Human exposure seems to be most common via consumption of commonly harvested shellfish such as clams, oysters, and mussels, although it has been proposed that exposure to lower levels of brevetoxins can take place following the consumption of certain planktivorous fish. Toxins will typically be found in the flesh of shellfish for up to 2–8 weeks following a HAB event, however there have been reports of toxin retention for nearly one year post-bloom. Notably, brevetoxins are tasteless and odorless and cannot be eradicated by rinsing, cleaning, cooking, freezing, or application of acid. To date, there is no reasonable means of preventing the uptake of toxins by shellfish, nor of removing the toxins from shellfish after harvest. Biochemistry/Toxicology
Brevetoxins are a group of greater than ten lipid-soluble cyclic polyethers that bind to a specific site on the voltage-gated sodium channel (VGSC), leading to an influx of sodium ions into the cell. This results in activation of nerves and spontaneous nerve cell membrane depolarization and firing. Due to their lipid-solubility, brevetoxins are able to pass through cell membranes and cross the blood-brain barrier. They are metabolized primarily by the liver and are excreted in the bile, although it is thought that urinary excretion plays a role in toxin clearance as well. Additionally, brevetoxins can bind a separate site on VGSCs, causing release of neurotransmitters (such as acetylcholine), resulting in tracheal smooth muscle contraction and widespread mast cell degranulation. Presentation and Diagnosis
Diagnosis of NSP is made based on clinical presentation as well as history findings including recent consumption of shellfish.
On average, symptoms begin 3-4 hours after consumption, but can begin anywhere from several minutes to 18 hours afterward. Symptoms typically include neurologic and gastrointestinal issues including:
nauseavomiting
diarrhea
numbness and tingling in the lips, mouth, face, and extremitiesThe latter has been described as “nerves being on fire” or “ants crawling and biting all over”.Other less common symptoms can include:
ataxia
loss of coordination
limb paralysis
reversal of hot and cold sensations
slurred speech
headache
pupil dilation
generalized fatiguePatients may be thought to be disoriented or intoxicated. Rarely, patients may experience respiratory distress requiring ventilatory support. Despite this seemingly severe constellation of symptoms, there have been no documented deaths due to NSP. NSP can present similar to other disorders such as:
common food poisoning
seafood allergy
paralytic shellfish poisoning
ciguatera fish poisoning
pesticide poisoning
alcohol intoxication
certain psychiatric disorders.Due to the extensive list of disorders with similar symptoms, a detailed food history is necessary to make the diagnosis. Management and treatment
Treatment for NSP is mostly supportive with monitoring and symptom management. Intravenous fluids and observation of respiratory function are the mainstay of treatment along with pain control. Activated charcoal can be given if the patient presents within four hours of consumption to decontaminate the gastrointestinal tract. Currently, there is no specific antidote for brevetoxins, however there may be a role for mannitol (the primary treatment for ciguatoxin, a dinoflagellate-produced toxin found in some species of fish) or brevatal, a natural antagonist of brevetoxin produced by K. brevis. Though public health policy differs by state, measures are taken to prevent shellfish poisoning outbreaks.