Morbillivirus neutralising antibodies in Scottish grey seals Halichoerus grypus: Assessing the effects of the 1988 and 2002 PDV epizootics

Phocine distemper virus (PDV) may have killed a small number of grey seals Halichoerus grypus in European waters in 2002, as it was thought to have done in 1988. PDV is known to affect harbour seal population and distribution patterns, but grey seal pup production trends did not correlate consistently with PDV outbreaks. Numbers of known mothers missing from study colonies do not increase in PDV years and pre-weaning pup mortality is similar to that in other years. Pup growth rates are similar in PDV and non-PDV years. Therefore, no direct evidence links PDV outbreaks to changes in grey seal reproductive parameters at the population, colony, or individual level. Investigation of exposure of grey seals to PDV used CDV virus-neutralisation tests on sera collected from breeding seals pre-1988, 1988, 2001 and 2002. No positive sera (≥titre 1:64) were detected prior to 1988. In 1988, 2001 and 2002 the prevalence was 96, 59 and 83 %, respectively. In 2001, prevalence in 'old' mothers (of breeding age by 1988, ≥13 yr old) was 63 % compared to 0 % in 'young' mothers (born after 1989, < 13 yr old). In 2002, prevalence was 88 and 93 % in old and young mothers, respectively. Many pups were seropositive by the end of lactation (North Rona = 36%, Isle of May = 54%). Mean log₁₀ antibody titres increased during lactation and were correlated with the mothers' titre. The high prevalence of seropositives in both adult age groups from geographically separated colonies suggests widespread exposure to a morbillivirus after 2001. This implies that PDV is highly infective and that contact was widespread before the 2002 breeding season.