Abstract
Marine mammal mortality events associated with harmful algal blooms appear to have increased in recent years, in concert with the increased frequency and expanded geographic distribution of harmful algal blooms and improved diagnostics for the identification of these toxins. These events typically present as large numbers of animals presenting ill or dead on beaches over a short time. Because of the significance of these events, veterinarians should possess knowledge of the potential toxic agents, clinical and postmortem findings, and appropriate sampling methods to assure that proper diagnostics can be performed.
To date, domoic acid and brevotoxin have been definitively linked to episodic marine mammal mortality events. Two other algal toxins, saxitoxin and ciguatoxins, are highly suspect for mortalities. Beginning in 1998, domoic acid, produced by the diatom, Pseudo-nitzschia australis, has been associated with extensive mortalities of California sea lions, dolphins, and southern sea otters on the California coast. A second class of algal toxin with definitive links to manatee and bottlenose dolphin mortalities is brevotoxin, produced by the dinoflagellate Karenia brevis.3,4 Saxitoxin has been implicated in the unusual stranding and mortality of humpback whales in Cape Cod Bay in 1989, and the mortality of more than 100 Mediterranean monk seals on the coast of Mauritania in 1997. Although not currently implicated in mortalities, ciguatoxins have long been suspected to be involved in the poor survival of the Hawaiian monk seal.1
Although the mechanisms of action vary, all these toxins are neurotoxins and both clinical and postmortem findings reflect this action. Routes of exposure also vary based on the toxin and species affected. Exposure occurs either directly through respiratory exposure or indirectly via food-web transfer. The susceptibility of marine mammals to algal toxins is, therefore, dependent not only upon the occurrence of toxin-producing algae within the habitat but often the co-occurrence of appropriate prey species at the time of an algal bloom to serve as vectors. For example, for domoic acid in the same geographic area, the primary vector for sea lion exposure is anchovies as compared to sea otters where invertebrates, specifically the spiny mole crab, are identified as the vectors of concern.
Because of the neurotoxic method of action of these toxins, history and gross findings are often suggestive but not diagnostic, for toxicity. Clinical presentation can vary from individual affected animals to mass mortalities. History and environmental assessments may include associated die-offs of birds or fish, and detection of pathogenic algal blooms within the area. Because of variations in vectors, it is possible for a single geographic area to have temporal variation in affected marine mammals associated with changes in the levels of toxic algae within the food chain.
Clinical signs associated with domoic acid intoxication vary by species affected and include head weaving, scratching, tremors, convulsions, vomiting, blindness, abortion, and sudden death. Gross examination findings are limited and include animals in good nutritional condition with empty stomachs from vomiting. Occasionally, areas of patchy cardiac pallor can be seen. Histologic review demonstrates hippocampal necrosis often with associated nonsuppurative multifocal encephalitis. Heart often demonstrates patchy multifocal myocardial degeneration and fibrosis.2
Brevitoxicosis clinical signs have been documented in manatees and include disorientation, inability to properly submerge, listing, listlessness, back flexing, lip flaring, and labored breathing. In manatees, gross findings reveal animals in good body condition typically with sea grass-filled stomachs. Histologic review reveals severe congestion of respiratory tissues, kidney, and brain.2
Clinical and postmortem sample collection from animals suspected to be affected by biotoxins should be based on available diagnostic modalities and biologic behavior of these toxins. Domoic acid can be detected in stomach content, serum, feces, and urine. Pregnant animals have had detectable levels in amnionic fluid. Collection of appropriate diagnostic samples is often impeded by the rapid clearance of the toxins by urination, defecation, and vomition. Because of this, collection of samples early in affected animals, and collection of a variety of fresh (non-fixed) postmortem samples is critical for proper diagnosis. Highest levels of toxin have been identified in urine and feces. Scanning electron microscopy of gastric content and fecal samples can identify the presence of P. australis frustules.
Brevotoxin can be found in stomach content, liver, kidney, and lung tissues of affected animals via receptor assay, ELISA, and HPLC-mass spectrometry. Additionally, immunohistochemistry utilizing anti-brevotoxin antibody performed on fixed tissues has identified positive staining in lymphocytes and macrophages in respiratory, renal, and nervous tissue.
Toxin level determinations are the key to both diagnosis and ecological assessment of marine biotoxin impacts. Because there are multiple biotoxins implicated and potentially affecting marine mammals, sample collection for analysis of unknown toxins should include multiple fresh or frozen samples. These should include gastric, intestinal, and colonic content; urine; bile; serum; CSF; brain; lung; liver; and kidney. As more biotoxins are identified, and as the frequency and distribution of blooms increases, it is likely that new biotoxins will be identified as the causes of marine mammal stranding and mortality. Further investigations remain needed in determining dose response and toxicokinetics, the impacts of multiple biotoxins, as well as chronic, low-level exposure impacts.
Literature Cited
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2. Gulland F. Domoic acid toxicity in California sea lions (Zalophus californianus) Stranded Along the Central California coast, May–October 1998. Report to the National Marine Fisheries Service Working Group on Unusual Marine Mammal Mortality Events. U.S. Department of Commerce. NOAA Technical Memo. NMFS- OPR-17; 2000:45.
3. Mase B, Jones, Wewing R, Bossart G, Van Dolah F, Leighfield T, et al. T. Epizootic of bottlenose dolphins in the Florida panhandle: 1999–2000. In: Proceedings from the American Association of Zoo Veterinarians and International Association of Aquatic Animal Medicine Joint Conference; 2000; New Orleans, LA.
4. Bossart GD, Baden DG, Ewing RY, Roberts B, Wright SD. Brevotoxicosis in manatees (Trichechus manatus latirostris) from the 1996 epizootic: gross, histologic, and immunohistochemical features. Environ Toxicol Pathol. 1998;26:276–282.