryan Wolker, a state veterinarian with the Arizona Department of Agriculture, in a statement released Wednesday.
Infesting cats and dogs
Despite being a fly, the screwworm is named after its horrific larval form. Unlike many species, screwworm maggots prefer to feed on living flesh. The flies lay eggs on a host via open wounds or mucous membranes, and then the maggots use their screw-like bodies and sharp mouth hooks to dig deep into the animal. This then causes extensive, potentially fatal, tissue damage to the host and raises the risk of serious secondary infections.
For decades, eradication programs in the U.S. and other countries in the Americas had pushed back the screwworm, creating a barrier as far down as Panama. This barrier has fallen in recent years, however, and the parasite has steadily been reclaiming its old territories throughout North America. In June, the screwworm officially returned to the U.S.
After a lull during the summer, the screwworm has resurged. As of October 29, there have been 74 total U.S. cases 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 that had been harvested by hunters. This week, there have been eight cases reported, including some more firsts.
Until now, Texas and New Mexico had been the only two states with documented screwworm. 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 screwworm in a cat, along with three additional cases in dogs.
Cause for concern
The New World screwworm is an especially grave threat to livestock, but it can also infest humans and their pets. In areas of Mexico where the parasite has resurged, there have been over 200 human cases reported since April 2025, with at least one death attributed to the infestation.
There have been no human cases reported in the U.S. during the current outbreaks. These latest cases in Arizona and non-livestock animals are still very concerning, since they indicate the fly is expanding its range and potential host reservoirs. And the more it spreads, the harder it will be to stop the parasite from reestablishing a foothold in the country.
Federal and local health officials say they are taking strong measures to prevent this from happening.
“Because of the extensive outreach, coordination, and surveillance already in place, Arizona producers, veterinarians, and pet owners know what to look for and how to report suspected cases. That preparation is allowing us to move quickly to protect animal health in the state,” Wolker said.
