The U.S. Department of Agriculture announced three new cases of New World screwworm, including the first cases in dogs and goats, on Monday, bringing the nation’s total case count to five. It also pledged to ramp up and expedite mitigation efforts for screwworm, a parasitic fly that the nation declared eradicated in the 1960s.
At a news briefing on Monday, federal and Texas state officials said that they were using A.I.-driven technology to monitor screwworm populations, training ranchers to recognize infections in their livestock and expanding the number of facilities that produce and disperse sterile flies, which are the primary tool for managing screwworm.
Officials are also considering whether to grant an emergency authorization of a new, genetically engineered strain of flies that could make sterile fly production faster and more efficient.
“We prevented and eradicated this pest before,” Texas Governor Greg Abbott said during the briefing. “We can do it again.”
The three new cases were identified in a calf in La Salle County, Texas, a goat in Gillespie County, Texas, and a dog in Lea County, N.M. It is not clear whether the dog acquired the parasite in the United States; although officials initially indicated that the dog may have recently been in Mexico, they later said its travel history was unknown.
The agency reported the first two cases, both in calves in Zavala County, Texas, last week.
Still, Secretary of Agriculture Brooke Rollins noted during the news briefing that the re-emergence of the New World screwworm was not unexpected.
For the past few years, the insect has been making its way north through Central America. Rollins credits monitoring and containment efforts with delaying the pest’s spread into Texas.
“Every model showed that the New World screwworm would be here in Texas by early last summer, so we bought ourselves an additional year to prepare for this moment,” Ms. Rollins said.
The New World screwworm is a blowfly that feeds on living flesh. Adult females lay their eggs in open wounds or orifices of warm-blooded animals. When the eggs hatch, the larvae burrow into the wound, consuming the animal’s tissue. Untreated, screwworm infections can kill animals within a week.
In humans, screwworm infections are rare. Last year, American health officials confirmed a travel-related case in a Maryland resident who had recently traveled to El Salvador, but no domestically-acquired human cases have yet been reported.
The parasitic fly was a serious livestock scourge in the southern United States in the early 20th century. Officials eradicated it using the sterile insect technique, which involved breeding enormous quantities of flies, sterilizing them by exposing them to radiation and then releasing them into the wild. The screwworm population plummeted as wild females, which mate only once in their lives, bred with the sterile males.
The technique relies on the release of sterile males, but separating the male and female flies produced in mass rearing facilities has not been practical. Consequently, both male and female flies are typically raised, sterilized and released.
A male-only strain would make the approach far more efficient. Recently, scientists have made strides toward creating a male-only strain with the NovoFly, a genetically engineered version of the New World screwworm.
“It’s going to allow us to almost instantaneously double the number of sterile flies that we put in the fight,” Scott Hutchins, Under Secretary for Research at the U.S.D.A., said of NovoFly at the news briefing on Monday.
The NovoFly contains genes for two unusual proteins. In males, these proteins have few known effects aside from sterility. But they cause females to die at the embryonic stage, so the only NovoFly to reach adulthood — unless an antidote to the genetic modification, tetracycline, is administered — is a sterile male.
“It’s never been tested in the field, and that has to be the next step,” said Maxwell Scott, an entomologist at N.C. State University, whose lab helped develop the NovoFly.
The NovoFly is considered a pesticide; typically it would be subject to a lengthy approval process before being released into the environment. However, because of the public health and economic dangers associated with a screwworm outbreak, the Environmental Protection Agency is considering an emergency exemption that could speed up its release.
That could make the NovoFly one of very few genetically engineered animals to have been released into the wild. In 2006, a modified version of the highly invasive pink bollworm was released in Arizona. More recently, the Oxitec mosquito — created to stymie the spread of malaria, Zika, yellow fever and other diseases — has been released in mosquito-plagued areas of the world, including the Florida Keys.
It is not yet clear whether NovoFly’s emergency exemption will be approved, but the U.S.D.A. has made it clear that it will continue to support the effort, and others that might bring the United States closer to total screwworm eradication.
“We are going to turn over every stone to find more sterile flies,” John Bellinger, the U.S.D.A.’s Senior Advisor for New World Screwworm Preparedness, said during the Monday briefing. “We have to be ready next spring.”
Emily Anthes is a science reporter, writing primarily about animal health and science. She also covered the coronavirus pandemic.
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