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We conducted a dose-response laboratory study to quantify the level of exposure to dietary Hg, delivered as methylmercury chloride (CH3HgCl), that is associated with suppressed immune function in captive-reared common loon (Gavia immer) chicks. We used the phytohemagglutinin (PHA) skin test to assess T-lymphocyte function and the sheep red blood cell (SRBC) hemagglutination test to measure antibody-mediated immunity. The PHA stimulation index among chicks receiving dietary Hg treatment did not differ significantly from those of chicks on the control diet (p = 0.15). Total antibody (immunoglobulin [Ig] M [primary antibody] + IgG [secondary response]) production to the SRBC antigen in chicks treated with dietary methylmercury (MeHg), however, was suppressed (p = 0.04) relative to chicks on control diets. Analysis indicated suppression of total Ig production (p = 0.025 with comparisonwise alpha level = 0.017) between control and 0.4 microg Hg/g wet food intake treatment groups. Furthermore, the control group exhibited a higher degree of variability in antibody response compared to the Hg groups, suggesting that in addition to reducing the mean response, Hg treatment reduced the normal variation attributable to other biological factors. We observed bursal lymphoid depletion in chicks receiving the 1.2 microg Hg/g treatment (p = 0.017) and a marginally significant effect (p = 0.025) in chicks receiving the 0.4 microg Hg/g diet. These findings suggest that common loon chick immune systems may be compromised at an ecologically relevant dietary exposure concentration (0.4 microg Hg/g wet wt food intake). We also found that chicks hatched from eggs collected from low-pH lakes exhibited higher levels of lymphoid depletion in bursa tissue relative to chicks hatched from eggs collected from neutral-pH lakes.

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We determined the distribution and accumulation of Hg in tissues of common loon (Gavia immer) chicks maintained for up to 15 weeks on either a control diet with no added methylmercury chloride (MeHgCl) or one containing either 0.4 or 1.2 microg Hg (as MeHgCl)/g wet-weight food. Total Hg and MeHg tissue concentrations were strongly positively correlated (r2 > 0.95) with the amount of Hg delivered to individual chicks throughout the course of the experiment. The pattern of differential Hg concentration in internal tissues was consistent within each treatment: Liver > kidney > muscle > carcass > brain. Feather Hg concentrations were consistently higher than those of internal tissues and represented an important route of Hg elimination. Feather mass accounted for 4.3% +/- 0.1% (average +/- standard error) of body mass, yet 27.3% +/- 2.6% of total Hg intake was excreted into feathers. Our calculations indicate that 26.7% +/- 4.9% of ingested Hg was not accounted for and, thus, either was never absorbed or was absorbed and subsequently eliminated in feces. With the additional excretion into feathers, 54% of ingested Hg was excreted. Demethylation was evident in the liver at all treatment levels and in the kidneys of chicks dosed at 1.2 microg Hg/g. Mercury concentrations were strongly positively correlated (r2 > or = 0.95) among internal tissues and with blood Hg concentration. Mercury concentrations of secondary feathers were moderately correlated (r2 = 0.82-0.93) with internal tissues. We supply regression models that may be used to provide perspective and a useful means of interpreting the variety of measures of Hg exposure reported in the literature.