Toxicity in Shellfish | What is Shellfish Poisoning?

If you live by the coast, clusters of mussels or oysters growing by the shore are probably a common sight. So what's stopping us from picking them ourselves instead of paying top dollar at markets? Well, eat the wrong one, and it could just kill you.

With species able to thrive across a wide range of conditions, shellfish are a sturdy staple consumed around the world.¹ And while a bad-smelling oyster is generally enough to make us pause and think twice, the ones that don’t smell can be just as harmful. For the most part, the danger is not the fault of the shellfish itself or even the bacteria inside it – in fact, almost all bivalves are edible.² The real culprits are invisible to the naked eye, noses and taste buds – potentially fatal toxins that no amount of cooking, steaming or boiling can remove.

Shellfish filter pollutants

Shellfish are ‘filter feeders’, meaning they filter vast volumes of water – up to 100L per day - to extract their food.³ This filtration can be incredibly effective at removing pollutants and metals from the water, and we even use it as a natural way of purifying waterways polluted by pesticides, agricultural run-off or industrial chemicals.⁴

But these pollutants don’t simply disappear. Instead, shellfish accumulate pesticides, metals, hydrocarbons, and anything else in the water around them, building up higher concentrations of contaminants in their body tissue the more they are exposed. In essence, if you eat shellfish anywhere near drainage areas, you are likely swallowing a concentrated dose of whatever chemicals exist in that water. In the unambiguous words of Achill Oysters Founder Hugh O’Malley, 'picking a wild untested oyster is a massive, massive gamble’.

Mussels can be up to 1000 times more sensitive to hydrocarbons in the water than the most advanced industrial sensors. This makes them incredibly effective bioindicators for detecting changes in ocean conditions. Some oil companies are even considering using mussels as an early warning system for oil leaks, while salmon farmers are currently researching their use as early alarms for incoming harmful algae!^5,6 

Causes of shellfish poisoning 

So what are these pollutants or toxins, and how serious can they be for your health? In the case of shellfish exposed to pollutants or harmful bacteria such as E. coli, the risk to our health is largely dependent on the amount of shellfish you eat: the more contaminated shellfish you eat, the more likely you are to increase your chances of falling ill. Howvever, even one batch of heavily loaded shellfish could lead to food poisoning.

The guilty party responsible for the majority of serious and potentially fatal cases of shellfish poisoning are microscopic ‘dinoflagellates’.⁸ While shellfish are largely unaffected, dinoflagellates produce toxins that can be perilous to both humans and fish, capable of killing a fully grown adult if ingested. Although shellfish will eventually purge these toxins, they can exist in harmful concentrations within the tissue for years following exposure.⁹ 

Risks: Be mindful of where your shellfish is sourced. If you eat mildly contaminated shellfish regularly over a long period, there could also be delayed and far more serious ramifications for your future health. Cumulative effects of consuming even low concentrations of heavy metals can lead to host of other health issues.⁷

What happens if you get shellfish poisoning?

So what happens if you eat an oyster containing these toxins? Depending on the species of dinoflagellates that shellfish consumed, there are three main types of poisoning you could be looking at.¹⁰ Spoiler - None of them are fun.

1. Paralytic Shellfish Poisoning (PSP)

Occurring rapidly after the meal, PSP will leave you nauseous, vomiting, and lightheaded. Within a few hours, you could face muscular paralysis that can progress to respiratory arrest and even death. 

2. Neurotoxic Shellfish Poisoning (NSP)

A few hours after ingestion, expect gastrointestinal (nausea, vomiting, cramps) and neurological (paresthesia, vertigo, hot-cold temperature reversal, loss of coordination) issues that can last weeks or even months. 

3. Amnesic Shellfish Poisoning (ASP)

Symptoms of ASP are characterised by nausea and vomiting, followed by a complex neurological syndrome (characterised by memory problems, confusion and physical weakness) with potentially permanent neurological repercussions.

How can farmers help prevent shellfish poisoning?

Buying shellfish in the shops is far safer than harvesting them wild. Europe has strict regulations to ensure shellfish is safe before it reaches the market: since 2006, standards have regulated the levels of micro-biological agents, phycotoxins, and some chemical contaminants allowed in shellfish. These regulations have been compiled into the "Hygiene Package" and broadly stipulate that no shellfish can legally be sold before being tested and meeting scientifically recommended maximum levels of contaminants.¹¹

Aside from regular testing by environmental regulators, there are a few key ways in which farm managers can reduce the risk of contaminated shellfish reaching consumers:

1. Depuration

Depuration is a process where harvested shellfish are placed in a clean ‘holding tank’ for hours or days before being sold. By controlling conditions like temperature and salinity of the holding tanks, shellfish are encouraged to expel contaminants and faeces.¹² Although effective for removing faecal contaminants like Salmonella typhi - a bacterium responsible for shellfish-associated Typhoid outbreaks in Europe and the US - it is still not a recognised solution for other contaminants like biotoxins or heavy metals.

2. Site selection

Choosing a water site is critical for farmers hoping to avoid losses in sales due to contamination. Farmers look for high flow waters, which are less susceptible to toxic algae build ups, and generally aim to remain at a recommended distance of 10km from possible sources of contamination (drainage outlets, harbours, industrial areas) to reduce the risk of contamination.¹³  

3. Temporary farm closures

Short-term closures may happen based on negative results from toxicity testing or educated predictions based on key environmental factors suggesting a higher likelihood for increased contaminant load during a certain time.¹⁴ If contaminant loads are high, closures ensure shellfish are not sold until levels have returned to meet safety standards and shellfish have purged contaminants.

Learn more about why shellfish farming can be highly sustainable.

Is it ever safe to harvest wild shellfish?

There are no safety guarantees when harvesting your own wild shellfish. But if you do choose to roll the dice, there are a few ways you can potentially minimise your risk of falling ill. 

1.  Check your government guidelines about recent farm closures and water quality

Most governments have a regulatory department where you can see recent or even live reports of your coastline’s water quality, and which areas pose a risk of contaminated shellfish. You can also check for Apps, like Norway’s ‘Blåskell’, which gives you a quick overview of waterways you should avoid harvesting from based on recent environmental tests.

2. Choose your season

Although shellfish can technically be harvested year-round, both concentrations of chemical contaminants and toxic algae in bivalves are known to fluctuate with the seasons.³ Warmer summer months may support a higher likelihood of toxic algae presence, while months of heavy rainfall can mean a higher risk of chemical contamination near run-off or drainage areas. 

3. Avoid eating raw

Although algae-based toxins will not be fully removed by cooking, you can significantly reduce your chances of contracting a number of other bacterial illnesses by avoiding raw or undercooked shellfish.²