The role of
morbillivirus infection as a cause of disease or death in harbour porpoises (Phocoena phocoena) from the German North and Baltic Sea was investigated by serology, histology and immunohistochemistry. Blood and tissue samples of lung, brain and lymph nodes from 74 stranded or by-caught harbour porpoises from German waters were collected between 1991 and 1997. According to dentinal growth layers and body length, animals were grouped into four age classes (neonates, 0-1, 1-4, 4-16 years of age).
Formalin-fixed,
paraffin-embedded sections were stained by
hematoxylin and
eosin (HE). Immunohistology was done in all lung tissues using the
avidin-
biotin-
peroxidase technique and a polyclonal canine distemper virus (CDV)
nucleoprotein-specific antibody, which cross-reacts with porpoise morbillivirus (PMV)
antigen. A virus neutralization assay for detection of (Onderstepoort-strain) CDV- and PMV-specific
antibodies was performed. Due to the cytotoxicity of some sera, only titres of 1:20 or greater were considered positive. PMV or CDV-specific
neutralizing antibody titres were found in 88 and 50% of the animals, respectively. Titres were always highest against PMV indicating
infection with a homologous porpoise virus strain. There were no significant differences in
neutralizing antibody titres between animals of the different age groups. No histological lesions specific for
morbillivirus infection were detected and by immunohistology all cases were negative for morbillivirus
antigen. The absence of morbillivirus
antigen and the lack of characteristic morbillivirus-specific lesions showed that
morbillivirus infection was not a cause of death or illness in the investigated population. However, the high incidence of PMV-specific
antibodies in all age groups indicated a continuous spread of
infection with a morbillivirus among harbour porpoises from the German Baltic and North Sea.