Sunday, 6 April 2014

Does Hg pollution increase risk of infection?

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 Pollution112(1), 33-40.


  1. This quite an interesting investigation, and especially with the background that anthropogenic pollution could be an indirect reason for the death of these porpoises. When I read your summary I questioned the limitations of the study as well. However, given the fact that significantly higher concentrations of Hg were present in infected porpoises it is quite possible that this caused a toxic effect to the animals and weakened their metabolism and as a result their immune system.
    I also agree with you - it would have been very helpful if they investigated and identified the microbial species that were responsible for the infections.

    Moreover, I think there also should have been a broader examination of the microorganisms present in the gut. The gut system acts as the first defence in most animals and a "disturbed" gut microbiota lowers the immunity response of the organism, and in return weakens the barrier functions of the intestinal wall. May be a disturbed gut microbiota was responsible for the Hg to enter the liver from the intestine? So the actually the gut microbiota was resonsible for the accumulation of Hg in the livers. This could also have caused a lowered immune response to pathogenic microorganisms which have then caused the lethal infections. If this was true, then this leads to the question of what caused this "damaged gut microbiota", especially when there was no difference between individual animals from different sites/different age, etc.

    I read somewhere that some bacteria can transform inorganic mercury to methylmercury which is a lot more toxic than mercury itself - I think it is a neurotoxin. Perhaps some microbes in the intestines or elsewhere could have produced it. Have they only investigated the livers of the porpoises? May be it would have been also worth looking in other organs such as the brain to look for the methylmercury. It could be possible that this has was the cause of death in some animals, and perhaps the death in some animals that was not directly linked to a pathogenic infection.

    However these are all just thoughts I had, and probably not possible to test anymore. But I think definitely more research should have been done on this, since it was to investigate the death of 37 marine mammals.

    1. Definitely agree with these thoughts. From what we have learnt this year about the importance of a healthy gut microbiota, and the role these microbes play in our overall immune response - it seems only natural to sample when possible. However, am unsure of the effects a stranding might have? The authors did say they only used 'fresh' corpses, but I fear that once the process of decomposition begins this would definitely have some sort of effect on the natural microbe community.

      The authors did analyse renal tissue, but did not publish this data - I think due to the fact there they did not have a large enough sample size. They did mention that Cd concentrations were higher in the renal tissues, so I wonder if by only investigating one tissue type we are getting a skewed view of which pollutants link to an increased chance of infection.

  2. That is true, I haven't given that thought too much attention, but now that you mentioned it; it is very possible that parasites on the shore potentially carrying microbes could have "contaminated" the dead corpses as well and caused a change in the overall microbiota of the animals- may be a wrong cause of death was concluded from this even. The unpublished data is quite unsatisfactory, I would have liked so much more information about state of the animals!