Are Pufferfish Poisonous? Tetrodotoxin Facts and Real Risks

How poisonous are pufferfish?

Tetrodotoxin, often shortened to TTX, is one of the most potent natural toxins known. In laboratory tests on mice, it was lethal when swallowed at a dose about 25 times smaller than that of potassium cyanide, and when injected, far smaller amounts still were deadly. A single pufferfish liver or set of ovaries can contain enough toxin to kill an adult.

How toxic a pufferfish is depends on many things. Toxicity varies between species, between individual fish, and with the season and the place where the fish was caught. In marine pufferfishes, the toxin is usually concentrated in the liver and ovaries, with smaller amounts in the gut and skin, while freshwater and brackish-water species tend to carry more of it in their skin. The flesh of some species, such as the northern puffer of the US Atlantic coast, is not toxic, while the flesh of others, including the white-spotted puffer, has been reported as highly toxic.

Because the amount of toxin is so unpredictable, there is no way for a nonexpert to tell whether a particular pufferfish is safe.

Where does the toxin come from?

Pufferfish do not make tetrodotoxin themselves. Most evidence shows that it is produced by bacteria, including species of Vibrio and Pseudoalteromonas, and that pufferfish accumulate it through a food chain that starts with these bacteria.

The strongest evidence comes from fish farms. Pufferfish raised on toxin-free food in tanks that wild prey cannot enter grow up nontoxic, while farmed pufferfish fed toxic food build up the toxin in their livers. Wild pufferfish appear to use the toxin as a defense. It makes them a dangerous meal for predators, and in some species even the tiny larvae carry toxin on their skin, which makes predators spit them out. Inflating is a second defense, explained in Why do pufferfish puff up?

How does tetrodotoxin work?

Nerves send signals by letting sodium flow through tiny gates in their cell membranes, called sodium channels. Tetrodotoxin plugs these channels from the outside, so nerve signals stop. Muscles no longer receive orders to contract, and sensation fades.

In people, symptoms usually begin within about 30 minutes of eating a toxic fish, although they can be delayed by a few hours. The first sign is often numbness and tingling of the lips and tongue, followed by dizziness, weakness and spreading paralysis. In severe cases the muscles used for breathing stop working, and a person can remain conscious while unable to move. No antidote has been approved for human use.

Anyone who may have eaten pufferfish and feels numbness or tingling in the mouth should seek emergency medical help immediately: call 911 in the US, 112 in the European Union or 999 in the UK.

Why do people eat fugu?

In Japan, pufferfish called fugu has been eaten for thousands of years, and it is also a delicacy in Korea and China. Because of the danger, its preparation is tightly controlled. Japanese fugu chefs must pass a licensing exam after years of training, and serving the most toxic organ, the liver, in restaurants has been banned in Japan since 1984.

These rules have greatly reduced deaths. In 1958, 176 people died from eating fugu in Japan. Between 1996 and 2006, the country recorded 20–44 poisoning incidents and 0–6 deaths a year, and from 2006 through 2009 there were 119 incidents involving 183 people, of whom seven died. In Tokyo, almost all recorded cases involved people preparing fish they had caught themselves rather than restaurant meals.

In the United States, a small number of licensed restaurants serve tiger puffer that has been cleaned of its toxic organs in Japan before being imported. The European Union bans the sale of pufferfish altogether.

Poisonous pufferfish in Europe and North America

Europeans now have a toxic pufferfish of their own. The silver-cheeked toadfish (Lagocephalus sceleratus), native to the Indian and Pacific Oceans, entered the eastern Mediterranean through the Suez Canal and has spread west as far as the Adriatic Sea and the Strait of Gibraltar. It is extremely poisonous, fatal poisonings have been reported in Egypt, Israel and Turkey, and authorities and scientists in Greece, Croatia and other countries have warned people not to eat or handle it.

In North America, poisonings have come from local puffers. Between 2002 and 2004, people who ate southern puffers caught near Titusville, Florida, fell ill, but the cause turned out to be saxitoxin, a different nerve poison that acts on the same sodium channels. Florida responded by banning the harvest of puffers from the affected waters.

Which animals can resist tetrodotoxin?

Pufferfish survive their own toxin because their sodium channels are built differently. A 2008 study of four pufferfish species found that each had evolved resistance through small changes in all eight sodium channel genes found in fish, and pufferfish also carry proteins that bind the toxin.

Tetrodotoxin is not unique to pufferfish. It is also found in blue-ringed octopuses, some newts, certain snails, flatworms and ribbon worms, and crabs. Some predators have evolved resistance too. The best-known example is the common garter snake in western North America, where some populations can eat rough-skinned newts loaded with the same toxin, as explained in How do garter snakes eat toxic newts? Some sharks and lizardfishes also eat pufferfish, apparently without harm.

Myths and facts about pufferfish poison

Pufferfish are poisonous, not venomous: their toxin harms animals that eat them, but they have no way to inject it. The difference is explained in Is a snake venomous or poisonous? The idea that tetrodotoxin from pufferfish was used to create “zombies” in Haiti became popular in the 1980s, but chemical tests of supposed zombie powders have not supported it. Videos of dolphins handling pufferfish are often said to show dolphins getting “high”, but this has never been tested. What is certain is that pufferfish should never be eaten unless prepared by a licensed expert.