Is it ever OK to purposely make a species go extinct? What if you could save millions of people around the world by eradicating a disease-carrying species of insect, like a mosquito? Could the lives saved outweigh the moral ambiguity of wiping out a species on purpose?

Now that gene-editing techniques such as CRISPR are available, we are closer to being able to take out an entire species at the flick of a genetic switch. 

However, as we’re currently on the brink of a sixth mass extinction (or it's already begun, depending on who you ask), we’re more used to the idea of saving a species from the edge of extinction, even when it looks like there is no hope, not ending one. 

If it was to happen though, there are two main contenders scientists are targeting: the flying hypodermic needle known as a mosquito and the utterly-dependent-on-its-human-host Guinea worm.

So, why these two?

Mosquitos: flying hypodermic needles of doom, and playing God

Mosquito-borne diseases kill millions every year. Zika, chikungunya, dengue, and yellow fever are all transmitted to humans by the Aedes aegypti mosquito. Malaria, spread by the Anopheles mosquito, kills over 430,000 people every year. More than half of the world’s population live in areas where these mosquito species are present. 

Scientists have been experimenting with genetically modifying mosquitos using various gene-driving techniques for years. Some aim to prevent them from carrying and passing on parasites, others attempt to stop them reproducing by sending out sterile males into the wild to compete for females, thus causing a population collapse.

In fact, there have been many examples of the successful modification of mosquitos to prevent them from breeding and ultimately transmitting these viruses to humans in the field, so why isn’t this mainstream yet?

Well first, the technology has to be perfected and practicalities dealt with; the number of modified mosquitoes that would have to be released is phenomenal and if the techniques are not perfect, cross-breeding and further adaptations could occur, potentially wiping out the wrong targets – the unintentional side effects of "playing God". 

There’s also the ecological argument that the consequences of the permanent removal of a species from its habitat are unknown and could be catastrophic. Mosquitos are pollinators, and a source of food for a vast number of animals, including fish, birds, turtles, frogs, lizards, bats, venus flytraps, dragonflies, and other insects. Wiping out a species could leave an ecosystem with a predator and no prey, or a plant with no pollinator.

However, a feature in Nature back in 2010 that debated a world without mosquitos from an ecological point of view concluded that they weren’t actually vital enough to make that much of a difference, and certainly didn’t have enough redeeming qualities to balance out the good it could do to remove them.

"They don't occupy an unassailable niche in the environment," entomologist Joe Conlon, of the American Mosquito Control Association, told Nature. "If we eradicated them tomorrow, the ecosystems where they are active will hiccup and then get on with life."

If the ecological and technical objections can be overcome, then it is unlikely sentimentality towards a creature that has ravaged humans will prevent the coming mosquitocide.

The Guinea worm’s mortal enemy is Jimmy Carter (yes, that Jimmy Carter)

When diagnosed with brain cancer in 2015 (he would go on to beat it), former President Jimmy Carter – founder of the Carter Center, a philanthropic NGO dedicated to alleviating human suffering – outlined a short list of accomplishments he would still like to achieve.

“I would like to see Guinea worm completely eradicated before I die,” Carter said. “I’d like for the last Guinea worm to die before I do.” 

When the Carter Center began its international campaign to eradicate Guinea worm disease back in 1986, there were an estimated 3.5 million cases each year in 21 countries. In 2018, the Center reported 28 cases across three countries. If eliminated, it would be the first parasite disease in history to be eradicated.

It’s a perfect target because Dracunculiasis, or Guinea worm disease, is a crippling, painful infection, but its diagnosis is easy and unambiguous, its transmission is seasonal, it has limited geographical distribution, and is transmitted exclusively by drinking stagnant water rather than being airborne, which makes controlling its transmission relatively easy and cheap.

Dracunculiasis occurs when people drink water contaminated with parasite-infected water fleas. It takes 10-14 months for the larvae to develop, migrate through the intestines, and grow into a mature worm, which can grow up to 80 centimeters (31 inches) in length. The worm burrows its way to the skin as an exit and the only way to remove it is to pull it out, which can take several days and is very painful.

A successful eradication strategy supported by the World Health Organization, the US Centers for Disease Control and Prevention, UNICEF, and later the Carter Center, has been in place since 1981. Without a vaccine or medicine that can treat it, it’s being wiped out mainly through educating people to avoid stagnant water sources and providing water filters, which has been hugely successful.

But the parasite is putting up a surprisingly good fight, and it is taking longer than expected. A recent study in The Lancet called the Guinea worm eradication date a “moving target”, with deadlines coming and going. The current target date is 2020, but that is likely to be pushed back again after a grim discovery: In 2016 it was discovered that in Chad – the largest source of guinea worms – the parasite has jumped to include canine and feline hosts, likely through the consumption of parasite-carrying fish and frogs. They cannot pass on the parasite to humans, but they can transfer them to water bodies that humans may frequent. Luckily, the number of cases has remained steady for the last couple of years, and if this can be controlled the elimination program may get back on target.

There are valid arguments against the purposeful eradication of guinea worms, ones that have been debated for decades. When a parasite goes extinct, it opens up a niche in an ecosystem that may be filled by another invasive parasite. This may create new parasite-host encounters, ones we may be completely unprepared for. However, the Guinea worm's reliance on a human host means its ecological significance is low.

Another argument posits we have much to learn from parasites like Guinea worms. There is evidence, for example, they produce morphine to dull their hosts' pain, which may explain how humans can carry such a large parasite without experiencing organ rejection. This could help with preventing organ rejection in transplant patients. If we wipe out the worm, scientists will never be able to study this. However, the effort that has gone into the eradication of this species, which will result in only the second infectious disease ever to be eradicated, appears to deem this a worthy sacrifice.   

The guinea worm may be one of the most endangered species on Earth, but who would mourn its loss?