Beware The Clam of Forgetfulness

Every day, PubCrawler emails to tell me about the latest papers that match various search terms. It means I never miss a relevant paper, but it also means I get told about an awful lot of irrelevant ones. Sometimes though, the title alone grabs my attention and demands a read. Such as yesterday's Risk assessment of the amnesic shellfish poison, domoic acid, on animals and humans. Shellfish causing amnesia?

It turns out that there's a neurotoxin, domoic acid, which can indeed cause brain damage including memory loss. It's produced by certain algae, and can accumulate inside shellfish, especially mussels.

Domoic acid is responsible for amnesic shellfish poisoning, which struck a cluster of over 100 people in Canada in 1987; 4 died, and several others suffered permanent neurological symptoms, including epilepsy and most notoriously, anterograde amnesia, the inability to form new memories.

Autopsies revealed prominent damage to the hippocampus and nearby temporal lobe areas. Domoic acid victims were therefore very similar to Henry Molaison (HM), the most famous amnesia sufferer, whose memory loss was caused by the surgical removal of the same areas.

Domoic acid is related to kainic acid, which neuroscientists will have heard of: it's widely used in epilepsy research to give animals seizures, amongst other things. Both are excitotoxins - they kill neurons by over-activating them, which opens ion channels allowing calcium to enter the cell and reach toxic levels. They're able to do this because of their chemical similarity to glutamate, the brain's most common neurotransmitter (and the one that the drug ketamine antagonizes).

Since 1987, there have been no further cases in humans, thanks to shellfish harvesting regulations. Marine animals and birds continue to suffer however, especially sea lions, although interestingly, sharks seem to be immune despite having the same glutamate receptors as mammals.

Overall, this is one more reason I'm glad to be a vegetarian. Although, that said, there are some equally nasty neurotoxins in plants...

Kumar KP, Kumar SP, & Nair GA (2009). Risk assessment of the amnesic shellfish poison, domoic acid, on animals and humans. Journal of environmental biology / Academy of Environmental Biology, India, 30 (3), 319-25 PMID: 20120452