Toxics DDT, Domoic Acid Linked to Sea Lion Seizures

The pesticide DDT and domoic acid, a neurotoxin produced by harmful algal blooms, has each been linked separately with poisoning during birthing season at the Channel Island sea lion rookeries. The threat caused by their interaction is only now being investigated.

The interaction between the two toxics, was studied by Dr. John Ramsdell, an environmental scientist at the NOAA National Center for Coastal Environmental Health and Biomolecular Research laboratory in Charleston.

The findings by Ramsdell and his team provide new insight into epileptic seizure patterns observed in California sea lions in Channel Islands National Marine Sanctuary and National Park.

"These findings will help us better understand the origins of this behavior and are the first to reveal the interaction between two noted stressors of this animal population," said Ramsdell.

The research report in the January issue of the journal "Environmental Health Perspectives" also identifies how the timing of pregnant animals' exposure to the toxics can affect the health of offspring.

The researchers observed that sea lion exposure to domoic acid during fetal development can lead to brain seizures when the young are two to four years old.

Sea lions swimming near Anacapa Island in California's Channel Islands group. (Photo by Chi Zhang)

Driven by climate changes and coastal pollution, algal blooms have been increasing in the sea lions' habitat over the past decade, resulting in more cases of acute poisoning. There is increasing concern over the long-term effects of algal toxins and how they interact with other changes in the marine environment.

Scientists with NOAA's Oceans and Human Health Initiative are studying how harmful algal blooms and marine ecosystem contaminants affect marine mammals to determine if humans exposed to the same toxins would experience similar symptoms.

Scientists working under Dr. Ramsdell conducted the first analysis of potential interaction of DDT and domoic acid during brain development in zebrafish, which are used as a laboratory model for epilepsy in humans.

The zebrafish were analyzed for seizure behavior under conditions that mimic both maternal transfer of DDT during the course of brain development and exposure to a harmful algal bloom shortly after brain maturation.

Young zebrafish that accumulated DDT concentrations that initially caused no observable effects showed increased sensitivity to domoic acid and more intense seizures, the scientists found.

The levels of DDT and domoic acid in the zebrafish were within the range found in fetal California sea lions.

Last year, Ramsdell reported in the online journal "Marine Drugs" that an increase of epileptic seizures and behavioral abnormalities in California sea lions can result from low-dose exposure to domoic acid as a fetus. His findings follow an earlier 2008 analysis led by Frances Gulland of the California Marine Mammal Center that showed this brain disturbance to be a newly recognized chronic disease.

The algae that produces the domoic acid is consumed by fish such as sardines, herring and anchovies, which form a large part of the sea lion diet.

Exposure during pregnancy concentrates the domoic acid toxin in the mother's amniotic fluid, which normally protects and aids in the growth of a fetus. In sea lions exposed to domoic acid, the fluid retains the toxin, subjecting the fetus to repeated direct absorption.

The latest study indicates that the presence of DDT increases the likelihood and intensity of seizures. With these studies, the scientists are learning about the effects of each individual pollutant and addressing the more difficult task of understanding how multiple stressors can interact to cause greater harm.

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