Introduction

The minimum single oral lethal dose of selenium for most animals is in the range of 1 to 5 mg/kg body weight. Concentrations of 25 ppm and greater in the total diet may be sufficient to produce acute poisoning in most animals (1).

Pathologic changes associates with selenium poisoning are varied and include swelling and congestion of the liver, congestion, degeneration, and necrosis of the renal tubular epithelium, congestion of the spleen with hyperplasia. of splenic nodules, hemorrhagic enteritis ranging from mild to severe, ulceration of the gastric mucosa, subserosal and subendocardial hemorrhages in the heart, edema and congestion of the brain with neuronal degeneration in the cerebral and cerebellar cortices, and focal symmetrical poliomyelomalacia (1-5).

Diagnosis of poisonings is usually made on the basis of selenium content in liver and kidney tissue. Concentrations of 4 to 25 ug/g may be seen in liver and kidney tissue in both acute and chronic selenium poisoning (1,4,6).

Blood concentrations may reach 25 ppm in acute cases, whereas they are more likely to be in the 1 to 4 ppm range in chronic cases of selenium poisoning. While there is considerable information concerning acute and chronic selenium poisoning in domestic land mammals and poultry, there is a paucity of information regarding selenium poisoning in marine mammals. Vitamin E deficiency has occurred in sea lions and dolphins and, Oklahoma State University, College of Veterinary Medicine previous analysis of dolphin (Tursiops truncatus) tissue has indicated normal selenium concentrations to be similar to domestic land mammals.

The following cases were confirmed as selenium poisoning in California sea lions (Zalophus californianus), and the blood and tissue content of selenium suggest a subacute or chronic syndrome.

Case Histories

Two California sea lions performed at a show on Saturday, October 24, 1988 and appeared normal that evening. One was then found dead Sunday morning, and the other exhibited convulsive seizures and died later that same day. The sea lions, both males, were flown to the Oklahoma Animal Disease Diagnostic Laboratory for postmortem evaluation.

The smaller of the 2 sea lions had 2 ulcers, 1 about 5 cm in diameter and the other about 1 cm in diameter, located in the cranial end of the stomach. The stomach was essentially empty. The gastric mucosa was reddened and contained a small amount of adherent mucus. The content of the intestinal tract appeared normal.

The stomach of the larger sea lion was relatively empty except for a small piece of black balloon-like material in the pyloric end of the stomach. The mucosa was red and there was brown necrosis of the tips of the leaves of the stomach. The spleen of the larger animal had a mottled (red and grey) appearance and was contracted.

The liver, kidneys, lungs, heart, adrenal glands and brain of both sea lions had no visible lesions.

Histopathology Observations

The brain of the largest sea lion showed areas of perivascular infiltrate of lymphocytes, foci of glial cells, neuronophagia, mild infiltrates of lymphocytes in the meninges and neuronal degenerations. The lesions were similar in the brain of the smaller animal but were milder. A nonsuppurative encephalitis was diagnosed in both sea lions.

The gross lesions of the stomach interpreted as necrosis were not confirmed microscopically. The ulcers in the stomach of the smaller sea lion were chronic and consisted of mucosal necrosis and fibroplasia.

The lungs of both animals were hyperemic and had no lesions of pneumonia. The liver, heart, kidneys, testes, pancreas, intestine and mesenteric lymph nodes of both animals had no significant lesion.

Brain tissue was cultured for viruses as encephalitis is thought to occur in San Miguel sea lions viral infections. Virology studies were negative.

Toxicology Studies

Blood and brain cholinesterase values in both animals were considered normal. There were no pesticide residues considered significant. There were no detectable residues of neutral, acidic or basic drugs. Heavy and trace metal analyses were performed by Zeeman atomic absorption spectrophotometry, with the exception of mercury which was performed by hydride generation atomic absorption spectrophotometry.

Selenium determinations were made by a modified Zeeman atomic absorption spectrophotometric procedure utilizing a pyrolytically coated graphite tube with a L'vov platform (7). The modifications include a cool-down step prior to atomization. Palladium nitrate is used as a matrix modifier in order to char the sample at 1400 C. The HGA-400 furnace conditions are shown in Table 1. Liver and kidney tissues were analyzed on a wet-weight basis. Tissue samples weighing 1 g were digested in 10 ml of 6 N nitric acid with 2 ml of 0.1% nickel nitrate added as a stabilizer. The samples were digested at 150 C for approximately 4 hr. The digested samples were then filtered into 100 ml volumetric flasks and diluted to volume with deionized glass distilled water.

Whole blood samples of 250 ul were diluted 1:3 with a solution of 0.1% TritonX and 0.1% nickel nitrate. The sample aliquot was 10 ul with an alternate volume of 10 ul of palladium nitrate solution (0.66 g palladium nitrate in 100 ml 5% nitric acid). The method had an expected recovery of 95% and a detection limit of 12 ppb. Results of selenium and other heavy and trace metal analyses are shown in Table 2.

Table 1. Perkin-Elmer HGA-400 Furnace Conditions for Selenium Determinations

Table 2. Heavy and Trace Metals Concentrations (ug/g wet weight) Detected in Liver and Kidney Samples from the Larger Sea Lion (A) and Smaller Sea Lion (B) of Cases 1 and 2

Postmortem blood from sea lion A had a selenium content of 5.2 ppm and from sea lion B 5.1 ppm. The brain mercury concentration from sea lion A was 0.015 ug/g, while the kidney of sea lion B had a mercury content of 0.78 ug/g.

Samples of the frozen fish the sea lions were being fed were submitted for assay with the carcasses of the 2 sea lions. The fish were thawed, and fluid from the thawed fish and homogenates of the thawed fish were analyzed for selenium. Fluid from the thawed fish contained 45.0 ug/g selenium and the thawed fish homogenate contained 1.28 ug/g selenium. Additional fish from the sea lions' diet were analyzed. The results are shown in Table 3.

Analysis of pool water where the sea lions were kept had a selenium content of 0.006 mg/L.

Total fish homogenate 1.01 1.06 0.70 Fish without viscera 0.65 0.77 0.63 Fish viscera 2.45 2.26 0.96.

Table 3. Selenium Concentrations (ug/g wet weight) of Fish in the Sea Lion's Diet

Discussion

Toxicologically significant selenium content for most land mammals (cattle, sheep and horses) are greater than 12 to 15 ug/g for liver, 3 to 6 ug/g for kidney and 3 ug/g for blood (1). While there is very little information regarding normal background concentrations of selenium in marine mammals, the authors considered the blood, liver and kidney selenium content in these 2 reported cases to be significantly elevated.

The selenium concentrations in the fish fed the sea lions greatly exceeded the dietary requirements of selenium for domestic livestock, which is in the 0.1 to 0.3 ug/g range (1). Further investigation is needed to elucidate the potentials for selenium toxicoses in captive sea mammals.

References

1.  Osweiler GD, Carson TL, Buck WB et.al. Clinical and Diagnostic Veterinary Toxicology, 3rd ed. Kendall-Hunt, Dubuque, Iowa (1985).

2.  Jubb KVF, Kennedy PC, Palmer N: Pathology of Domestic Animals, 3rd ed. Academic Press, New York (1985).

3.  Bludgett DJ, Bevill RF: Acute Selenium Toxicosis in Sheep, Vet Hum Toxicol 29: 233-236 (1987).

4.  Morrow DA: Acute Selenite Toxicosis in Lambs. JAVMA, 152: 1625-1629 (1968).

5.  Shortridge EH, O'Hara PJ, Marshall, PM. Acute Selenium Poisoning in Cattle, NZ Vet J 19: 47-50 (1971).

6.  MacDonald DW, Christian RG, Strauss KL et.al.: Acute Selenium Toxicity in Neonatal Calves, Can Vet J 22: 279-281 (1981).

7.  Edwards WC, Blackburn TA: Selenium Determinations by Zeeman Atomic Absorption Spectrophotometry, Vet Hum Toxicol 29: 12-13 (1986).