The accidental sale of potentially deadly fugu in Japan has sparked a health scare – and the same poison is now found in European species

Gamagori city in Japan was put on alert this week after toxic fish went on sale in a local supermarket. Pufferfish are considered a delicacy in Japan, often eaten raw as sashimi or cooked in soups. But if the fish are not carefully prepared they can be deadly.

The supermarket in Gamagori failed to remove the liver from the fish before putting them on sale, and unfortunately the liver is one of the organs that can harbour the potent neurotoxin tetrodotoxin. In an effort to recall the potentially poisonous fish sold, loudspeakers across the city have been warning citizens of the danger; at the time of writing, three of the five packs of fish sold had been traced.

The risks of eating pufferfish are widely known in Japan and the preparation and sale of the fish is tightly regulated. Nevertheless, poisonings happen every year but, because the symptoms are recognised, effective treatment can be very successful and fatalities are rare.

Facebook Twitter Pinterest A package of pufferfish containing the potentially deadly liver, top right. Photograph: AP

However, pufferfish are widely distributed in tropical waters – and those consumers and those fishing outside Asian countries aren’t always aware of the dangers. The fish has also invaded new territories in recent years posing new problems, especially for fishermen not familiar with the species. Relatively recently, the fish have arrived in the eastern Mediterranean.

The Suez Canal when it opened in 1869 brought dramatic changes to world trade. It also brought with it invasive species from the Red Sea: 925 non-native species have been recorded in the eastern Mediterranean since the opening of the canal. Migration of species in the opposite direction is relatively rare because the Red Sea is comparatively saltier and nutrient poor. One of the invading species, the silver-cheeked toadfish (Lagocephalus sceleratus), is a type of pufferfish most usually found in the Pacific and Indian oceans. The toadfish appears to be thriving off the Turkish coast and is of particular concern for several reasons.

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The main cause for concern is its economic impact. The silver-cheeked toadfish has strong teeth that it can use to bite through fishing lines and nets to get to the trapped fish. The damage to fishing gear is one thing, but the toadfish will then go on to feast on the abundance of food it finds inside the nets, meaning several kilos of the catch are often unsaleable. The financial losses to fishermen, who are already often on very low incomes, have been increasing in recent years.

The other concern is the toxicity of the fish. Tetrodotoxin is found in several of the silver-cheeked toadfish organs, including the ovaries and liver. This powerful neurotoxin, when pure, can kill an adult human at a dosage of as little as two milligrams. It is far more potent a poison than arsenic or cyanide.

In the body, tetrodotoxin blocks sodium channels that enable nerves to fire. It kills because nerves no longer trigger muscle movement controlling breathing and the victim suffocates. The toxin can also induce heart failure. There is no antidote and treatment is to support breathing artificially until the body excretes the toxin naturally. The treatment is very successful, with many people making a full recovery.

There is no market for pufferfish as a delicacy in Turkey but the fishermen often eat the fish they catch because they can’t sell it elsewhere. A recent survey found that 38% of surveyed fishermen had consumed pufferfish and of that group, 11% had shown symptoms of intoxication. Clearly, messages about the dangers of this fish have not been getting through.

The poison is not produced by the fish itself. Rather, it is a product of environmental bacteria that accumulate in the fish. Pufferfish are not the only species that can accumulate tetrodotoxin in this way. There is evidence that tetrodotoxin-contaminated seafood is spreading northwards – it has even been found off the southern coast of England. In this case it the fish involved wasn’t the silver-cheeked toadfish, as these are likely to remain in the eastern Mediterranean where the water is much warmer. Instead, tetrodotoxin was detected in molluscs – pacific oysters, native oysters, mussels and hard clams.

The studies carried out so far have been small but substantial enough to highlight a potential problem and give hints of the causes and circumstances to be wary of. For example, not all species in all conditions are affected in the same way. Pacific oysters in the summer months from shallow, low saline sites show the highest levels of the toxin. Sea temperatures of at least 15°C seem to be needed to produce detectable levels of the toxin.

Sea temperatures around the UK coastline have already shown an increase of over 0.7°C over the last 30 years. These temperatures are likely to increase, as is rainfall, due to global warming. The speculation is that conditions will become increasingly favourable for the tetrodotoxin-producing bacteria. What is certain is that more studies need to be carried out to understand the nature of any potential problem and identify what can be done.

Educating people about the dangers is certainly one important aspect of poisoning prevention. Screening can also help stop contaminated food from going on sale, but there might be ways of reducing the levels of toxin in the first place. For molluscs this may not be easy, but there are ways of removing the silver-cheeked toadfish from the Mediterranean and reducing the threat of poisoning there.

Fishing for the toadfish and removing it from the environment is one method, but there are few incentives to do this. There is no market to sell the fish for food, so bounties have been proposed for each fish landed. Unfortunately, the amount offered per fish is unlikely to be enough to give financial incentive for Turkish fishermen to catch the species.

Fortunately, there might be another market for this fish: medicine.

Tetrodotoxin, whilst proving extremely dangerous in high doses, may have medical benefits in lower doses. Studies have shown that in some circumstances the toxin may relieve pain, particularly in cancer patients. At a time when we are increasingly looking for alternatives to opiates for pain relief, tetrodotoxin may be one candidate; it has even been shown to reduce the pain of withdrawal from opiates. It may ultimately be the case that this invasive species could provide valuable benefits.