Exposure to heavy metals and infectious disease mortality in harbour porpoises from England and Wales
Have been on the lookout for some papers supporting the theory that pollution may lead to immunosuppression, leading to an increased risk of disease and this is what I found.
The harbour porpoise (Phocoena phocoena) is undergoing a decline in population size in the southern North Sea and English factor. Many factors have been proposed, most importantly (for the purpose of our module) the question of whether environmental pollution is causing immunosuppression, hence increasing the likelihood of infection. As porpoises are at the top of their food chain they are at risk of bio-accumulating substances such as mercury (Hg)
Throughout 1990-1994 the authors carried out a systematic post-mortem investigation of 86 stranded porpoises in order to establish their cause of death. They only included freshly dead or moderately decomposed animals, and measured both the concentration of metals (Hg, Cr, Ni, Cu, Zn, Se, Cd and Pb) and the cause of death (physical trauma or infectious disease, the latter of which was ID’ed via histological, bacteriological or virological examination)
49 porpoises had died due to acute physical trauma, and were healthy at the time of death. These porpoises were used as controls. 37 porpoises died due to infectious diseases caused by bacterial, fungal and viral pathogens. The majority of the porpoises exhibited severe parasitic pneumonia which is commonly associated with a secondary bacterial infection. Seven porpoises died of generalised micro-parasitic infections (6 cases of bacterial septicaemia, 1 case of morbillivirus infection). I would have preferred some more specific information on what species were causing each infection, especially considering the various tests carried out on each body.
Liver concentrations of Hg, Se and Zn were significantly higher in porpoises that died of an infectious disease when compared to ‘control’ animals (those who had perished due to trauma). However Cu, Cr, Ni, Pb and Cd concentrations did not differ between the two groups.
Little is known about the effects of Zn on the marine mammal immune system, but in humans and rodents Zn is essential for the development and functioning of the immune system. The higher Sn concentrations in infected organisms correlated with Hg levels, this is due to the fact that marine mammals have a mechanism to deal with Hg toxicity, which involves an antagonistic interaction between Se and Hg.
Hg concentration did not vary/ was not associated with differences in sex, region found, season/year of death, state of decomposition and storage method although Hg concentration has been previously reported to increase with age, due to long-term bioaccumulation. The authors concluded that ther was a definite correlation between Hg levels and cause of death (infection) although could not prove any causation (See attached Figure).
Overall the authors are aware of their studies limitations, and concluded that they could not reject the hypothesis that ‘Hg exposure may influence health and contribute to mortality’. I believe this study is a good basis with which to continue our research into answering the question of whether Hg exposure/pollution can lead to an increased risk of infection due to a lowered immune system. I was also impressed by their method of study, this is a difficult question to answer due to the problematic nature of getting data (How do you catch a group of live porpoises, test them for infection and take Hg samples?), but by using the information the sea freely offers (stranded porpoises) we can glean information that will help us understand the big picture.
Bennett, P. M., Jepson, P. D., Law, R. J., Jones, B. R., Kuiken, T., Baker, J. R., ... & Kirkwood, J. K. (2001). Exposure to heavy metals and infectious disease mortality in harbour porpoises from England and Wales. Environmental Pollution, 112(1), 33-40.