What is Shellfish Toxicity?
Oysters, clams, mussels, scallops, and, on rare occasions, certain crustaceans, snails, and fish, might be tainted by toxins from microscopic sea algae known as dinoflagellates or cyanobacteria. When people eat these shellfish, they can get shellfish poisoning, which can cause problems with the nervous system and upset stomachs. The toxins attack important parts of our bodies’ cells, like ion channels, kainate receptors, and protein phosphatases, that are necessary for nerves and muscles to work properly. When they disrupt these parts, it causes the symptoms we see in patients with shellfish poisoning.
There are four different types of shellfish poisoning, each caused by a different toxin and showing different symptoms. These are paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), amnesic shellfish poisoning (ASP), and diarrheic shellfish poisoning (DSP). PSP is linked to a toxin called saxitoxin, NSP is caused by brevetoxin, ASP happens because of domoic acid, and DSP is from okadaic acid. These toxins and the illnesses they cause often happen when there’s a harmful overgrowth of algae in the water.
What Causes Shellfish Toxicity?
Shellfish poisoning mostly happens during a specific time known as the “red tide,” which is when certain algae multiply rapidly, creating what’s called an algal bloom. This type of poisoning mostly comes from eating contaminated food, although some toxins, like brevetoxin, can also be breathed in. Algal blooms happen more frequently in the summer, which means there’s a higher chance of encountering marine poisons during this time.
The poisons that make humans sick are made by certain kinds of algae called dinoflagellates and a type of bacteria named cyanobacteria. These molecules build up in shellfish that people often eat. Some of these toxins, including saxitoxin, neosaxitoxin, and ganyautoxin, are grouped together and called “paralytic shellfish toxins” (PSTs). PSTs are not destroyed by heat and are produced by specific types of dinoflagellates (Alexandrium, Gymnodinium, and Pyrodinium) and certain cyanobacteria.
Another toxin, brevetoxin, is made by an algae named Karenia brevis. A type of algae called a diatom, specifically from the genus Pseudonitzschia, makes a poison called domoic acid. There are also toxins named okadaic acid and its similar toxins, the dinophysistoxins (DTXs), which are made by types of algae known as Prorocentrum lima and Dinophysis.
Risk Factors and Frequency for Shellfish Toxicity
Shellfish toxicity poses a risk for those who eat shellfish, especially those caught along tropical and temperate coasts during the summer. This has led to cases of Paralytic Shellfish Poisoning (PSP) in numerous countries including Canada, Chile, France, Guatemala, Malaysia, Nicaragua, the Philippines, Portugal, Spain, Tasmania, the United Kingdom, and the United States. There have also been instances of toxins identified in shellfish worldwide, even in the absence of reported human exposure. Of all types of shellfish toxicity, PSP has caused the most deaths.
Blooms of a microscopic alga called ‘K. brevis’, which causes neurotoxic shellfish poisoning (NSP), are common in the Gulf of Mexico. Cases of NSP have been documented around the world, but most recorded instances are from the southwest United States. As well as affecting the digestive system, there have been cases of lung irritation due to airborne toxins from this alga in the same region.
The largest report of Amnesic Shellfish Poisoning (ASP) cases came from Prince Edward Island, Canada in 1987. Individuals exposed to the toxin responsible for ASP over a long period have experienced changes in their memory. Since the 1987 outbreak, several public health measures have been introduced to prevent future ASP outbreaks.
Diarrhetic Shellfish Poisoning (DSP) cases have been reported in Argentina, Canada, Chile, Denmark, France, Ireland, Japan, New Zealand, Norway, Portugal, the UK and the US.
Signs and Symptoms of Shellfish Toxicity
Shellfish poisoning can be a severe and sometimes lethal condition. Symptoms usually begin with difficulty in breathing, unresponsiveness or lack of pulse, which require immediate medical help. Initial treatment involves stabilizing the patient’s airway and circulation. A more thorough assessment can be done once immediate threats to life are managed.
Shellfish poisoning is often linked to the recent consumption of contaminated shellfish or seafood. It can happen at any time of the year but is more common in summer due to frequent algal bloom. Signs of shellfish toxins range depending on the specific type of poisoning: Paralytic, Neurotoxic or Diarrhetic Shellfish Poisoning, or Amnesic Shellfish Poisoning.
Paralytic shellfish poisoning (PSP) may initially cause symptoms like facial tingling or numbness, unsteadiness, blurred vision, and muscle weakness. The patient may also report a “floating sensation”. Early stomach upset, such as vomiting or diarrhea, isn’t as common in PSP as in other types of shellfish poisoning. These symptoms can start from 20 minutes to 5 hours after eating the toxic shellfish and the severity of the symptoms depends on the quantity consumed. Symptoms can last from 30 minutes to up to 10 days, but typically, they resolve within 24 hours. Death can occur 3.5 to 8 hours after the onset from respiratory failure.
Neurotoxic shellfish poisoning (NSP) symptoms may be both stomach-related (like mild-to-moderate vomiting, diarrhea) and neurological (like numbness in face and limbs, reversed hot-and-cold sensations). Symptoms usually start within 1 to 3 hours but can delay up to 18 hours after exposure. In general, NSP has less risk of muscle weakness, respiratory problems, and death compared to PSP. Officials have noted that NSP can also result from inhaling sea spray containing the toxin, which can cause coughs, wheezing, and asthma flare-ups.
Amnesic shellfish poisoning (ASP) most commonly causes early upset stomach, but symptoms can range from a slight stomach ache to severe nausea and vomiting. Some patients also reported a headache (43%), and about 25% of them experienced short-term memory loss. About three people have died from ASP, according to data from a past Canadian outbreak. Some patients who have consumed razor clams contaminated with domoic acid have reported long-term memory loss.
Diarrhetic shellfish poisoning (DSP) could cause patients to experience nausea, stomach pain, vomiting, chills, and fever. Symptoms often appear between 30 minutes to 4 hours after consuming contaminated shellfish and may last up to three days. These symptoms are usually mild.
Physical examination for shellfish poisoning might reveal signs of dehydration such as rapid breathing and heart rate, low blood pressure, a dry mouth, and shriveled eyes. For severe cases involving vomiting or diarrhea, additional signs could include poor skin color return (capillary refill) and amplified bowel sounds. Those with toxin-related illnesses could have low oxygen levels in the blood and abnormal lung sounds.
Patients with suspected shellfish poisoning should undergo a full neurological evaluation and should be closely monitored, as the symptoms can sometimes be delayed. Doctors should be ready to perform intubation in patients suspected to have PSP. A negative inspiratory force test could be useful if respiratory muscle weakness is suspected.
Testing for Shellfish Toxicity
If you are suspected of having shellfish poisoning, your doctor will start by asking you lots of questions about your recent activities and the food you’ve been eating. They’ll be especially interested if there have been any local warnings about harmful algae or contaminated shellfish in your area. If you’ve eaten shellfish during such an alert, it’s more likely that this could be causing your symptoms.
To test for shellfish poisoning, doctors can use specialized tests that can detect the presence of different types of toxins in the shellfish. These tests include high-pressure liquid chromatography (HPLC) and enzyme-linked immunosorbent assays (ELISA). However, these tests are quite advanced and might not be available in smaller clinics or healthcare centers. If these tests can’t be done in time, the doctors will have to make their decision based on your symptoms and what you’ve told them.
In addition to these specific tests, your doctor may want to do some general blood tests. These could include a complete blood count (CBC), which checks for the number and types of cells in your blood; coagulation studies, which see how well your blood clots; and a complete metabolic panel (CMP), which checks for levels of various chemicals and electrolytes in your blood. Looking at these tests can also help your doctor find out if you are dehydrated, which can be a sign of shellfish poisoning.
If you are dehydrated, your blood may show high levels of substances like blood urea nitrogen, while other levels, like creatinine, might be normal. If both these levels are high, it could signal that your kidneys are being affected by dehydration. The CMP can also show how your liver is functioning, which can be important since some of the shellfish toxins are eliminated from your body through the liver.
If you’ve been inhaling brevetoxins, which is one type of toxin that can come from harmful algae, your doctor might want to take an X-ray of your chest. The X-ray might not show anything specific, but it can help rule out other conditions, like pneumonia, which need different treatments.
Treatment Options for Shellfish Toxicity
The primary approach to treating shellfish poisoning is supportive care, which means treating the symptoms not the cause itself. Activated charcoal, a substance that can help absorb toxins, might be considered if the person hasn’t vomited up most of the shellfish. However, it’s unclear how well activated charcoal can absorb shellfish toxins. The practices of using laxatives or rinsing out the stomach (gastric lavage) aren’t universally recommended because there isn’t enough evidence that they benefit patients with shellfish poisoning.
For those with PSP, a type of shellfish poisoning that causes nerve problems, close monitoring of breathing is crucial. If breathing becomes difficult, a tube may be inserted into their windpipe (endotracheal intubation), and they may need help breathing (ventilatory support). This could last several days until their muscle weakness gets better. While there have been tests in animals on antibodies that reverse breathing failure caused by the toxin in PSP, it hasn’t been tested in humans yet.
In the case of NSP, another type of shellfish poisoning, managing stomach issues and changes in the senses is an important part of the supportive treatment.
Treatment with mannitol, a type of sugar, might be considered for brevetoxin poisoning due to its use in treating ciguatera fish poisoning since both toxins are similar. However, there’s limited data on its use in ciguatera and none for its use in NSP. Brevenal, a compound found in certain algae that naturally hinders brevetoxin, might be a theoretical treatment but it hasn’t been tested.
If someone gets sick from inhaling brevetoxin, they could potentially be treated for narrowed air passages or worsening asthma with albuterol, a medication that helps open up the airways. Animal studies also show that drugs countering the effects of histamine and atropine, a medication for certain types of nerve agent and pesticide poisonings can block brevetoxin-induced narrowed airways. Patients usually get better quickly once they leave the area where they’ve inhaled the poison. Advising people to avoid such places might help prevent such incidents.
For ASP and DSP, other types of shellfish poisonings, supportive treatment is all that we currently have as no specific antidote has been identified for either condition yet.
What else can Shellfish Toxicity be?
If you’re suffering from shellfish toxicity, there are various conditions your doctor might consider to make sure your symptoms aren’t due to something else:
- Botulism
- Tetrodotoxin toxicity
- Conotoxin toxicity
- Sting from a Crown-of-thorns starfish
- Seabather’s eruption
- Ciguatera toxicity
- Scombroid
- Infectious enterocolitis
- Food poisoning
- Guillain-Barre syndrome
- Seizure disorder
A thorough medical check-up can help your doctor decide which of these conditions might be causing your symptoms.
Preventing Shellfish Toxicity
To avoid getting sick from eating shellfish, there are a few steps you can take:
- Firstly, always buy from trusted places known for their good quality shellfish.
- Stay informed about any local warnings or advisories on seafood safety in your area.
- Don’t collect or buy shellfish during times when there’s a ‘red tide’ – an event that often increases the risk of shellfish being contaminated.
- Store and refrigerate your shellfish appropriately to ensure it stays safe to consume.
- Always cook your shellfish thoroughly before you eat it.
- Lastly, if you find any shellfish that appear damaged or dead, it’s best to throw them away.
If you experience any unusual symptoms after eating seafood, it’s crucial to seek medical help quickly. Prompt attention can help rule out any serious conditions and ensure you get the right treatment as soon as possible.
