Why Snails Are Deadlier Than Crocodiles and Sharks

It’s true that data on deaths by animal is spotty, but the slow-moving slimy snail presents a hazard that mega-predators simply do not

Ruth Schuster
Ruth Schuster
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'You're toast.'
'You're toast.'Credit: Kurit afshen/Shutterstock.com
Ruth Schuster
Ruth Schuster

Which are the most dangerous animals in the world – to us, that is? Hats off to the mosquito. Its bite is nothing compared with that of an alligator, for instance, but hundreds of thousands of people die each year from mosquito-borne diseases such as malaria and dengue.

Second on the list is the snake and third is the dog. And then at number four we have the freshwater snail, totally humiliating mega-predators such as the crocodile (eighth), lion (12th) and shark (14th).

The list of deadliest animals in 2018 was published by the Statista website and publicized by AllAboutCats.com, presumably to stress the relative benignity of the lion, which killed only 22 people that year. Fear not the king of beasts.

In the case of the mosquito and snail, the cause of death is third-party microbes. Neither the insect nor the mollusk can rip out your jugular, but they count as dangerous animals because direct contact between you and them is necessary for your death to ensue.

This is not so in the case of the unknown animal in which the coronavirus allegedly originated. The virus is suspected (but not proven!) to have originated in an animal such as a pangolin or bat that was eaten in China. When was the last time you personally physically encountered a pangolin or bat, whether in nature or on a plate?

Before continuing, some caveats. The numbers mislead in the sense that the big animals capable of attacking us in ways we notice are dying out. If lions, other big cats and wolves, sharks and giant reptiles weren’t in so much trouble, they might have achieved many more fatalities.

Also, data on deaths by animal is spotty. Not every hippo or shark encounter is recorded, but you can bet that every dog fatality is (“Toddler mauled to death in horrific dog attack” appeared just last Saturday, as did a story of a lady in northern India who was killed by a pitbull).

Also, jellyfish are notably absent from Statista’s dangerous-animal list – but again, the statistics out there are unreliable. People encountering deadly jellyfish in the sea may drown and their cause of death remains unrecognized. Scientists estimate that jellyfish kill hundreds of people a year. In 2019, The Washington Post even suggested that people planning seaside vacations factor in jellyfish forecasts.

So: the point isn’t that mosquitoes suck or that you should flee screaming at the sight of a snail. The point is that if you don’t provoke them and stay out of their mouths, giant reptiles, sharks and their toothy ilk will generally leave you alone. According to the Florida Museum’s watch on global shark fatalities, there were only six in 2018 and 11 in 2021. So one could shriek that they doubled (almost) or one could get a grip and decide that both are very small numbers when considering that millions upon millions of people swim in the ocean every year.

“Sharks don’t want to eat you,” Israeli shark expert Dr. Ziv Zemah Shamir of the University of Haifa reassures: their natural food is fish, not terrestrial mammals.

A freshwater snailCredit: Shrimplake/Shutterstock.com

Beware the snail

Unfortunately, we are the natural food of certain mosquitoes, and it is when certain mosquito species penetrate our skin with their mouthparts that they may infect us with microbes in their saliva – an extraordinarily complex secretion chock-full of proteins (some of which remain a mystery to science).

But what is the deal with the freshwater snail? It couldn’t prey on us even if it wanted to because it couldn’t catch us and has no teeth in the traditional sense, only a radula, like a rasp down its little throat with thousands of micro-teeth.

The problem is when the freshwater snails carry the helminth (worm) causing schistosomiasis, aka bilharzia or “snail fever.”

Several species of snail can carry the disease, and there are even several types of bilharzia parasite. Bilharzia is a chronic disease, but it can and does turn deadly. Estimates of deaths from it range from about 20,000 to 200,000 a year.

How does this work? It’s a cycle, so its starting point is arbitrary. But let’s start with an infected person relieving themselves in freshwater – a lake or river or even a puddle – and releasing eggs of the worm in their urine or feces. In the water, the eggs hatch and larvae ensue and infect snails. The larvae reproduce inside the snail.

Inquiring minds want to know if becoming an ambulatory incubator for bilharzia larvae hurts the snail. Well, they’re not talking, but one small study found that prawns were more likely to eat bilharzia-infected snails than noninfected ones. Why is not clear, but infected snails seemed to have a depressed reaction to the predation threat.

Freshwater snails carry the helminth (worm) causing schistosomiasis, aka bilharzia or “snail fever.”Credit: SHS.photography/Shutterstock.com

Back to your innards. Let us be clear that the bilharzia worm has two stages of reproduction in its life cycle: one through cloning, and one through sex.

Inside the snail, the larvae reproduce asexually, rendering the snail more likely to be eaten itself, as we saw. Such is life. Uneaten infected snails release the larvae clones into the water.

This is where you come in. Step into the water, swim in it, wash clothes in it and the larvae may penetrate your skin and enter the bloodstream, travel around and infect your lungs, liver and more.

Inside you, they mature to adulthood. Not all helminths even have male and female forms, but this one does – and your innards are where they mate and produce eggs. Their eggs pass through the membranes of the bladder and intestines for you to secrete out into the lake or wherever, and thus the cycle of bilharzia continues.

In any case, bilharzia does not pass person-to-person; the whole cycle with the snail must be involved.

What to do? Those growing up in bilharzia territory were long advised to avoid standing freshwater where the snails might hang out. They do not hang out in rushing river water. In any case, these days the condition is usually treatable, but medical authorities note that one must complete the medication course because it works best on adult helminths, and your system would likely be hosting young ones as well.

If bilharzia is treatable, why are the snail and its disease on board so dangerous as to shame the crocodile? For one thing, they are widespread and not everybody knows they have it or seeks treatment, can afford treatment or gets treated successfully.

Bilharzia has been identified in Africa, where it is most common, but also in Asia and South America, and cases have been noted in the Caribbean and Yemen. The World Health Organization estimates that around a quarter-billion people have it at any given time.

And at least in history, it may have been prevalent in the Middle East.
Evidence in ancient Egyptian and Assyrian medical texts report symptoms that sound like schistosomiasis, and the infection has been detected in several Egyptian mummies. In fact, archaeologists first noticed this in 1910. The disease still exists in Egypt, but at least it’s become rare in the lands whence the great armies of Assyria spread throughout the region spreading terror and, possibly, parasites.

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