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Editor:
Sonars, Gas Bubbles, and Cetacean Deaths
There is, at present, no definitive answer to the intriguing issue of cetacean strandings and deaths in relation to military sonar use.7 The "Brief Communication" on this topic recently published in Nature by Jepson et al.3 may be considered of great concern, but it also raises some crucial questions.
A number of peculiar postmortem findings were observed in the cetaceans under study. Ten beaked whales were found stranded in the Canary Islands shortly after a military naval exercise occurred nearby. This interesting paper suggests that gas bubble formation, possibly in response to either rapid decompression ("decompression sickness," or DCS equivalent?) or exposure of nitrogen-supersaturated tissues to sound waves2 was the likely cause of death.3 The authors report that intravascular gas bubbles and fat emboli, presumably sonar induced, were found in "vital organs" without further specifying what these organs were. Were the heart and brain involved, as it would be reasonable to believe? And, if so, why not consider also the possibility that one or more large-sized gas bubbles may have caused, at least in some animals, sudden heart failure by means of "pure" mechanical compression, either on the myocardial vagal network or on nuclei in the brain from which vagal nerve fibers originate?
Apart from their bacteriologic status, no other data regarding either the age or the preexisting health of the whales are provided, nor are the tissue levels of highly lipophilic persistent organochlorine pollutants (POPs), such as poly-chlorinated biphenyls (PCBs),1 reported. We would like to suggest that gas bubble formation within the subcutaneous adipose tissue and widespread vascular embolization of fat material caused sudden spikes in the blood and tissue levels of PCBs or related xenobiotics. A similar pathogenetic mechanism could also have been involved in the cases of three delphinids and one beaked whale found stranded in the UK between 1992 and 2003.3 The same authors report that these animals had gas-filled cystic cavities in their livers and other parenchymatous organs. The liver is another site for the accumulation of PCBs in cetaceans.5 Apart from their immunosuppressive activity,8 PCBs are known to be endocrine disrupters that have been associated with morphologic changes in several hormone-producing glands, such as the adrenal and thyroid glands. These effects have been clearly demonstrated in beluga whales from the St. Lawrence Estuary and Hudson Bay, Canada.4,6
In conclusion, while recognizing the validity of the contribution made by Jepson et al.,3 we still believe that a number of acute and chronic disease factors and mechanisms, acting alone or in combination, may have also been involved in the deaths of these whales.
Department of Comparative Biomedical Sciences
Faculty of Veterinary Medicine
University of Teramo
Piazza Aldo Moro, Teramo, Italy
References
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