EPIZOOTIOLOGY OF MORBILLIVIRUS INFECTION IN NORTH-AMERICAN HARBOR SEALS (PHOCA-VITULINA) AND GRAY SEALS (HALICHOERUS-GRYPUS)

A longitudinal study of morbillivirus infection among harbor (Phoca vi tulina) and gray (Halichoerus grypus) seals on the Atlantic coast of N orth America was carried out between 1980 and 1994. Serology also was carried out on harbor seals from the Pacific northwest coast collected in 1992 and 1993. The prevalence of morbillivirus neutralizing antibo dies was significantly (P < 0.0001) higher in gray (73%, n = 296) than in harbor seals (37%, n = 387) from the Atlantic. Titers were signifi cantly (P < 0.0001) higher against phocine distemper (PDV) compared to any other morbillivirus. Antibodies were not detected in serum from P acific harbor seals. During the winter of 1991 to 1992 an epizootic oc curred among harbor seals on the northeast coast of the United States. The event was characterized by an increase in strandings and by a sig nificant (P = 0.001) increase in PDV antibody prevalence to 83% (n = 3 6) in seals stranded that winter. Morbillivirus lesions and antigen we re observed in six animals found stranded from southern Maine to Long Island, New York (USA), between November 1991 and April 1992. In addit ion, morbillivirus encephalitis was detected in tissues from a harbor seal that stranded in 1988. Enzootic infection appeared to be present in both seal species, although with a different prevalence of disease. We propose that enzootic infection among gray seals is facilitated by population size, high annual recruitment and innate resistance to cli nical disease. Infection may be maintained in the smaller harbor seal population through casual contact with gray seals.