Exposure to heavy metals and infectious disease mortality in harbour porpoises from England and Wales

We investigate whether long-term exposure to heavy metals, including immunosuppressive metals like mercury (Hg), is associated with infectious disease in a wild cetacean. Post-mortem investigations on 86 harbour porpoises, Phocoena phocoena, found dead along the coasts of England and Wales revealed that 49 of the porpoises were healthy when they died as a consequence of physical trauma (most frequently entrapment in fishing gear). In contrast, 37 porpoises died of infectious diseases caused by parasitic, bacterial, fungal and viral pathogens (most frequently pneumonia caused by lungworm and bacterial infections). We found that mean liver concentrations of Hg, selenium (Se), the Hg:Se molar ratio, and zinc (Zn) were significantly higher in the propoises that died of infectious disease compared to healthy porpoises that died from physical trauma. Liver concentrations of lead (Pb), cadmium (Cd), copper (Cu) and chromium (Cr) did not differ between the two groups. Hg, Se, and the Hg:Se molar ratio were also positively correlated with age. The association between Zn concentration and disease status may result from Zn redistribution in response to infection. Further work is required to evaluate whether chronic exposure to Hg may have presented a toxic challenge to the porpoises that succumbed to infectious disease.