ABSTRACT
Marine mammals inhabiting polluted coastal waters are known to accumulate high concentrations of environmental contaminants, and this has been related to several abnormalities, including skeletal deformations, reproductive toxicity and hormonal alterations. Recent outbreaks of previously unidentified morbilliviruses have led to mass mortalities among several species of marine mammals. In 1988, approximately 20 000 harbour seals (Phoca vitulina), representing up to 60 per cent of local populations, and several hundred grey seals (Halichoerus grypus) died in north-western Europe due to an outbreak of a previously unknown virus (Mahy et al., 1988; Osterhaus and Vedder, 1988), which was subsequently named phocine distemper virus (PDV). Affected animals suffered from fever, cutaneous lesions, gastrointestinal dysfunction, nervous disorders and respiratory distress (Osterhaus and Vedder, 1989). Although the identification of a virus as the aetiological agent appeared to solve the puzzle as to the cause of the event, chemical contaminants were suspected as possible cofactors. The appearance and outcome of an epizootic reflects the sum of many interacting factors, including genetic background and diversity, social behaviour, population density, immunological memory (prior exposure), nutrition and the presence of the pathogen. Outbreaks of morbilliviruses among previously unexposed populations have been known to result in elevated mortality rates (Harder et al., 1995), but the additional stress of immunotoxic chemicals may exacerbate infection.
