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 the prevalence was 96, 59 and 83%, respectively. In 2001 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 log 10 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.
KEY WORDS: Morbillivirus · Serology · Epizootic · Phocine distemper virus · PDV · Grey seals
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 287: [241][242][243][244][245][246][247][248][249][250] 2005 from Europe, of which 3084 occurred in UK waters and approximately half were found in England (Hall et al. 1992).In 1988 PDV had a dramatic effect on the abundance and distribution of UK harbour seals, with estimated mortality rates of up to 52% in areas such as the Wash on the east coast of England (Thompson et al. in press). By contrast, local mortality rates in 2002 were not as high, with the Wash experiencing < 22% mortality (Thompson et al. in press). These temporal and regional differences in harbour seal mortality have been attributed to different exposures and/or different immunity levels within local harbour seal populations (Lawson & Jepson 2004).Both recent PDV outbreaks in Europe have shown that many harbour seals die from the effects of PDV, but the conspecific grey seal Halichoerus grypus population appeared to be affected to a much lesser extent (Hall et al. 1992, Lawson & Jepson 2004. While most of the harbour seal deaths in England were found to be due to PDV, only half of the Scottish mortalities tested positive for PDV (by RT-PCR or immunoperoxidase test, England 63/69, 91%; Scotland 6/12, 50%; Lawson & Jepson 2004). Similar findings were reported earlier f...