Review of the article: Schwacke et al., 2010. Eosinophilia and biotoxin exposure in bottlenose dolphins (Tursiops truncatus) from a coastal area impacted by repeated mortality events. Environmental Research 110: 548-555.
Over the past two decades unusual mortality events of bottlenose dolphins (Tursiops truncatus) have taken place in the northern Gulf of Mexico. Strandings spanning the Florida Panhandle have resulted in hundreds of bottlenose dolphin mortalities and have coincided with the deaths of fish, sea turtles and seabirds. The brevetoxin producing dinoflagellate Karenia brevis has been attributed to some of these deaths due to the presence of brevetoxin in high levels in the stomach contents of sampled stranded animals. However, blooms of K. brevis have been relatively rare in the Panhandle area, compared to other parts of the Florida coast where blooms are common and dolphin mortality events are low. The purpose of this study was to identify why dolphins in the Panhandle area appear to be so susceptible to the toxic effects of brevetoxin, in an area where K. brevis blooms are infrequent, by evaluating dolphin health and assessing background exposure to biotoxins.
Capture-release efforts were conducted around St. Joseph Bay in 2005 and 2006, blood, urine, faeces, gastric fluid and blowhole swab samples were taken. The urine, blood and faecal samples were analysed for the presence of brevetoxin and domoic acid (DA) using an enzyme-linked immunosorbent assay (ELISA), and/or liquid chromatography/ mass spectrometry (LC/MS), and by receptor binding assay. Functional immune assays were also conducted on the blood samples to assess lymphocyte proliferation and phagocytosis, for example.
Interestingly, brevetoxin was not detected in dolphins sampled in 2005 and only ≤ 1.1 ng/ ml was detected in 78% of blood samples from the 2006 sampling effort. Domoic acid was found in urine and blood samples from both years, at a higher concentration in the urine samples with the highest recorded values of 201 ng/ ml in 2005 and 28.6 ng/ ml in 2006. Immune assays identified eosinophilia cases representing just less than one quarter (23%) of the dolphins sampled, furthermore 21% of individuals showed elevated levels of lactate dehydrogenase (LDH). The authors used a G-test of independence to identify whether eosinophilia and elevated LDH were associated, 57% of eosinophillic animals also demonstrated elevated LDH, an association that was statistically significant.
The results suggest that baseline levels of brevetoxin are very low, therefore perhaps not responsible for the unusual mass mortalities observed in this area of the Gulf of Mexico. However, DA levels were relatively high, although variable, which could reflect chronic exposure to this biotoxin. Without determining the levels of toxins during the mass mortality events it is not possible to identify the role that brevetoxin and DA have in dolphin mortalities. Nonetheless it is possible that DA exposure contributes to the onset of eosinophilia, as observed in Californian sea lions, since blood eosinophilia in humans is reported to be related to non-infectious causes such as toxins and allergic reactions. Moreover, acute and chronic exposure to DA as been associated with degenerative cardiomyopathy which could lead to elevated levels of LDH, as observed in this study.
Unfortunately the authors were unable to determine a definitive cause and effect relationship between eosinophilia and DA exposure in this investigation. Problems attributed to the rapid elimination rate of DA coupled with the 8 day maturation time of eosinophils in the bone marrow, prior to migration to the blood system, meant that eosinophilia cases did not have concurrent urine samples for DA analysis. The authors suggest that unless exposure to DA is constant, DA is likely to be undetectable when eosinophilia is manifested. Some evidence exists suggesting that DA alters immune function, based on in vivo and in vitro investigations using mice, and therefore establishing the effect of DA exposure on the manifestation of eosinophilia (or other immunological changes) in bottlenose dolphins, could provide important information as to the effect of background toxin exposure on this species.
Posted by Lauren Messer, February 2011
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