Abstract

Herpesvirus is implicated as the causative agent in a significant number of mortalities of harbor seal pups undergoing rehabilitation. In a seven year study (1990-1996), 46% of adrenal tissues collected from 193 animals had histologic evidence of PHV-1 infection characterized by multifocal areas of necrosis with cells containing intra-nuclear inclusions. Phocid herpesvirus, type-1 (PHV-1), an α-herpesvirus, was isolated in 1996 from adrenal tissues of affected harbor seals at The Marine Mammal Center (TMMC). The aim of this current project was to further establish PHV-1 as the causative agent of these deaths by monitoring the humoral immune response to PHV-1 in harbor seal neonates and correlating this response with histological lesions, clinical data and molecular evidence of PHV-1 infection. During the 1998 pupping season (February-July), serum samples were collected at weekly intervals from 91 harbor seal neonates admitted to TMMC. Using a newly developed indirect ELISA, PHV-1-specific immunoglobulin (IgG) was measured. In addition to serum, post-mortem tissue samples were also collected from animals that died. These samples were submitted for histologic evaluation and probed for the presence of PHV-1 genetic sequences by using a combination of PCR and Southern blotting.

The majority animals examined during January-April were seronegative, although some pups sampled at admission had low serum concentrations of PHV-1 specific IgG. Interestingly, these PHV-1-specific antibody concentrations declined over subsequent samples, suggesting that this antigen-specific IgG represents passively transferred antibody of maternal origin. During a 4 week period (late April, early May), dramatic increases in PHV-1-specific IgG were measured in a large number of animals. This correlated with the maximum incidence of adrenal lesions determined by histologic examination and viral presence in adrenal tissue by PCR. By late May, samples from all animals at TMMC were seropositive. This timing of seroconversion was independent of pup age and length of time that the animals had previously spent at the TMMC. To date, PCR of adrenal tissues has identified the presence of PHV-1 in seronegative, but not seropositive animals. This indicates that humoral immunity is important in the successful protection against PHV-1 infection of adrenal tissues. Collectively, these findings suggest that it may be possible to reduce the mortality of PHV-1 associated disease in harbor seal neonates by increasing the serum concentrations of PHV-1 specific IgG in at-risk animals.