Hippocampal Pathology in California Sea Lions with Domoic Acid-Induced Temporal Lobe Epilepsy

In 1987, over 100 Canadians became ill after eating cultivated mussels from Prince Edward Island. Symptoms included the typical gastrointestinal issues, but serious neurological findings such as disorientation, confusion, and memory loss were also observed (Perl et al., 1990). In the worst cases, the patients developed seizures or went into coma. Three elderly people died. The cognitive changes were persistent, and had not resolved within a two year follow-up.

The toxin was identified as domoic acid, which received the well-deserved moniker of Amnesiac Shellfish Poison. Domoic acid is a potent excitatory amino acid that activates kainate and AMPA receptors, the binding sites for the ubiquitous excitatory neurotransmitter glutamate. It acts as an excitotoxin by overstimulating these receptors, causing a flood of calcium ions into the cells. Particularly vulnerable are neurons in medial temporal lobe structures such as the amygdala and the hippocampus, which is critical for memory.

Postmortem examination of four brains revealed hippocampal pathology that could account for the clinically significant anterograde amnesia seen in other (still living) patients (Teitelbaum et al., 1990). The pattern of neuronal loss was consistent with the damage observed in kainic acid animal models of epilepsy.

Fig. 3 (modified from Teitelbaum et al., 1990).  Panel A: Section of hippocampus from a patient who died 24 days after mussel-induced intoxication, showing severe loss of neurons in all fields except CA2 (arrow), and tissue collapse is evident in part of field CA1 (double arrow).  Panel B: Control Subject.

What was the source of the Amnesiac Shellfish Poison that had accumulated in the mussels?  A "red tide" of phytoplankton created a harmful algal bloom that produced domoic acid, which accumulates not only in shellfish but also in fish such as anchovies and sardines.
This is where the California sea lions make their noisy entrance...

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Bonus! Live Sea Lion Web Cam at Pier 39 in San Francisco.


Domoic Acid Toxicity in California Sea Lions

The Marine Mammal Center in Sausalito, California rescues and rehabilitates sick, stranded, and malnourished marine mammals, including seals, sea lions, and cetaceans. An up-to-date list of their current patients is available here. They are the premiere institution for the diagnosis, treatment, and scientific study of domoic acid toxicity in California sea lions:

The Marine Mammal Center was the first group to definitively diagnose DA posioning in marine mammals because of a large outbreak in California sea lions in 1998. In September 2004, the Center received a grant from the Oceans and Human Health Initiative to study the long term effects of domoic acid in sea lions. This project studied the impact of DA on health, survival, and reproduction. Part of this project focused on the neurological effects of DA. Effects were evaluated using magnetic resonance imaging (MRI), cognitive behavior tests (how the animal behaves), and histopathology (tissue samples from dead animals).

Their website on the topic is highly recommended, and contains links to published papers such as Magnetic resonance imaging quality and volumes of brain structures from live and postmortem imaging of California sea lions with clinical signs of domoic acid toxicosis [PDF].

Most recently, a team of researchers from Stanford University collaborated with the Marine Mammal Center to conduct a detailed neuropathological investigation of the brains of sea lions who suffered from seizures due to domoic acid toxicity (Buckmaster et al., 2014). Unfortunately, this is not an uncommon occurrence, since the current census of pinniped patients includes five sea lions diagnosed with acute domoic acid toxicity. In the chronic state, the animals can experience recurrent seizures, leading to a failure to thrive and poor prognosis. The authors hypothesize that the animals develop temporal lobe epilepsy, which can serve as an unfortunate accidental model of temporal lobe epilepsy in humans.

The researchers examined the brains of 14 domoic acid-exposed (DA) animals and 9 control animals. Five of the affected sea lions were admitted in status epilepticus, a state of continual seizure that can cause severe brain damage and even death. The study expanded on earlier work by using stereological methods to obtain an unbiased estimate of the total number of neurons in each hippocampus (left and right hemispheres).

In control sea lions, Buckmaster and colleagues (2014) estimated that each hippocampus contains over 6 million neurons! For the comparative hippocampal anatomy aficionados, sea lions had a relatively small proportion of neurons in the dentate gyrus granule cell layer relative to other mammals (i.e., macaque monkeys, squirrel monkeys, dogs, rats, and mice), and the granule cell layer was thinner than in other species.




Importantly, the authors observed significant neuronal loss in the DA-exposed animals, with substantial variation across the hippocampal subfields (see Fig. 3). And interestingly, the damage was unilateral in most cases: the left hippocampus in four, the right hippocampus in seven, and bilaterally in only three.

Fig. 1 (modified from Buckmaster et al., 2014). Nissl-stained cell bodies in the hippocampi from (A) control and (B-D) chronic domoic acid sea lions. Note the increasing levels of neuron loss in the three chronic DA cases. All were admitted in status epilepticus with DA toxicity. In (A), lines indicate border between the hilus (h) and CA3 field. g, granule cell.


In addition, the authors compared the pattern of neuronal loss in sea lions to that observed in human patients with temporal lobe epilepsy, using tissue obtained at autopsy or after temporal lobe resection (for seizure control):

Substantial neuron loss was evident in all hippocampal subfields of patients with temporal lobe epilepsy and chronic DA sea lions compared with controls (Fig. 3B). In sea lions neuron loss was more severe in the hilus, CA3, and CA2 subfields compared with humans. In humans neuron loss was more severe in CA1. Sea lions and humans displayed similar levels of granule cell loss.

Fig. 3 (modified from Buckmaster et al., 2014). Neuron loss in hippocampal subregions. (B) Neurons per affected hippocampus for chronic DA sea lions (mean + SEM) and neuron densities reported in 11 previously published studies for patients with temporal lobe epilepsy. Symbols indicate results from individual studies. Bars indicate averages.


As we saw in the earlier cases of Amnesiac Shellfish Poisoning in Canada, the CA1 region of the hippocampus was especially vulnerable, and this is also true in cases of hypoxia or anoxia. However, it's notable that significant neuron loss was observed throughout the hippocampus.

Why the difference from sea lion CA1? The reasons are unclear. Nonetheless, when examining the brain as a whole, it is remarkable that the hippocampus shows such qualitatively similar pathology in sea lions and humans poisoned by domoic acid, and humans with temporal lobe epilepsy. The authors speculate that the misfortune of chronic DA sea lions may yield an opportunity to test new anti-seizure treatments, for the benefit of both marine and terrestrial mammals.


References

Buckmaster, P., Wen, X., Toyoda, I., Gulland, F., & Van Bonn, W. (2014). Hippocampal neuropathology of domoic acid-induced epilepsy in California sea lions. Journal of Comparative Neurology, 522 (7), 1691-1706 DOI: 10.1002/cne.23509

Perl, T., Bédard, L., Kosatsky, T., Hockin, J., Todd, E., & Remis, R. (1990). An Outbreak of Toxic Encephalopathy Caused by Eating Mussels Contaminated with Domoic Acid. New England Journal of Medicine, 322 (25), 1775-1780 DOI: 10.1056/NEJM199006213222504

Teitelbaum, J., Zatorre, R., Carpenter, S., Gendron, D., Evans, A., Gjedde, A., & Cashman, N. (1990). Neurologic Sequelae of Domoic Acid Intoxication Due to the Ingestion of Contaminated Mussels. New England Journal of Medicine, 322 (25), 1781-1787 DOI: 10.1056/NEJM199006213222505