Yesterday a Public Health warning was sent to the Emergency Departments of Southern Tasmania regarding an algal bloom in the D’Entrecasteaux Channel and the accompanying risk of Paralytic Shellfish Poisoning (PSP). I thought I’d use this announcement as an opportunity to review the implicated saxitoxin (STX) poisoning.
The communique from the Director of Public Health reads thus:
“Consumption of shellfish that feed on affected algae may be associated with paralytic shellfish poisoning (PSP). The toxins responsible for PSP are commonly known as saxitoxins. Shellfish include oysters, mussels, scallops with roe on, periwinkles and clams. Scallops eaten without the roe do not pose a risk. This warning does not apply to shellfish from outside the affected areas, nor doses [sic] it apply to abalone or rock lobster meat, but does apply to the gut from these species.
To our knowledge clinically significant cases are rare in Tasmania, but it remains a distinct possibility that cases may occur in individuals or small groups who gather shellfish for personal consumption. Commercial shellfish activities in the affected area have been closed to harvesting.”
What is STX?
STX is a heat stable Na-channel-blocking neurotoxin produced by dinoflagellates, particularly the species that proliferate during so-called “red-tide” blooms. The photo below is of a Noctiluca bloom in an aquaculture area of the South D’Entrecasteaux channel. Noctiluca itself is not noted for saxitoxin production, but other species in these blooms pose a risk (particularly several Alexandrium and Gonyaulax species). Seafood harvesting bans typically are imposed when STX exceeds a safe action level (typically 80 microg STX eq 100 g(-1) tissue). There is currently such a ban in place for the D’Entrecasteaux shellfish industry.
How does STX cause human toxicity?
STX is produced by dinoflagellates and then becomes concentrated by filter-feeding molluscan bivalves (oysters, clams, mussels, periwinkles, etc.). There is some evidence that “non-traditional” vectors such as rock-lobsters and some fish concentrate STX in their gastrointestinal tracts. We sit atop the food chain, and if we consume shellfish that have been feeding on these dinoflagellate blooms, we are exposed to potentially dangerous levels of the toxin. STX is heat- and acid-stable, therefore not eliminated by normal cooking methods.
The mechanism of human toxicity is essentially voltage-gated Na-channel blockade resulting in impaired conduction in neurones and myocytes, leading to rapid onset of paraesthesias (often lips and face, then extremities), bulbar palsy, other cranial nerve palsies, ataxia and ultimately ascending motor paralysis. I would assume that ECG changes resemble those of the other Na-channel blocking agents (i.e. tachycardia, widened QRS, dominant R in aVR with R:S ratio>0.7 in aVR), but it is not clear from my reading if ensuing ventricular dysrhythmias play a prominent role in the morbidity and mortality from STX. Non-specific symptoms include headache, nausea and vomiting, diarrhoea. The main concern is respiratory failure secondary to respiratory muscle paralysis. Symptoms typically commence 10-60 minutes post ingestion. The acute phase of PSP lasts variably around a few days, but muscle weakness can be persistent for some time after.
Other shellfish poisonings include:
- Neurological Shellfish Poisoning (NSP) caused by brevetoxin (essentially is a similar syndrome to PSP minus the paralysis)
- Diarrheal Shellfish Poisoning (DSP) caused by okadaic acid
Amnestic Shellfish Poisoning (ASP) caused by domoic acid
What is the real risk?
Hard to say. Fatality rates from PSP range from 1-12% in isolated outbreaks. The high mortality rates in some areas are almost certainly caused by poor access to advanced life support (i.e. in densely populated developing countries). As far as I am aware, there have been no severe cases in Tasmania, and no deaths associated with PSP. Mortality is almost certainly limited by Public Health initiatives such as the current one operating in Tasmania. As stated in the communique above, non-commercial shellfish harvesters are likely to be most at-risk. We should be vigilant for neurological presentations in the context of recent shellfish ingestion. A good history is clearly key!!!
Can we treat STX PSP?
There is no antidote to STX. Treatment is supportive – the mainstay being recognition of the pathophysiology (the irritating phrase “high index of suspicion” springs to mind) and being vigilant for the risk of respiratory muscle paralysis. Intubation and ventilation are all we’ve got, but if initiated in time, should ensure survival.