Should we wipe out the pests now that we can?

Felicola (Lorisicola) isidoroi is a creature that is probably either extinct or on the verge of being so. In the past, it must have been present throughout most of the Iberian Peninsula, but the last time scientists encountered one was in 1997. For biologists, this represents a loss of biodiversity. For everyone else, it’s just another bug. Felicola (L.) isidoroi is a louse that lives by sucking blood. Its unique feature is that its only host is the Iberian lynx. Specific to the most endangered feline on the planet, it shared the latter’s path to extinction until humans decided to save the feline, but not its parasite.

The lynx recovery program includes deworming specimens released in the wild, a procedure that is also conducted in the event of a capture. Jesús María Pérez, a zoologist and expert in pests and parasites at the University of Jaén in southern Spain, believes that the louse is still a much rarer species than the lynx itself, and should also be saved because it is part of biodiversity: “As a unique product of evolution, it has incalculable value.”

The dilemma posed by the lynx louse is the same one generated by many other parasites, pests, and species that, like some mosquitoes, are not pathogens themselves but vectors that carry the cause of various diseases. A few days ago, a group of biologists, ecologists and sociologists published an opinion piece in the journal Science whose title makes it clear what it’s about: Deliberate extinction by genome modification: An ethical challenge.

Humans are good at extinguishing life. In the last five centuries, 73 entire branches of the evolutionary tree have disappeared. But we’re not so good when it comes to eliminating pathogens. Despite all the advances of the medical revolution of the late 19th and mid-20th centuries (hygiene, vaccines, antibiotics and so on), the number of eradicated infectious diseases can be counted on the fingers of our hands, and there would still be several fingers left over. And even in these cases, as for instance with onchocerciasis or river blindness, what is actually being eradicated is the disease, not the causative agent. In that sense, the lynx louse case is truly exceptional.

Throughout the recent history of our war against insects harmful to humans, all sorts of techniques have been used, including traps, the introduction of other species, broad-spectrum insecticides such as DDT, next-generation insecticides, rodenticides, and the irradiation of mosquito populations to sterilize them. And these days we’re also thinking about genetic modification.

By taking advantage of the existence of certain genetic drivers that do not follow Mendel’s laws, certain traits can be selected, giving the modified gene a greater than 50% chance of being passed on to the next generation. With CRISPR genetic cut-and-paste technology, the desired mutation can be passed on to all offspring and remain present in successive generations. Never before had it been possible to interfere in the destiny of a species in such a radical way.

“There must be a very strong moral justification, and that justification will require more than economic self-interest,” says in an e-mail Gregory Kaebnick of the Hastings Center for Bioethics located in Garrison, New York and co-author of the Science article. “The species most likely to be eradicated are those that cause great suffering,” he adds. In any case, “the decision requires collaboration with the societies affected by the disease in question, and we maintain that this is both a local and global problem.”

The cattle screwworm, whose image opens this article, is one of the candidates for elimination from the face of the Earth. It is the larval stage of the fly Cochliomyia hominivorax. The flies lay their eggs in sores, damaged mucous membranes and wounds of a wide variety of mammals, including humans.

“A female can lay around 500 eggs,” notes Pablo Martínez Labat, head of the parasitology department at the Cuautitlán School of Higher Studies at the Autonomous University of Mexico (UNAM). In two weeks, the eggs will hatch and the maggots will feed on the host’s tissues. “Living tissues, but contaminated with bacterial agents,” explains Martínez Labat. If nothing is done to treat it, the infected person can die.

The fly was present in all warm regions of the Americas, with a high incidence in livestock-raising regions. But the beginning of the atomic age marked the beginning of the end for this parasite. After years of theorizing, in 1959 American researchers released thousands of adult males on the Caribbean island of Curaçao. The flies had been irradiated to sterilize them in their pupal stage.

At a rate of 200,000 per week and 300 individuals released per square kilometer, they wiped out C. hominivorax within weeks. The success was such that Florida ranchers pushed for a large-scale program. They released 50 million irradiated flies over several weeks, wiping out 90% of the wild population in three months. The rest of the U.S. livestock-raising states followed suit, eradicating the fly from their own territory in the 1970s. The same was done in northern Mexico to create a buffer zone. The cattle screwworm at that point disappeared from Mexican territory and some Central American countries.

“The plant they installed in Mexico had the capacity to breed 500 million flies a week,” recalls the UNAM parasitologist. But in 2018, it was dismantled and moved to Panama. At first, it didn’t seem like a bad idea; once it had been eradicated from North America, it had to be stopped on the southern flank. But for various reasons, cases in the north are now beginning to reappear. The influx of people from countries like Colombia and Venezuela, where the screwworm is abundant, could be behind it. There are new plans to combine irradiation with genetic modification, which could spell its final doom.

Meanwhile, plans against disease-carrying mosquitoes do not seek their total extinction, but rather their local eradication where necessary. The combination of radiation and gene drives (a process to propagate certain genes within a population) has reduced the incidence of dengue fever in several areas of the Americas thanks to mosquitoes created by the company Oxitec. However, the lethality of dengue fever is nowhere near that of malaria, which kills half a million people each year. For this reason, the Science article singles out Anopheles gambiae, its main vector, as an ethically acceptable target.

“In recent years, we’ve had great success in the laboratory creating gene drive-modified mosquitoes (GDMMs) that could one day be used as tools to control malaria in Africa,” says John Connolly, a researcher at Imperial College London and chief scientist for Target Malaria, a project funded by the Bill Gates Foundation.

Currently, two GDMM strategies are being considered. One seeks to reduce the mosquito population by affecting female fertility — by releasing sterile males, for example, or mosquitoes modified to carry a mutation that makes their offspring dependent on something they won’t find in nature.

Another approach seeks to introduce genetic traits into the population that will reduce the viability of the pathogen they carry. It’s important to remember that the causative agent of malaria is a protozoan parasite of the Plasmodium genus. So far, no releases or field trials have been conducted.

“This would only happen when laboratory data, mathematical modeling, and a rigorous risk assessment indicated that gene drives for malaria control had a safety profile that would allow regulatory approval and the buy-in of stakeholders in Africa,” Connolly clarified.

Rubén Bueno, technical and R&D director at Laboratorios Lokímica, admits that genetic engineering “has allowed us to ask these questions.” However, he believes that new tools could be combined to exert population control below the level of damage “without having to reach extinction.”

From Mexico, UNAM professor Martínez Labat points out that “even monsters have a function in nature.” Kaebnick, co-author of the Science article, offers one last thought: “Species are valuable in and of themselves, even beyond their contribution to the rest of the ecosystem. However, sometimes, weighing the suffering they cause to us or the animals in our care might justify their eradication, if the suffering cannot be addressed otherwise.”

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