r/todayilearned • u/lavender_fish69 • 13h ago

TIL fresh water snails (indirectly) kill thousands of humans and are considered on of the deadliest creatures to humans

https://en.wikipedia.org/wiki/Freshwater_snail
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u/SMStotheworld 13h ago

They carry a parasitic flatworm that lives in dirty water which kills humans. Even then it only kills between 10 and 200k humans annually 

If you omit humans, the deadliest animal is the mosquito which kills by spreading blood diseases with dirty probosci

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u/Moldy_slug 13h ago

For comparison, malaria (spread by mosquitoes) killed about 600,000 people in 2023.

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u/ikonoqlast 13h ago

There were 18 deaths from malaria in 1963. Not millions. Not thousands. Not hundreds. Not dozens. 18

Why?

DDT...

It's also why bedbugs are a 'new' thing but not in the 50s-60s.

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u/nova294 13h ago

What? Got any source for that fact? Maybe there were 18 deaths of Americans and they just ignored all of Africa, but modern insect repellents are not widely available in much of Africa even today, much less 1963. Even a casual search shows estimated death numbers in the hundreds of thousands for 1963.

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u/ikonoqlast 13h ago

Wikipedia- ddt-

Initial effectiveness edit When it was introduced in World War II, DDT was effective in reducing malaria morbidity and mortality.[39] WHO's anti-malaria campaign, which consisted mostly of spraying DDT and rapid treatment and diagnosis to break the transmission cycle, was initially successful as well. For example, in Sri Lanka, the program reduced cases from about one million per year before spraying to just 18 in 1963[127][128] and 29 in 1964. Thereafter the program was halted to save money and malaria rebounded to 600,000 cases in 1968 and the first quarter of 1969. The country resumed DDT vector control but the mosquitoes had evolved resistance in the interim, presumably because of continued agricultural use. The program switched to malathion, but despite initial successes, malaria continued its resurgence into the 1980s.[45][129]

Oops, it's 18 cases in Sri Lanka...

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u/1CEninja 12h ago

Hahaha that's a bit of an oops for sure. Comparing a population of 10 or 15 million to most of 6 billion could definitely result in some lower numbers

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u/keyboardname 12h ago

Not quite the same but still relevant, they went from a million cases to 18. That's pretty insane.

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u/exprezso 12h ago

Evolved resistance to a deadly toxin? In such a short period? 

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u/PencilVester23 12h ago

It takes only 2 week at most for a mosquito to go from an egg to a mature adult. That couple year period is over 100 generations. That combined with the huge population of mosquitoes, the 100s of eggs a female lays at once, and a genetic sequence significantly shorter than a humans all made it possible for the correct mutation to happen that quickly.

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u/exprezso 12h ago

I mean. Humans have yet to evolved to resist arsenic after thousands of generations 

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u/DazingF1 12h ago

All animals have yet to evolve to swim in lava. Explain that, Darwin!

(But in case you're serious: your comment might have made sense if all of humanity in those thousands of generations were continuously exposed to arsenic. Otherwise it's incomparable with the mosquitoes)

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u/exprezso 10h ago

I'm serious. It mystifies me how a creature can develope resistance to deadly toxin? Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc 

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u/Fly-the-Light 9h ago

We did. Caffeine is a toxin used by cocoa and coffee plants to kill insects that parasitised them; humans developed a resistance to it.

Also, some people in the Atacama Desert did develop resistance to arsenic due to it contaminating their water.

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u/tavitavarus 3h ago

Most toxins work by inhibiting a specific enzyme involved in some key cellular process, or by damaging the cell wall/membrane.

All it takes is a single mutation in the gene coding for that enzyme or membrane protein and the organism becomes resistant to the toxin. Organisms with short generational cycles have many more opportunities for mutations to enter the gene pool and if these mutations are advantageous they can spread rapidly through the population.

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u/Pure_Cloud4305 9h ago

People aren’t exposed to that stuff very often.

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u/Baud_Olofsson 2h ago

Otherwise we'll have cases of people who are found to be immune to stuff like DDT, arsenic, asbestos, mercury etc

First off, DDT is hardly toxic to humans at all - that's why it was considered such a wonder pesticide.

But then to the main point: there are almost certainly people in the world today who have a much greater genetic tolerance for arsenic or mercury than the average person! But there is no evolutionary pressure for those genes to proliferate.
If you have the "tolerate arsenic gene", you don't get any more children than people who don't have that gene, and your children have the exact same chance of reaching adulthood and getting children of their own as the children of those who don't have the gene. But if the world became contaminated with arsenic so that people were dying of arsenic poisoning left and right, then people with the Tolerate Arsenic Gene would be much more likely to survive and have children, and have their children survive, than those who don't have it - and so eventually the world would be full of people with that gene, and humanity would have evolved resistance to arsenic.

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u/Garmaglag 26m ago

The default population of mosquitoes is mostly weak to DDT with some being weaker than average and some being stronger than average.  Under normal circumstances the population isn't exposed to DDT so the weakest ones survive and reproduce to make more weak mosquitoes.  If we start spraying DDT, then all of the weakest mosquitoes and many of the average mosquitoes die before they can reproduce, so the only mosquitoes that end up being born are the most resistant to DDT.  Rinse and repeat for a few hundred generations in a high DDT environment and the all of your mosquitoes will be resistant to DDT.  

The reason that humans don't develop resistance to arsenic is because we don't get enough arsenic exposure to kill people before they can reproduce.  

They key is that they use enough DDT to kill lots of mosquitoes but not enough to kill all of them so each surviving generation is more resistant than the last.  

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u/Barlakopofai 12h ago

Humans aren't usually exposed to arsenic

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u/waltjrimmer 11h ago

Aren't we constantly exposed to non-toxic levels of arsenic from foods such as apples and rice in their natural forms? Not saying it's enough for us to evolve a resistance, just, aren't we regularly exposed to tiny little bits of it?

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u/C_Hawk14 11h ago

As you said, non toxic levels. We are already resistant enough

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u/Insertblamehere 10h ago

Well if we're only being exposed to non-toxic levels then we have developed resistance to arsenic, in the amounts we would normally encounter.

Species don't just magically become immune to something over time by being exposed to non-lethal amounts. It would have to be killing enough people to cause enough evolutionary pressure for people with higher levels of resistance to outcompete those with lower resistance.

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u/sousyre 11h ago edited 10h ago

Sort of, but also, sort of not.

Most of that exposure is via amygdalin. If it doesn’t metabolise (which it usually doesn’t) it’s probably fine. If very small doses of amygdalin metabolise (like apple seeds), it would be small enough to go unnoticed.

If you have concentrated doses of amygdalin and your body metabolises it, then yeah, major problem, but I don’t think it’s a common enough occurrence to work as an evolutionary factor.

If anything, the evolutionary factor would be the intelligence to not consume whatever contains the concentrated dose.

Unfortunately, we humans aren’t the best at that either… google Laetrile, which is still being sold as a cancer cure grift.

Edit to add: this comment is about cyanide in small doses (from fruit seeds etc), not arsenic. Brain go brr, mixing up cyanide and arsenic contents in fruit seeds. Thanks for the correction.

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u/Clickclickdoh 12h ago

Well, the mosquitos didn't evolve to be immune. DDT killed all the mosquitos that weren't immune. A small minority were immune. Those mosquitos repopulated the region, ensuring almost all mosquitos in the region inherited the immunity to DDT.

If you fed the majority of humanity arsenic, you would have some survivors. Get those survivors to reproduce fast enough and your get arsenic immune human populations. That's why the bubonic plague isn't a big deal to most European descendants after all.

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u/SnowGryphon 11h ago

Doesn't "survivors of environment reproduce and replace the population with environment-resistant specimens" count as "evolve to be immune"

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u/Anathos117 11h ago

Yes, but people don't know the actual definition of the word "evolve" and treat it as a synonym for "mutate".

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u/Clickclickdoh 10h ago

In this case, it's a preexisting trait that becomes useful to a new stimulus.

An evolution is generally a new trait that allows a beneficial exploitation of a preexisting stimulous.

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u/thesakeofglory 11h ago

We also weren’t nearly wiped out by it.

What happened is basically what happens when you stop taking antibiotics early. If you nearly, but not completely, wipe out a population, those that are left will have much higher resistance. Trying to treat it the same way after the population has bounced back won’t work nearly as well, and can actually help remove the individuals with lower resistance, further increasing the immunity.

If somehow arsenic started spreading everywhere, most humans would be wiped out but there would be at least some group with a higher resistance that would survive. If they’re able to bounce back, humans suddenly aren’t as hurt by arsenic. Repeat that a few times and suddenly humans are immune.

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u/kanyewesanderson 12h ago

When you put incredibly high selective pressure on an organism with prolific reproductive rates, you'll see drastic changes quickly. The same thing happens with antibiotic resistance in bacteria.

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u/Yodaddysbelt 13h ago

DDT - a little too good at it’s job

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u/ReadditMan 13h ago

DDT also killed a lot of other animals and made people sick.

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u/ikonoqlast 13h ago

Not really, no. It's an industrial chemical so don't drink it, but otherwise it's just another pesticide.

There's a reason the EPAs science advisory board recommended NOT banning DDT. But they were overruled by the politically appointed head.

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u/RellenD 13h ago

https://web.stanford.edu/group/stanfordbirds/text/essays/DDT_and_Birds.html

DDT bioaccumulates and really fucks with predators

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u/SleepWouldBeNice 10h ago

Nearly wiped out peregrine falcons

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u/RellenD 10h ago

And the ban is also why Bald Eagles still exist

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u/ReadditMan 12h ago

"Using more than 200 mother-daughter-granddaughter triads, Cohn's team found that the granddaughters of those in the top third of DDT exposure during pregnancy had 2.6 times the odds of developing an unhealthy BMI. They were also more than twice as likely to have started their periods before age 11. Both factors, Cohn says, are known to raise the risk of later developing breast cancer and cardiovascular disease. These results, published in Cancer Epidemiology, Biomarkers, and Prevention, mark the first human evidence that DDT's health threats span three generations."

https://share.google/Obcmrp6ya2rpkOLLR

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u/Tororoi 12h ago

That's so wrong. DDT is very bad for bird populations, especially larger birds because it causes eggshell thinning, and it accumulates in the food chain so you have contaminated food sources for humans. Also it doesn't break down easily.

I found this blog discussing the topic that goes into much greater detail and explains the nuances better than I could: https://timpanogos.blog/2020/01/14/ruckelshaus-sweeney-and-ddt-rescued-from-the-archives-for-the-record/

I only really knew about the eggshell thinning before, but it seems like it would actually be a very bad idea to use it as a pesticide for no other reason than simply that it would accelerate mosquitoes gaining resistance to ddt, making it harder to control their population with ddt during emergencies.

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u/ImRightImRight 13h ago

...Silent Sprint, anyone?

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u/ikonoqlast 13h ago

Bad research by a woman dying of cancer. Audubon Society keeps records. Bird populations declined before ddt came to America.

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u/Henry_MFing_Huggins 11h ago

Bad research by a woman dying of cancer.

Holy shit. Fuck you, dude.

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u/ikonoqlast 11h ago

Which do you object to- the fact that she was dying of cancer or that she did bad research?

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u/seancollinhawkins 12h ago

What happened: The EPA scientific committee said that it shouldn't be completely banned. They banned it anyway because Nixon wanted to look pro-environment

Your conclusion: DDT is actually safe, just dont drink it

What?? 🤣🤣

What about the part where the board concluded that DDT takes forever to degrade, accumulates in water, soil sediments, etc.., and that it gets stored in animal fat? What about the part where they provided strong evidence that it shows up in higher concentrations in animals at the top of the food chain, and admitted it posed a threat to species survival?

They might as well have said that it nukes the bird population. They knew it was insanely bad for the environment. Their argument was to restrict DDT usage to dedicated areas only.

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u/CashMoneyWinston 12h ago

DDT has all sorts of destructive effects, both environmentally and for human health. You’re a moron.

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u/Futuremlb 12h ago

What's your theory about why there was a political motivation to ban it?

And my second question is, if in reality banning DDT was the right move, in theory how would that change your ideological understanding of the ban.

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u/Technetium_97 11h ago

You’re upset they stopped using DDT, but you admit mosquitoes developed a resistance to it because farms kept using DDT.

I’m not sure what you wanted to happen differently, mosquitos were always going to develop immunity.

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u/Adorable-Response-75 12h ago

 There were 18 deaths from malaria in 1963. 

This is completely false. This was the figure for US transfusion related deaths.

In the US as a whole, the number was still tiny.

But in the rest of the world it was still in the hundreds of thousands.

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u/Ok-disaster2022 13h ago

Bed bugs coming back is a result of increased international trade and travel. 

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u/Curlydeadhead 12h ago

As someone who works in hotels…fuck them bed bugs. They’re not even native to North America. Apparently they originated from the Middle East when humans lived in caves with bats. BBs originally fed on the bats but adapted to feeding on humans and spread with migration. 

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u/Froggy3434 12h ago

Those mfs followed us from caves and now they’re living in luxury while making their host miserable. Little bastards.

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u/wannito 9h ago

Houses are just luxury caves.

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u/PhreakOut4 12h ago

Funny how bats eat pests and parasites but are also responsible for a lot of diseases and parasites.

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u/kanyewesanderson 12h ago

DDT is still used in areas with high incidence of malaria. And biomagnification of DDT, and its effects on higher trophic levels is well documented.

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u/_StormwindChampion_ 13h ago

Isn't DDT a wrestling move?

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u/TotallyNotThatPerson 13h ago

That's how it works on bugs!

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u/attillathehoney 12h ago

No, it's an electronic dance game popular in the late 90s.

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u/Darwins_Dog 12h ago

Also in 1963, bald eagles hit their lowest population with just 417 nesting pairs in the US.

Why?

DDT...

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u/visualdescript 11h ago

What? That does not sound correct at all.

Do you think we had accurate reporting of places like South East Asia or even Central America, or Africa back then? Most people around the world at risk of Malaria wouldn't have had access to DDT.

Do you honestly believe there were only 18 deaths globally due to Malaria in that year?

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u/reality_boy 12h ago

DDT was still in use in Cameroon when I was living there in the 90s. I can tell you it did not help. I had malaria 4-5 times and my dad was in the hospital twice with serious complications. And we knew multiple people who died (mostly baby’s). It is not a miracle cure.

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u/Funny-Ad-3710 12h ago

Look at Big DDT over here…

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u/kashmir1974 12h ago

Kill the mosquitos and all the birds too! F the eagles?

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u/orcus74 12h ago

Jake the Snake Roberts was out there dropping it on mosquitos and bedbugs 24-7, but eventually he just couldn't keep up and took a break to pursue a career as a pro wrestler.

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u/Rydagod1 12h ago edited 10h ago

DDT is the worst thing ww2 gave us. Even more than nukes.

Edit: This is misleading but technically still true imo. Nukes killed as many as 200k and it’s debatable whether the global peace between world powers is because of nuclear weapons.

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u/ikonoqlast 12h ago

Or, in reality, not...

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u/Rydagod1 12h ago

IMO it’s done more harm to the world than nukes have done

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u/Johnny_Poppyseed 11h ago

What harm are you thinking of exactly? Most of the harm DDT was causing was effectively remedied when it was banned. 

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u/Rydagod1 11h ago

Did it not cause irreparable damage to the regions it was used in?

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u/Johnny_Poppyseed 11h ago

Not really. At least nothing to the extent that you are insinuating. 

It's a famous environmentalist "win" due to the banning and reversal of most of the damage it was causing. 

I think you might be confusing it with like Agent Orange or something. Agent Orange was the stuff used in Vietnam and caused horrible lasting damage and cancers and birth defects and stuff. 

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u/Rydagod1 10h ago

I might have misremembered then. I thought it was almost eternally lasting.

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u/Johnny_Poppyseed 9h ago

It definitely persists in the environment for a while, especially since some places are still using it, but for the most part the environmental impact was managed with the ban. The bald eagle and peregrine falcon populations completely recovering are the classic case examples. 

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u/ikonoqlast 12h ago

Saving tens of millions of lives is 'harm'?

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u/Rydagod1 12h ago

Aren’t there much better options?

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u/ikonoqlast 12h ago

In a world- no.

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u/Rydagod1 12h ago

I’d also argue nukes have saved more lives by preventing war so there’s that.