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In vitro assays were developed for studies concerning the functioning of the immune system of the harbour seal (Phoca vitulina). Proliferative responses of peripheral blood mononuclear cells (PBMC) were measured after stimulation with different concentrations of the mitogens concanavalin A (Con A), pokeweed mitogen (PWM), phytohaemagglutinin (PHA) or lipopolysaccharide from Salmonella typhimurium (LPS). Con A and PWM induced strong proliferative responses, while PHA and LPS induced comparatively low proliferative responses. Responses of mitogen stimulated PBMC to recombinant human interleukin-2 (rhIL-2) and in vitro immunoglobulin production by mitogen stimulated PBMC were measured to discriminate between stimulation of T cells and B cells. It was found that Con A and PHA stimulate phocine T cells, PWM stimulates both T cells and B cells and LPS predominantly stimulates phocine B cells. Antigen-specific immune responses were measured after immunization of seals with an inactivated rabies vaccine and/or with tetanus toxoid. Antigen-specific proliferation of PBMC and the presence of antigen-specific antibody forming cells were demonstrated for both antigens in the PBMC of immunized animals. The responses measured in vitro correlated well with the development of specific serum antibody titers to these antigens.

During a recent disease outbreak among harbour seals (Phoca vitulina) in the North and Baltic seas, more than 17,000 animals have died. The clinical symptoms and pathological findings were similar to those of distemper in dogs. Based on a seroepizootiological study, using a canine distemper virus (CDV) neutralization assay, it was shown that CDV or a closely related morbillivirus (phocid distemper virus-PDV) was the primary cause of the disease. The virus was isolated in cell culture from the organs of dead seals and characterized as a morbillivirus by serology (immunofluorescence neutralization and enzyme-linked immunosorbent assays) and by negative contrast electron microscopy. Experimental infection of SPF dogs resulted in the development of mild clinical signs of distemper and CDV-neutralizing antibodies. The disease was reproduced in seals by experimental inoculation of organ material from animals that had died during the outbreak. However, seals that had been vaccinated with experimental inactivated CDV vaccines were protected against this challenge. This fulfilled the last of Koch's postulates, confirming that the morbillivirus isolated from the seal organs, was the primary cause of the disease outbreak. The recent demonstration of the presence of a similar virus in Lake Baikal seals (Phoca sibirica), which infected these Siberian seals 1 year before the northwestern European seals were infected, raises new questions about the origin of this infectious disease in pinnipeds.

Recently morbilliviruses were isolated from harbour seals (Phoca vitulina) in North West Europe (phocid distemper virus-1: PDV-1) and from Baikal seals (Phoca sibirica) in Siberia (phocid distemper virus-2: PDV-2) during outbreaks of severe disease which resembled distemper in dogs. PDV-1 and PDV-2 were passaged in SPF dogs, in which they caused distemper-like disease symptoms, and were subsequently passaged in Vero cells in which they caused cytopathic changes. PDV-1, PDV-2, and canine distemper virus (CDV) were compared with respect to their biological, morphological, physical, protein chemical, and antigenic properties. It was concluded that PDV-1 should be considered a newly recognized member of the genus Morbillivirus, whereas PDV-2 proved to be quite similar if not identical to CDV.

Two inactivated canine distemper virus (CDV) vaccines--an adjuvanted whole inactivated virus and a subunit ISCOM preparation--were tested for their ability to induce protective immunity in harbour seals (Phoca vitulina) against phocid distemper, a disease that recently killed greater than 17,000 harbour seals in the North and Baltic seas, and was shown to be caused by infection with a newly discovered morbillivirus, which is antigenically closely related to CDV. Four CDV seronegative harbour seals were vaccinated three times with the whole-virus vaccine, two with the ISCOM subunit vaccine and two were sham-vaccinated with an antigen-free preparation. Ten days after the last vaccination, when all six vaccinated animals had developed CDV neutralizing antibody titres ranging from 300 to 3000, all eight animals were challenged by the oculonasal and the peritoneal routes, with an organ suspension from dead seals. None of the six vaccinated animals developed clinical signs. The two sham-vaccinated seals died on days 14 and 18, respectively, after having shown a body temperature rise, respiratory symptoms and weight loss. In organs from both dead animals morbillivirus antigen was demonstrated with an enzyme-linked immunosorbent assay and an immunofluorescence assay. One of these two animals had developed a low titre of CDV-specific antibodies just before death. These data clearly indicate that seals can be protected from fatal challenge with the phocid distemper virus (PDV), by vaccination with certain inactivated CDV vaccines. They also reconfirm that infection with PDV should be considered the primary cause of the recent epizootic in seals.