by Aspen Ford, Washington State Standard
June 17, 2026
A parasitic fly that poses a deadly threat to livestock and pets has re-invaded the United States for the first time since 1968.
Washington state agriculture officials say it’s unlikely that the New World screwworm will spread here. It doesn’t thrive in colder climates, and the state recently implemented new inspection requirements to help keep the flies out.
The screwworm feeds on the tissue of warm-blooded animals and “screws” open a wound in flesh to lay its eggs. If left untreated, its larvae can be fatal.
The pest has infected at least 12 animals in Texas and New Mexico, including cattle, goats, a sheep and a dog, in the span of two weeks. On Tuesday, the U.S. Department of Agriculture said it was funding $105 million of projects aimed at controlling and eradicating the screwworm.
New World screwworm maggots can cause “painful, foul-smelling wounds” in people as well as animals, according to the U.S. Centers for Disease Control and Prevention. But the CDC said earlier this month that the risk to people in the U.S. remains low.
Livestock entering Washington state now must undergo a veterinary inspection within five days prior to travel, aligned with federal requirements. Before the detection of screwworms in the United States, that timeline was longer.
No cases of the New World screwworm have been reported in Washington, according to the state’s Department of Agriculture.
While the flies develop best in warm tropical regions, changing weather patterns could increase the likelihood of them surviving seasonally further north.
Eastern Washington, in particular, may be more susceptible to temporary establishment due to the warm temperatures there during part of the year, according to a state Department of Agriculture spokesperson.
The department’s greatest concern is the importation of infested animals into Washington, but there are several protocols in place to limit the risk.
In addition to veterinary inspections before entering the state, livestock are also inspected by the Washington Department of Agriculture’s brand inspectors before leaving the state, during any change in ownership, before going to sale, and when entering a feedlot.
At slaughterhouses, livestock are inspected by both state officials and federal officials with the Food Safety and Inspection Service. The U.S. Department of Agriculture says that the presence of the fly is not a food safety issue.
States with infected animals have implemented “stop-movement controls, and no livestock has left an infested zone without oversight,” according to a Washington State Department of Agriculture spokesperson.
The agency encourages owners to monitor their animals for unusual wounds and report concerns promptly.
The parasite is not an infectious disease and doesn’t spread animal-to-animal, the agency said. There are preventative measures and treatments that can be used to keep animals safe from the screwworm.
“At this time, Washington has strong animal health monitoring and response systems in place, and state officials continue coordinating closely with federal and industry partners to support preparedness and response efforts,” a state Department of Agriculture spokesperson said.
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