Paralytic Shellfish Poisoning

By BW

Have you ever heard the culinary rule, “Never eat shellfish in a month which does not contain the letter ‘R’?” The origin of this phrase is contested, but many people believe that this is related to red tide algal blooms, which are most prevalent in the warm summer months (1). Red tide algal blooms are large oceanic populations of microorganisms. These blooms contain photosynthetic dinoflagellates, which are small, unicellular eukaryotic organisms. These organisms can inhabit shellfish, and produce a wide variety of toxins which can lead to severe illness in those who ingest the contaminated shellfish. There are four main kinds of shellfish poisoning: amnesic shellfish poisoning (ASP), diarrheic shellfish poisoning (DSP), neurotoxic shellfish poisoning (NSP), and paralytic shellfish poisoning (PSP) (2). PSP is the most lethal of these illnesses, as it has an estimated 8-9.5% mortality rate (3). It is caused by a dinoflagellate called Alexandrium tamarense, which produces a neurotoxin called saxitoxin (4). Due to the increasing global temperature and acidification of the ocean, a result of climate change, red tide may become a more relevant danger to the seafood industry, as these oceanic conditions favor the generation of red tide algal blooms.

Structure of a dinoflagellate. The transverse flagella wraps around the body of the organism,
while the longitudinal flagella extends behind the organism. Image obtained from here.

Dinoflagellates are unicellular eukaryotic organisms with a few unique characteristics. They have both a transverse and longitudinal flagella. The transverse flagella spans around the circumference of the organism, and allows for rotational and forward motion. The longitudinal flagella is located at the back end of the organism, and mainly acts as a rudder to help with control of movement, though it does provide some forward motion. Many dinoflagellates are photosynthetic and live in the water. They contain a wide range of photosynthetic pigments, ranging in color from green to a reddish brown. When certain species of these dinoflagellates appear in high concentration in the water, they cause the surface of the water to have a reddish brown appearance, leading to the classification of these algal blooms as ‘red tide’. As stated above, these dinoflagellates can cause people and animals alike to become ill if they ingest their toxins, which can accumulate in shellfish which live near the red tide (3). \

Alexandrium tamarense is one of these red tide dinoflagellates, and the etiology of the illness which it causes has to do with the toxin which it produces, saxitoxin. Saxitoxin is a neurotoxin, as it causes the malfunction of neurons in the human body by altering the function of sodium gated channels in the neuron which are vital to neuronal firing. This can cause feelings of numbness and tingling throughout the body, nerve dysfunction, nausea, vomiting, and in severe cases leads to death by respiratory failure, as the muscles which contribute to breathing are paralyzed (5). PSP can be diagnosed by examining the suspected infected tissues of a shellfish vector which was consumed by a patient experiencing PSP like symptoms, and subjecting this to an enzyme-linked immunosorbent assay (ELISA), which uses antibodies to detect the presence of the toxin (6). Treatment of PSP generally involves symptom management. If the patient is seen and diagnosed very early after ingestion of the contaminated food, then stomach pumping and administration of activated charcoal or a dilute bicarbonate solution can help to relieve the severity of the symptoms. If the patient is experiencing respiratory symptoms, then the patient is typically given respiratory support, which involves ventilatory support and administration of an anticurare agent, a class of drug which reverses neuromuscular paralysis (3).

Due to the relative isolation of PSP, as algal blooms typically form in small pockets along an oceanic coast, cases are very rare, but come in clusters. One of the largest outbreaks of PSP occurred in the summer of 1987 in Guatemala. A total of 187 cases of PSP with 26 associated deaths were reported at this time. It was found that the vector for this poisoning was Amphichaena kindermani, a species of clam which is commonly used to make clam soup. Children under the age of 6 had a much higher fatality rate than adults, at around 50 and 7% respectively. The lethal dose calculated for children was 140 µg/kg, which was much lower than previously reported lethal doses, and suggested that children may be more sensitive to PSP than adults. The contaminated clams were obtained by the victims of PSP through private fishing, meaning that they did not undergo commercial testing for the toxins produced by red tide dinoflagellates (7). This illustrates the dangers of private fishing, because most private citizens do not have the means to test their food for dangerous compounds prior to eating them, leaving them susceptible to shellfish poisoning.

Largest red tide algal bloom ever observed, near the western coast of the United States in 2015.
This photo was taken off the coast of La Jolla San Diego, California. Image from here.

Due to the rising global temperature, many experts believe that marine biotoxins may increasingly become a problem for the seafood industry in the coming years. Last year, in 2015, scientists discovered a toxic algal bloom off the coast of Washington, which they believe may be the largest ever encountered. Traveling from California to British Columbia, the bloom posed a threat to the numerous shellfish beds in that stretch of coast, which are harvested commercially for eating. The outbreak was thought to be due to the abnormally warm weather in that area last year, as well as the increasing acidity of the ocean, a problem which marine biologists believe is likely a consequence of climate change. The main biotoxic components of the bloom were domoic acid, DSP toxins, and PSP toxins, which was the first time these toxins were all observed off the coast of the state of Washington at one time. The coastal shellfish manager for the Washington Department of Fish and Wildlife, Dan Ayers, has been working with scientists and others in his field to attempt to develop a means to predict these toxic algal outbreaks, much like the tools which are used to predict hurricanes, which are based on characteristic weather patterns which have produced hurricanes in the past. Aside from acidity and water temperature, many other factors influence the growth of red tide algal blooms, including ocean current and wind speed and direction. However, Ayers notes that they are still significantly far away from an effective, predictive model (8). More information about the environment which produces toxic algal blooms is needed, so that we can predict and prepare for these algal blooms before it is too late.

While paralytic shellfish poisoning is a rare ailment, it may become increasingly relevant as climate change effects the marine ecosystem. For this reason, it is important that we know the warning signs of an Alexandrium tamarense outbreak, and that we understand what to do if someone ingests shellfish containing saxitoxin. It is also important to be mindful of the risks associated with ingesting shellfish which is not commercially tested for marine biotoxins, as the consequences could be lethal.

References

1. O’Connor, A. “The Claim: Never Eat Shellfish in a Month Without an R.” The New York Times. 22 July 2008. Web.
2. FAO Corporate Document Repo. Marine biotoxins. Food and Agriculture Organization. Web.
3. Fleming, L. E. Paralytic Shellfish Poisoning. NIEHS Marine and Freshwater Biomedical Sciences Center.
4. Hansen, P. J. (1989). The red tide dinoflagellate Alexandrium tamarense: effects on behaviour and growth of a tintinnid ciliate. Mar. Ecol. Prog. Ser. 53, 105-116.
5. Strichartz, G., Rando, T., Hall, S., Gitschier, J., Hall, L., Magnani, B., Hansen Bay, C. (1986) On the Mechanism by Which Saxitoxin Binds to and Blocks Sodium Channels. Ann. NY Acad. Sci. 479, 96-112.  
6. Garet, E., Gonzalez-Fernandez, A., Lago, J., Vieites, J. M., Cabado, A. G. (2010) Comparative Evaluation of Enzyme-Linked Immunoassay and Reference Methods for the Detection of Shellfish Hydrophilic Toxins in Several Presentations of Seafood. J Ag. Food Chem. 58, 1410-1415.
7. Rodrigue, D. C., Etzel, R. A., Hall, S., De Porras, E., Velasquez, O. H., Tauxe, R. V., Kilbourne, E. M., Blake, P. A. (1990) Lethal paralytic shellfish poisoning in Guatemala. The American Journal of Tropical Medicine and Hygiene. 42, 267-271.  
8. Doughton, S. “Toxic algae bloom might be largest ever.” The Seattle Times. 15 June 2015. Web.