Quick, what’s the deadliest non-human animal in the world? The one that kills the most people, I mean. You might guess something predatory or venomous. Or you might be savvy enough to say the mosquito, the standard answer that I too would give if I were playing Trivial Pursuit. But the truth is more complicated than a factoid. “The mosquito” isn’t a single species, and the skeeters that spread malaria aren’t even in the same genus as the ones that carry, say, West Nile. Besides, surely the cause of death in these cases is not the insect but the malaria parasite or the West Nile virus. Not to downplay the negative effect of mosquitoes, but at best it’s an indirect role.

So if not mosquitoes, then what?

It’s a tricky question, in part because these deaths mostly occur in developing countries with poor record-keeping. Take a look at this infographic (source).

Let’s start at the bottom and go up. After mosquitoes and humans comes “snake”, but that encompasses several reptile families. No single snake genus, be it the saw-scaled vipers, the cobras, the kraits, or the mambas, kills as many as 10,000 people a year. Then we have some more vectors of parasites and viruses. Again, don’t blame the carrier. But “freshwater snail” is different. It doesn’t transmit a virus or a protozoan. It transmits another animal: the blood fluke or schistosome, a flatworm related to those cute planarians. Here’s an animal that kills us directly. It really should be in the image instead of the snail. And 10,000 is a very low estimate; other sources place the number as high as 200,000 deaths per year. Not to mention that a thousand times more folks are infected, often remaining sick for years, without dying from the disease. There are several species of this parasite, but they are all in the same genus (Schistosoma), and most deaths (up to 150,000) are due to a single species, S. haematobium. Quite the assassin, although I’d still prefer to share a bathtub with a single blood fluke than with a single great white shark.

How are all these people getting infected? The parasite hatches from an egg in fresh water and swims to find an aquatic snail, burrowing into its flesh. Inside the snail it divides asexually, and soon the snail is shedding thousands of clones back into the water. These clones seek the skin of a mammal, be it a rat or a human, and penetrate straight in. A person doesn’t have to drink the contaminated water, they just have to wade or swim in it. The flukes seat themselves in the host’s abdominal veins, where males and females mate for life and release eggs that ultimately take the back exit. If you dump your sewage in the same water you will be splashing through later, the life cycle is complete. The adult worms can remain in the host for years, causing a chronic, debilitating illness called schistosomiasis featuring belly-distending inflammation, incapacitating anemia, painful and bloody urination, and other unpleasant symptoms.

This is a fixable problem. It only persists because it primarily affects the poor in the developing world, who are frankly a low priority for the Americans and Europeans with the means to address it. Improved sanitation would be a huge help. Drug treatments are being applied, although this is a double-edged sword since the parasite can easily evolve resistance to the drug. I work on the genetics of the snail immune system, in the hopes that this will lead to strategies for boosting the immunity of wild snails. But for all of these efforts, the funding is partly compared to that for diseases affecting wealthy westerners.

The solution may be the complete elimination of these parasites. That is to say, deliberately driving one or more animal species to extinction. Most of the recent discussion about purposeful extinction has focused on mosquitoes, but blood flukes are another obvious target. Now, this may not even be possible. After all, the worms can hide out in other mammals even if all human infections are cured. And though the prospect of creating resistant snails is exciting, it’s sobering that snails have failed to evolve immunity to flukes for hundreds of millions years. But suppose we could do it. I see a lot of hand-wringing over such ideas. Isn’t every species valuable? Who are we to choose who lives and who dies? As a staunch environmentalist, my conditioned knee-jerk response is to oppose any kind of extinction. And these worms are special. They co-evolved with us and helped to make us who we are. A scientist 500 years from now might reasonably want to study them, so I would vote to keep them in captivity. But if that weren’t possible, would we really axe them all?

In the end, such deliberations are hypocritical. If your child were at risk for schistosomiasis, instead of some kid in Sudan, you would kill as many worms as you needed to. Besides, we are already choosing who lives and dies on a massive scale. I don’t see nearly as much concern over the thousands of other invertebrates that we are wiping out for no good reason. In the case of blood flukes or mosquitoes, it makes us uncomfortable because the slaughter would be premeditated, not accidental. It reminds me of the Trolley Problem, in which you need to murder a bystander with your own hands in order to stop a runaway trolley that would otherwise flatten several victims. Most folks would refuse to push a fellow human onto the tracks, despite the clear utilitarian benefit. This gut reaction is a good thing, because murdering others for the perceived greater good is the kind of thing totalitarian dictators do. But a worm is not a person. The value of a species depends on its ecological interactions with other species around it, something we can measure and assess with a cool head. Like it or not, we run the controls on this planet. We can let nature take its course or we can intervene, but either way we are responsible for the consequences.

I framed the question as the deadliest non-human animal, not the deadliest animal. That’s no fluke, as it were. We hold each other’s fragile lives in our hands, and though we are often careless with them, we need not be. The blood flukes aren’t driving this train. They ride aboard us. We decide which track to follow.