Oct 8, 2018: What causes the “Red Tides” occurring in Florida?



Credit: Cristobal Herrera/EPA, via Shutterstock, New York Times

The Red Tides are the result of the proliferation or “bloom” of dinoflagellates (primarily marine plankton which is a collection of organisms ((bacteria, archea, algae, protozoa)) and drifting animals that live in large bodies of water and are unable to swim against a current.)

During the Red Tide, dinoflagellates produce toxic algae containing saxitotoxin which kills marine life.  The algae is filtered and concentrated by bivalve mollusks (mussels, scallops, clams).  Ingestion of the mollusks (containing saxitotoxin) causes paralytic shellfish poisoning (PSP).  Predators of bivalve mollusks (shellfish, lobsters, crabs, fish) may also be vectors of saxitotxins.

Saxitotoxins bind to the cardiac sodium channel, and possibly the calcium and hERG K+ channels impacting depolarization and repolarization.  These toxins suppress AV nodal conduction, peripheral nerve conduction and depress the medullary respiratory center. 

The initial symptoms of PSP are tingling around mouth and lips (minutes to 2 hours) after shellfish consumption.  In more severe cases, the numbness/tingling spreads to the neck and face.  Headache, abdominal pain, nausea, vomiting and diarrhea may accompany neurologic symptoms which include weakness, dysarthria, paresthesia, double vision, ataxia, vertigo.  Dysarthria and dysphagia are the strongest indicators of PSP.  In severe cases, respiratory distress occurs.  In most cases recovery is rapid and complete with symptoms resolving within 24-72 hours.  Treatment is supportive.

There is still much unknown about saxitotoxin.  In animal studies, high doses cause hypotension and cardiopulmonary arrest. 

Did you know…

Questions about this topic are frequently on the emergency medicine board exam.


I am interested in any questions you would like answered in the Question of the Week.  Please email me with any suggestion at donna.seger@vanderbilt.edu


Donna Seger, MD

Medical Director

Tennessee Poison Center


Poison Help Hotline: 1-800-222-1222