The disease that killed more than 18 000 harbour seals Phoca vitulina and a small number of grey seals Halichoerus grypus in the North Sea, the Kattegat-Skagerrak and the southern Baltic in 1988 has now been well described for all afflicted areas, and the pathological and virological findings are In good concordance from all seal groups studied. The descriptions vary in detail but render l~ttle doubt that the disease had identical characteristics, and that a newly recognised virus, phoclne

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... mper virus (PDV), of the genus A4orbillivirus, was the primary cause of the disease. A number of viral and bacterial agents as well as parasites were identified in the dead seals, but none of them seem to be of primary importance for the development of the disease syndrome. The high mortality can probably b e explained by the fact that a highly pathogenic vlrus was introduced into a n a v e population with no specific immunity to the infectious agent. It is also possible that factors such as organochlorine pollution or crowding of seals a t haul-out sites may have exacerbated the impact of the disease in some areas. However, there are currently insufficient data to determine the potential role of these other factors in the severity of the outbreak. Antibodies to infections by canine distemper virus (CDV)-like viruses are present in both North Atlantic and Antarctic pinnipeds, but the immunological response is usually distinct from that to PDV. The pathogenicity of these infections is unknown, but likely less than for the virus that infected North Sea harbour seals in 1988. However, harp seals Phoca groenlandica a r e frequently infected by PDV and they have recently invaded the North Sea where they have been observed in the vicinity of harbour seal haul-out sites. Hence, migrating harp seals are a plausible vector of the virus. PDV has been transmitted to terrestrial carnivores causing distemper outbreaks in Danish farmed mink Mustela vison in 1989, and an epizootiological link cannot be excluded. There a r e no ep~zoot~ological links between the seal epizootic in North Western Europe In 1988-89 and the disease outbreaks in Baikal seals Phoca sibirica and high Arctic sled dogs in 1987-88.