Investigations on the development of the health status of harbour seals in the year of the PDV seal die-off in 2002

More than 21,500 harbour seals were killed by Phocine Distempter Virus in the North Sea and adjacent waters in 2002. Of those more than 3,340 seals were found dead on the coast of Schleswig-Holstein. After the second seal die-off had ceased, seals were captured alive for a health check and also stranded seals were examined for their health status. Blood status and chemistry tests were performed, microbiological, parasitological, histological, serological and cytological investigations were conducted.
During the first catch after the seal die-off animals seemed to be in clinically good condition although hematological disturbances (leucocytosis as well as leucopenia) were detected. Bacteriologically, Bordetella bronchisptica was isolated, a bacterium which was exclusively cultured during the seal die-off. At later catches hematological values normalized and also the normal bacterial flora was cultured from vagina and anus. The serological investigations for morbillivirus-specific antibodies revealed positive titers in 100% of the seals. Morphologically, there was no evidence for a continuing morbillivirus infection by both, histology and immunocytochemistry. The pathomorphological changes were similar to those before the epizootic including the predominance of parasitic and bacterial bronchopneumonia, emaciation and dermatitis.
Contrary to the first seal die-off in 1988/89 an increased number of large skin wounds on the ventral side and infections of the umbilicus were not seen. This may indicate that the seal population in Schleswig-Holstein was generally in better health condition at the second seal die-off compared to the first one. The continued investigations will allow a further assessment of the development of the seal population in the Wadden Sea.