Puffer-fish poisoning – Poison, poison… Tasty fish!

As I’ve said before, the definition of disease is fairly open and, in a sense, poisoning is a disease. Does it really matter whether a toxin is produced by something that’s in the body as opposed to outside the body? Perhaps. I don’t know. I don’t really care. Here’s this week’s article.

For a long time, the Japanese have known that people who ate puffer-fish liver would, on a somewhat regular basis, become paralysed, stop breathing and die. This became such a widespread problem that Japan banned the sale of puffer-fish liver in restaurants and markets in 1983. Despite this, people have not stopped eating it. In fact from 1997 to 2007, 470 people were admitted into hospital for puffer-fish poisoning, 30 of whom died. People eat these livers thinking they have special ways of neutralising the toxin. Experimentally (as seen as above) these ways do not work.

OK, Japan. Come here a minute, I just want to talk to you. Now I enjoy food. I’ll eat deliciously deep-fried food that will sever a good couple of months off my life. I’ll go on the occasional bender and increase my risk of cirrhosis. Hell, I’ve eaten at McDonald’s when it wasn’t even necessary. But really, Japan, there is no fish innard tasty enough to warrant risking dying a slow, frightening death by suffocation.

Junk food and alcohol. Probably still not as bad as puffer-fish liver... maybe. (Picture taken by uhh... someone, I don't remember)

I want my fugu!

So scientists long knew that puffer-fish (fish of the Takifugu family) contained this toxin called tetrodotoxin (TTX). TTX stops nerves and muscles cells from conducting electricity by blocking sodium channels in the cell membrane. It’s like putting a plastic cap over an electrical terminal. Once you stop the flow of charge, you have no more electricity. This stops nerve action and you get paralysis.

Also, this plastic cap is shaped so that it only fits particular electrical terminals. Heart muscles have slightly different sodium channels that TTX can’t block. So, TTX-poisoned patients don’t die from a simple heart attack (which is what would happen if they were blocked); instead they are fully conscious as their diaphragm is slowly paralysed and they cannot draw more breath. But, if you don’t die, you eventually recover as your nerve cells produce more sodium channels so that current can flow and muscles can work again.

TTX is generally concentrated within the puffer-fish’s liver, ovaries/testes, skin and intestine. As long as you don’t eat these bits of the fish, you’ll be fine. The meat is OK to eat and apparently get a nice tingle on your tongue due to the low concentrations of TTX blocking the nerves on your tongue.

All this was well and good. Scientists thought that puffer-fish just make this toxin and this stopped animals from eating them, giving them an evolutionary advantage. It all fit in with their model. But when they looked closer, scientists found that not all puffer-fish species would produce TTX. Even weirder, fish of the same species in one area would make a lot of TTX, but in another area they would not.

[Side note: This may have led people to believe their traditional ways of neutralising toxins were working; they may not have been ingesting any TTX at all]

Curiouser and curiouser

In 1964, other scientists found TTX in the eggs of the California newt. Soon, they were finding TTX all over the place; in flatworms, in blue-ringed octopus, in starfish, in frogs, in crabs, in sea snails. It was a puzzle, only certain species in completely different families made TTX.

Maybe they all came up with a common defence against the common problem of being eaten. However, TTX as a molecule is pretty damn complicated. It’s hard to synthesise if you don’t have the prerequisite materials, and these animals didn’t seem to; at least, not any more than their TTX-free relatives.

So maybe it was from the environment. A simple experiment showed that this was the answer. A group took 6000 puffer-fish from farms. All farm-raised fish were TTX-free. They were kept in tanks, fed a normal diet and were still TTX-free. Then, they started feeding them bits of liver from wild puffer-fish, which contained low levels of TTX. Amazingly, these fish started concentrating TTX in their organs up to highly toxic levels. Other toxins were also fed to these fish, but they weren’t concentrated to the same level. Non-puffer-fish were also fed the same diet of liver, but not only excluded but also detoxified TTX. This conclusively showed that puffer-fish can selectively concentrate TTX from the environment.

But where was it coming from? A whole lot of studies looked at different bacteria dredged up from the sea and it turns out a lot of bacteria from different families all produced TTX. This is not all that surprising because bacteria (even of different species) have sex all the time and exchange bits of DNA. They were only produced in small amounts in these bacteria, but a phenomenon called bioamplification occurred.

Bioamplification in an ecosystem happens when there’s a substance that is efficiently absorbed by organism, but only slowly excreted. OK, so the TTX-making bacteria only make a little bit of TTX, but planktons eat a lot of these bacteria. So planktons have a higher concentration of TTX in them. In turn, various worms and shrimps eat a lot of plankton. TTX concentration is higher again in these animals. If you go up the food chain up to the puffer-fish, you’ve now got a metric butt-ton of the amount of TTX you started with. One more step to humans and you’ve got a toxic amount. This same mechanism of bioamplification has in the past caused toxic amounts of mercury to build up in sharks and other fish.

So there is the story of puffer-fish toxin. And, as with most of the Disease of the Week articles, the moral is: Don’t be an idiot. Stop eating poisonous things.

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11 Comments

Filed under Thomas' Corner

11 responses to “Puffer-fish poisoning – Poison, poison… Tasty fish!

  1. Chris

    Fantastic article Thomas! Does this mean that Japanese people would do well to avoid whale meat also, as certain whales eat plankton too and presumably would contain even larger amounts of TTX?

    • thomastu

      Interesting question, Chris. I haven’t heard anything about any TTX poisonings from eating whale. Most likely it wouldn’t be a problem though. As I said in the article, only some animals store TTX in their bodies, even though many closely related species do not. The reason why some animals do and others don’t, I don’t know.

      Also, as I understand it, only the whale blubber and meat is eaten, while TTX seems to localise in the innards (mostly the liver). This makes it unlikely that, even if whales did store TTX in their bodies, people eating whale would not really be affected.

      TT

      • Other aniamls are known which concentrate toxins from their food as an anti-predator defence. I remember a National Geographic article about passion vine butterflies, which have a lot of cyanide in them. Dendrobatid (poison-arrow) frogs also get their toxin from ants so the ones raised in captivity on a diet of fruit flies and crickets aren’t poisonous. I don’t recall reading about any mammals that do that though.

        I am not familiar with the details but you mention that other toxins fed to puffer-fish are not concentrated to the levels that TTX is. If they’re actively transporting it into those organs, doesn’t it mean that it’s not the same phenomenon as the other thing you mention, bioaccumulation of mercury in sharks? Eating large predator fish and whale blubber can be somewhat hazardous for the latter reason, heavy metals and some organic pollutants accumulate in the fat.

      • Cheers for those other examples!

        I think that it is still bioaccumulation of TTX because it is concentrated up the food chain. It occurs whenever there is a slower excretion/breakdown of a substance compared to intake. Just because other toxins are either broken down or excreted at a faster rate doesn’t mean TTX isn’t bioaccumulating.

        TT

      • A recent article outlining how some birds can poison people by the same mechanism, if any of you are still interested – http://scienceblogs.com/tetrapodzoology/2010/06/death_by_toxic_goose.php

        TT

  2. I’d say the ability to concentrate small amounts of TTX gives pufferfish an evolutionary advantage! Plus it saves them the hassle of making it themselves.

  3. Pingback: Cholera – Love in the time of rice-water stool and high infant mortality « Disease of the Week!

  4. Briana Ferguson

    Can you site your article for me, so I can use some of the information in my research paper?
    Send it to my e’mail? Thanks

  5. Pingback: Might get a figure 8 puffer...

  6. joshua

    i thout it killed you and you ar not really dead you are just shut down and come back to life?

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