Shellfish related illness can be infectious (viral or bacterial), due to heavy metal contamination or poisoning due to the accumulation of toxic microalgae or dinoflagellates. Shellfish poisoning is rare and has become even less frequent because of increased controls in the shellfish industry.

There are four types of shellfish poisoning:

1. Paralytic shellfish poisoning: This is similar to TTX poisoning, but results from other potent sodium channel blockers such as saxitoxin, gonyautoxins and their derivatives. It is the commonest shellfish poisoning and has a high fatality rate because of paralysis and respiratory failure.
2. Neurotoxic shellfish poisoning: This is rare and causes neuroexcitatory effects that are sometimes similar to ciguatera. There is no specific treatment except supportive care.
3. Encephalopathic shellfish poisoning: This is due to domoic acid and has only been reported once as an outbreak in North America.
4. Diarrhoetic shellfish poisoning is similar to infectious gastroenteritis, but is rapid in onset because it is due to ingested toxins. There are no neurological effects. It can cause severe diarrhoea, fluid loss and hypovolaemic shock. It requires aggressive fluid resusciation and symptomatic treatment.