Persistent, lipophilic polyhalogenated aromatic hydrocarbons (PHAHs) a
ccumulate readily in the aquatic food chain and are found in high conc
entrations in seals and other marine mammals. Recent mass mortalities
among several marine mammal populations have been attributed to infect
ion by morbilliviruses, but a contributing role for immunotoxic PHAHs,
including the polychlorinated biphenyls (PCBs), polychlorinated diben
zo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) was no
t ruled out. We addressed this issue by carrying out a semi-field stud
y in which captive harbour seals were fed herring from either the rela
tively uncontaminated Atlantic Ocean or the contaminated Baltic Sea fo
r 2 years, We present here an overview of results obtained during this
study, An impairment of natural killer (NK) cell activity, in vitro T
-lymphocyte function, antigen-specific in vitro lymphocyte proliferati
ve responses, and in vivo delayed-type hypersensitivity and antibody r
esponses to ovalbumin was observed in the seals fed the contaminated B
altic herring. Additional feeding studies in PVG rats using the same h
erring batches suggested that an effect at the level of the thymus may
be responsible for changes in cellular immunity, that virus-specific
immune responses may be impaired, and that perinatal exposure to envir
onmental contaminants represents a greater immunotoxic threat than exp
osure as a juvenile or adult. Together with the pattern of TCDD toxic
equivalents of different PHAHs in the herring, these data indicate tha
t present levels of PCBs in the aquatic food chain are immunotoxic to
mammals. A review of contaminant levels in free-ranging harbour seals
inhabiting polluted areas of Europe and North America suggests that ma
ny populations may be at risk to immunotoxicity. This could result in
diminished host resistance and an increased incidence and severity of
infectious disease.