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Uruguay wants to genetically eradicate devastating screwworm flies

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Uruguay wants to genetically eradicate devastating screwworm flies

On a warm, sunny day in Montevideo, the capital of Uruguay, the air is smog-free and fresh. In a highly secure facility at the National Institute for Agricultural Research (INIA), there are tens of thousands of genetically modified flies, their bright blue wings fluttering against the walls of their small, white net cages. They represent a new weapon that could soon be used against an enemy that kills cattle and costs the livestock industry millions of dollars each year: the New World screwworm fly, a parasite common in parts of South America and the Caribbean. The female flies often lay their eggs in wounds on cattle. They hatch into worm-like larvae that screw themselves into the flesh of the host animal and not only damage the skin on their way, but also eat meat. If left untreated, infected animals eventually die in horrific agony.


To combat the screwworm fly, INIA veterinarian Alex Menchaca and his colleagues used CRISPR genetic scissors to develop a genetic construct that gradually makes all female flies sterile. The construct manipulates the fly’s reproductive process so that it can spread preferentially and more quickly than ordinary genes. That’s why it’s also called “Gene Drive”. “With gene drives we can control these pests in a precise and effective way,” says Menchaca.

Gene drives also occur in nature, but the technology for targeted production is new and still quite controversial. CRISPR allows scientists to cut specific genes in the DNA of any organism and replace them with new sequences. This can alter an animal’s DNA in ways that affect the survival of the species, often by rendering females infertile.

Some organizations have attempted to develop gene drives to eradicate mosquitoes. Target Malaria, supported by the Seattle-based Bill & Melinda Gates Foundation (BMGF), is currently the most advanced gene drives project in the world. But here too we have not yet progressed beyond cage experiments. The approval process for release into the wild is slow.

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In 2020, INIA researchers received permission from the Uruguayan government to test their gene drive techniques as part of the national screwworm fly control program. They are currently experimenting in the laboratory with various components of the gene drives in gene-edited screwworm flies.

The plan is to create a population of male screwworm flies with modified versions of genes that are important for the fertility of female flies. When the manipulated males are released into the wild, they should mate with females and pass on the gene. As generations pass, more and more female screwworm flies will inherit copies of the gene drive and become sterile, leading to a population collapse.

The INIA researchers are currently close to the next stage of cage experiments in the laboratory. The Inter-American Development Bank (IDB) has granted them funding of $450,000.

“The interesting thing is that you can interrupt the development of females if you implant a gene into them,” says entomologist Maxwell Scott of North Carolina State University, who is working with the Uruguayan team. “This is potentially a very efficient system.”

In July last year, Panama declared a state of animal health emergency due to screwworm fly outbreaks across the country. And in February of this year, more than 200 cases of screwworm fly infestations on animals were reported in Costa Rica, prompting the government to also declare a state of emergency. In Uruguay, the screwworm fly costs the livestock industry between $40 and $154 million annually. Agricultural exports are the linchpin of the Uruguayan economy. Over 80 percent of the goods the country exports are agricultural products. That includes beef, which makes up 20 percent of that, worth $2.5 billion a year.

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This makes it all the more important for the country to find new tools to combat the pests, says Carmine Paolo De Salvo, rural development expert at the IDB. “The government [Uruguays] is under constant pressure to do something about it,” he says.

Scientists have been trying to combat screwworm flies for decades. In the 1950s, researchers at the US Department of Agriculture developed the so-called sterile insect technology (SIT). The male screwworm flies are sterilized using radiation. The DNA-damaged males are then dropped over the infested area using aircraft. When they mate with wild female flies, the eggs do not hatch, slowing population growth and preventing the parasite from spreading.

This approach has proven successful in many countries, including parts of Central America, freeing millions of animals and wildlife from the pests’ painful grip. In the USA, a nationwide eradication program using SIT worked so well that in 1966 the US Department of Agriculture declared the insects eradicated within the country’s borders. The benefits to the livestock industry were immense: producers saved up to $900 million, and the health of wild and livestock animals improved.

But even with sterile males, eradicating screwworm flies remains a stubborn challenge. To prevent the parasites from returning, the United States, along with Central and South American countries, operate a permanent sterile fly exclusion zone on the Panama-Colombia border, requiring a continuous supply of billions of flies each year. This effort is too costly and simply not enough to eradicate screwworm flies in South America. The pests are too entrenched and too difficult to monitor, researchers say. So alternative methods were sought.

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