Impaired immunity in harbour seals [Phoca vitulina] fed environmentally contaminated herring.

In recent years, mass mortalities among seals and dolphins have been attributed to infections with different morbilliviruses. In all cases, these marine top predators were exposed to high levels of persistent lipophilic environmental contaminants accumulated through the food chain. This observation led to the hypothesis that a contaminant-related suppression of the immune system might have contributed to the severity of the virus outbreaks. We conducted a semi-field feeding experiment, in which we fed two groups of harbour seals (Phoca vitulina) fish with different levels of contaminants. During a period of 2 1/2 years, blood samples were taken at regular intervals, and the functioning of different compartments of the immune system was monitored and compared. We found impaired natural killer (NK) and specific T cell responses in the seals fed contaminated fish. This is the first demonstration of immunosuppression in mammals following chronic exposure to environmental contaminants at ambient environmental levels.