Neurotoxin in Shark Fins: A Wider Threat?

Megan Brooks

March 30, 2012

March 30, 2012 — An analysis of fin clips from sharks in Florida waters found high concentrations of a neurotoxin that has been linked to neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis (ALS), and parkinsonism dementia.

The neurotoxin, β-N-methylamino-L-alanine (BMAA), is produced by cyanobacteria, also known as blue-green algae.

Deborah C. Mash, PhD, from the University of Miami Miller School of Medicine, Florida, who worked on the study, said the concentrations of BMAA in the samples are a "cause for concern," not only in shark fin soup, considered a delicacy in Asian cuisine, but also in dietary supplements and other forms of shark cartilage ingested by humans.

Their report was published online February 21 in the journal Marine Drugs.

Dr. Deborah C. Mash

However, the risks of exposure to this neurotoxin may go beyond shark products. Commenting on these findings, Frederick L. Tyson, PhD, from the National Institute of Environmental Health Sciences (NIEHS), Research Triangle Park, North Carolina, points to "other routes of BMAA exposure that we should be concerned about."

"People are exposed to it in fresh water systems," Dr. Tyson, who was not involved in the shark study, said in a telephone interview with Medscape Medical News. "It's blue-green algae, so there certainly is dermal exposure and the possibility of ingesting it while you are in the water."

"The other concern is that it does accumulate in crop plants so if you have crops that are being irrigated by reservoirs or dam systems or whatever that have a high cyanobacterial biomass there is potential there for some human risk as well," he added.

Sharks Bioaccumulate BMAA

Dr. Mash and her colleagues sampled fin clips from 7 different species of sharks in the waters off the South Florida coast: blacknose, blacktip, bonnethead, bull, great hammerhead, lemon, and nurse sharks. They detected BMAA in the fins of all species examined, with concentrations ranging from 144 to 1836 ng/mg wet weight.

"Sharks, because they are long-lived, bioaccumulate environmental toxins over the lifespan, just like humans," Dr. Mash told Medscape Medical News. "Sharks have mercury levels that are elevated and our work shows a second neurotoxin — BMAA."

"It's been suggested," Dr. Tyson said, "that BMAA can bioaccumulate in nerve cells that are not going to reproduce so it doesn't get cleared."

The concentrations found in shark fins overlap with the concentrations the researchers observed previously http://www.ncbi.nlm.nih.gov/pubmed/19254284 in postmortem brain tissue from patients who died with sporadic Alzheimer's disease and ALS.

The BMAA concentrations in shark fins also mirror the BMAA levels found in fruit bats in Guam. The fruit bats accumulate BMAA from their diet of cycad seeds. Ingestion of fruit bats has been linked to severe ALS and parkinsonism dementia in indigenous people of Guam.

Association Merits Further Research

Dr. Mash told Medscape Medical News, "Further work is needed to determine the risk to human health."

Dr. Tyson agrees. "Research has shown an association between BMAA and neurodegenerative disease," he said, "but there is no hard data that it's causal. No one has made a mechanistic connection."

"It does merit research in terms of how much of a human risk it poses, but the jury is still out until we can show some kind of mechanistic connection at least in animal models and we have some applications (at NIEHS) that are certainly asking those questions," Dr. Tyson added.

The study was funded through a donation from the Herbert W. Hoover Foundation. The authors and Dr. Tyson have disclosed no relevant financial relationships.

Mar Drugs. 2012;10:509-520. Abstract

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