Domoic acid, a potent excitatory neurotoxin produced by the diatom Pseudo-nitzschia australis, has recently been reported as a cause of stranding in California sea lions (Zalophus californianus). Clinical signs in affected sea lions include ataxia, disorientation, scratching behavior, seizures, coma, and death. Lesions caused by domoic acid in these animals have included neuronal necrosis, particularly in the area of the hippocampus. Since first being reported in 1998, approximately 400 live-stranded sea lions suspected of having domoic acid toxicity have been admitted to the marine mammal center, a private nonprofit marine mammal rehabilitation center in central California. Treatment of affected animals has been directed at attempting to stop seizures with the use of benzodiazepines such as diazepam and lorazepam and preventing further seizure activity with the use of phenobarbital. Rapid initiation of therapy in affected sea lions may have been responsible for increased survival rates reported in a previous study. Few studies have evaluated the use of anticonvulsants in marine mammals, and dosages used are frequently empirically derived by extrapolation from other species. However, absorption, distribution, and elimination in different species may be highly variable. Although phenobarbital dosing is often titrated to clinical effect, clinical response is difficult to assess in sea lions affected by domoic acid toxicity because clinical signs of domoic acid toxicity including lethargy, disorientation, and ataxia may closely resemble signs associated with phenobarbital overdose. The pharmacokinetics of oral phenobarbital were investigated in order to determine a safe and efficacious dosage for use in treating seizures induced by domoic acid in California sea lions.
Nine California sea lions (six male, three female) weighing between 29 and 43 kg were included in the oral phenobarbital study. At the time of the study, the sea lions were judged to be in good body condition, showed normal behavior, and had normal hematology and serum chemistry parameters. None of the animals had been suspected as having been affected by domoic acid toxicity at the time of stranding. The animals did not receive any other medication during the course of the study. Sea lions were housed in three groups of three animals each and had access to a freshwater pool at all times. The animals were fed a diet consisting of whole, frozen Atlantic herring (Clupea harengus) thawed and given at a rate of approximately 5% of the body weight of each animal per day. The sea lions were fed approximately every 12 hours at 08:00 and 20:00. Each group of three sea lions was given a dosage of 2, 4, and 6 mg/kg respectively of phenobarbital (Qualitest Pharmaceuticals, Inc., Huntsville, AL 35811) orally BID for 7 days. Blood samples were drawn from the caudal gluteal vein using 1.5-inch, 21-g needles directly into blood collection tubes containing serum separation gel and clot activator (Vacutainer Systems, Becton Dickinson, Franklin Lakes, NJ 07417). Between 3 and 5 ml were drawn per sample. Blood was drawn on day 0, 4, 7, 10, and 14 of the study. Animals in the third group, which was given the highest dosage of phenobarbital, were also sampled on day 16. Mentation was scored using a standardized protocol. Oral dosages of phenobarbital (2 mg/kg) resulted in peak plasma concentrations of 10–30 mg/L which are considered therapeutic levels for most species receiving phenobarbital for treatment of seizures. Dosages above 2 mg/kg (4 and 6 mg/kg) resulted in plasma concentrations above 30 mg/L and dosages of 6 mg/kg were associated with significant central nervous system side effects. The time to maximum plasma concentrations (11.9–78.2 mg/L) was 7 days suggesting that a loading dose is required to bring plasma concentrations into therapeutic range rapidly. The average terminal half-life was 5.8 days, indicating that a minimum of 24 days (four half-lives) is necessary before steady state can be achieved. In conclusion, a loading dose of phenobarbital followed by an oral maintenance dosage of 2 mg/kg appears to achieve plasma concentrations in the range of 10–30 mg/L which may be therapeutic without further exacerbation of domoic acid induced central nervous system side effects.