Round table on morbilliviruses in marine mammals

Molecular analysis of parapoxvirus from a spotted seal Phoca largha in Japan

A spotted seal Phoca largha with nodular and scab lesions on the whole body was brought to an aquarium in Nagoya, Japan. We extracted DNA from the lesions and used the polymerase chain reaction (PCR) method for detecting orthopoxvirus and parapoxvirus DNA. Parapoxvirus but not orthopoxvirus DNA was detected. The partial nucleotide sequence of the envelope gene was determined from the PCR product, and the sequence was seen to be closely related to 2 parapoxvirus strains from spotted seals in Alaska, showing 100% identity at the amino acid level, with one nucleotide substitution. Virus-neutralizing (VN) antibody against canine distemper virus (CDV) was not detected in the serum, indicating that this individual was not infected with CDV or phocine distemper virus (PDV), which both have a high mortality rate for marine mammals. These results suggest that the lesions were caused by infection with pinniped parapoxvirus, and that the viruses spread and are maintained within the habitat range or populations of spotted seals from the Bering Sea to the Japan Sea. This is the first report of molecular analysis of parapoxvirus in marine mammals in Japan.

Histologic and immunocytochemical characterization of canine distemper-associated metaphyseal bone lesions in young dogs following experimental infection

The proximal metaphyses of the humerus of weanling gnotobiotic dogs experimentally infected with canine distemper virus (CDV) were investigated histologically and immunocytochemically between 4 and 41 days after infection. Viral antigen was demonstrated in hematopoietic marrow and bone cells at postinfection day (PID) 5 and PID 7, respectively. Between PID 8 and 27, CDV antigen was abundantly present in marrow cells, osteoclasts, and osteoblasts and less frequently in osteocytes. Immunopositive cells in both osseous tissues and bone marrow declined between PID 29 and PID 36 and were absent by PID 41. Chondrocytes of the growth plate were negative for viral antigen throughout the observation period. In bone, viral antigen was more frequently observed in bone cells of the primary spongiosa than in the secondary spongiosa. There was a strong correlation between occurrence of CDV antigen and osseous changes. Associated metaphyseal bone lesions were mild and most prominent between PID 8 and PID 32. Lesions consisted of necrosis of osteoclasts, which was associated with subsequent persistence of the primary spongiosa (growth retardation lattice). Atrophy and necrosis of osteoblasts and marrow cells were also noted. Infection of metaphyseal bone cells appears to be common in young dogs with experimental systemic distemper. Bone cell infection is preceded by infection of marrow cells, and infected bone cells may experience degeneration and necrosis. This subtle viral effect may result in defects in bone modeling in CDV-infected dogs.

Intestinal lesions in experimental phocine distemper: light microscopy, immunohistochemistry and electron microscopy

The involvement of the intestinal mucosa and of the gut-associated lymphoid tissue in phocine distemper was studied in six severely diseased harbour seals 11 to 16 days after experimental infection. Five seals exhibited a mild or moderate enteritis in the small or large intestine. In all the seals, a moderate to severe depletion of submucosal lymphoid follicles was found. Likewise, antigen of phocine distemper virus (PDV) was demonstrated immunohistochemically in the intestinal wall of all the seals. Most antigen was found in the submucosal lymphoid follicles, followed by the crypt epithelium and follicle-associated epithelium (FAE). Ultrastructurally, intracytoplasmic tubular structures were detected in the FAE and interpreted as morbilliviral nucleocapsids. The results indicate a direct cytopathogenic effect of PDV on intestinal lymphoid and epithelial cells and suggest an important role of the intestinal tract in phocine distemper and, by analogy, in other morbillivirus infections as a regular site of virus replication, virus shedding and immunosuppression.