Mercury exposure and neurochemical impacts in bald eagles across several Great Lakes states

Abstract

In this study, we assessed mercury (Hg) exposure in several tissues (brain, liver, and breast and primary feathers) in bald eagles (Haliaeetus leucocephalus) collected from across five Great Lakes states (Iowa, Michigan, Minnesota, Ohio, and Wisconsin) between 2002-2010, and assessed relationships between brain Hg and neurochemical receptors (NMDA and GABA(A)) and enzymes (glutamine synthetase (GS) and glutamic acid decarboxylase (GAD)). Brain total Hg (THg) levels (dry weight basis) averaged 2.80 μg/g (range: 0.2-34.01), and levels were highest in Michigan birds. THg levels in liver (r(p) = 0.805) and breast feathers (r(p) = 0.611) significantly correlated with those in brain. Brain Hg was not associated with binding to the GABA(A) receptor. Brain THg and inorganic Hg (IHg) were significantly positively correlated with GS activity (THg r(p) = 0.190; IHg r(p) = 0.188) and negatively correlated with NMDA receptor levels (THg r(p) = -0245; IHg r(p) = -0.282), and IHg was negatively correlated with GAD activity (r(s) = -0.196). We also report upon Hg demethylation and relationships between Hg and Se in brain and liver. These results suggest that bald eagles in the Great Lakes region are exposed to Hg at levels capable of causing subclinical neurological damage, and that when tissue burdens are related to proposed avian thresholds approximately 14-27% of eagles studied here may be at risk.

Environmental contaminants in excrement of Iowa's nesting and wintering Bald Eagles (Haliaeetus leucocephalus)

“…For most contaminants there were several instances of samples with very high levels which may indicate that some Bald Eagles in our study area were being exposed to potentially toxic levels of contaminants. This is consistent with other studies on Bald Eagles in the Midwest that have documented elevated levels of contaminants such as lead and mercury in blood and tissue samples (Rutkiewicz et al 2011, Nam et al 2012, Warner et al 2014.…”

“…Following a ban on the use of DDT in 1972, complemented by conservation and recovery efforts, the Bald Eagle has undergone a spectacular recovery across much of its range including Iowa (Shepherd 2013). However, there are concerns about exposure of Bald Eagles to environmental contaminants, particularly lead and mercury (Rutkiewicz et al 2011, Haig et al 2014. The ingestion of spent lead ammunition, lead fragments embedded in food items, and lead fishing tackle has been linked to morbidity and mortality in several avian species including Bald Eagles (Fisher et al 2006, Cade 2007, Bedrosian et al 2012.…”

“…Like lead, mercury is a nonessential element. It is a toxin with documented negative effects on bird health including neurological and reproductive impairment (Eisler 1987, Rutkiewicz et al 2011. In an assessment of several bird species, Evers et al (2005) found that piscivores had the highest mercury levels and that Bald Eagles were species at particularly high risk in aquatic systems.…”

Assessing the neurotoxicological risk of methylmercury exposure for beluga whales (delphinapterus leucas) harvested in the Mackenzie Delta Estuary.

“…Previous studies have suggested that the interaction o f MeHg or inorganic Hg (iHg, Hg2+) with cysteine residues may inhibit, stimulate or damage components o f the dopaminergic (Gomes et al, 1976), cholinergic (Castoldi et al, 1996), GABAergic (Huang and Narahashi, 1997;Narahashi et al, 1994), and glutamatergic (Albrecht and Matyja, 1996) signaling pathways. In wildlife studies, MeHg exposure was associated with significant reductions in y-aminobutyric acid type A receptor (GABAa-R) binding in lab-exposed mink (Mustela vison) and decreased N-methyl-D-aspartate receptor (NMDA-R) binding in several brain regions of MeHg-exposed bald eagles (Haliaeetus leucocephalus) (Rutkiewicz et al, 2011;Scheuhammer et al, 2008), mink (Basu et al, 2007c), and polar bears ( Ursus maritimus) (Basu et al, 2009). It is believed that such MeHg-associated neurochemical changes may have consequences to the whole organism as these receptors are components o f the main inhibitory and excitiatory pathways in the central nervous system, and they play important roles in animal behaviour, memory and motor function (Popescu, 2005;Reis et al, 2009;Vicini and Ortinski, 2004).…”

“…Concentrations o f MeHg below ~ 2-5 mg kg"1 ww were likely below thresholds o f overt MeHg intoxication in mink (Suzuki, 1979;Wobeser et al, 1976). However, HgT concentrations measured in the central nervous system (CNS) o f Arctic beluga whales exceeded concentrations associated with neurochemical variation in wildlife and avian studies (Basu et al, 2005c;Basu et al, 2007b;Basu et al, 2007c;Basu et al, 2009;Basu et al, 2005e;Rutkiewicz et al, 2011;Scheuhammer et al, 2008).…”