The occurrence of poxviruses in cetaceans has been well documented for decades. However, there are few molecular data of cetacean poxviruses. This report describes poxvirus infection in a rough-toothed dolphin (Steno bredanensis) with serious fungal dermatitis confirmed by pathological examination and PCR.
A subadult rough-toothed dolphin stranded alive on the western shore of Taiwan in July 2006. The dolphin died after 3-week medical treatment for pneumonia, malnutrition and skin lesions at the aquarium. During the first week, slightly raised firm, tan nodules with round or elliptical light gray border were noted on the skin of head, flippers, dorsal fin, and fluke. Several skin samples, including epidermis and dermis, were excised and fixed in neutral 10% buffered formalin. Microscopic examination revealed mild hyperplasia, ballooning degeneration and necrosis of epidermal epithelial cells. There were few foci of epidermal erosion, focal necrosis of the superficial dermis and microabscesses of epidermis. Fibrin, necrotic cellular debris and degenerate neutrophils were found in superficial epidermal layers and lining the erosive areas.
The nodular lesion then rapidly progressed to become necrotic, ulcerated, and edematous. In the last week, the affected skin became thick and sloughing. After its sudden death, the necropsy was performed immediately. Heavy cestode infestation in intestine, dilated ventricles, ascites, hydropericardium, serous atrophy of subcutaneous and internal body fats and were noticed. The somatic lymph nodes were enlarged to two- to three-times their normal sizes and congestive. Representative skin and other tissue specimens were frozen in -20°C or fixed in neutral 10% buffered formalin following by routine process and stain with haematoxylin and eosin for light microscopic examination. Microscopically, the lesions were composed of marked epidermal hyperplasia, ballooning degeneration, pustule formation, and subacute inflammation of the underlying dermis. Throughout the affected foci, epithelial cells contained intracytoplasmic eosinophilic inclusion bodies, and there were 5-7 micron fungal hyphae within the epidermis and overlying serocellular crusts. In pustule areas, the fungal hyphae penetrated through the superficial surface of the epidermis with focal invasion of underlying dermis. Large intradermal infiltrates of neutrophils, histiocytes, and scattered focal hemorrhages were present throughout the superficial dermis.
Frozen skin samples were processed to extract total DNA using the DNeasy kit. We amplified the DNA polymerase gene fragments of the expected size and proved that total DNA from these lesion contained poxvirus genomic DNA. The sequences of poxvirus DNA polymerase gene from this dolphin showed high homology with previously described cetacean poxvirus 1 (91~97%) and cetacean poxvirus 2 (84%). A phylogeny based on poxvirus DNA polymerase gene sequences of cetaceans from GenBank and this study showed the sequence from the rough-toothed dolphin and cetacean poxvirus 1 formed a monophyletic group separating from cetacean poxvirus 2 with strong bootstrap support (100%).
The prevalence of poxvirus infection in cetaceans of the western Pacific is unclear. Previous studies indicate that poxvirus infection is likely endemic in some cetacean species, and poxvirus-affected dolphins may be associated with stressful conditions such as poor water quality, drastic drop in water temperature, and environmental contaminant accumulation. Investigation of poxvirus infection in cetaceans of western Pacific may offer a path for our understanding the health status of cetaceans and their habitats.