Effects of methylmercury exposure on the immune function of juvenile common loons (Gavia immer)

Abstract

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.

Toxicity Reference Values and Tissue Residue Criteria for Protecting Avian Wildlife Exposed to Methylmercury in China

“…The effects of MeHg on growth (Kenow et al 2003 ) , behavior (Kenow et al 2010 ) , immune function (Kenow et al 2007a ) , and the biochemical index (Kenow et al 2008 ) of juvenile common loons were investigated during which individuals were exposed for 15 weeks. Neither adverse effects on growth or survival occurred in loons that were exposed to three doses, 0.1, 0.5, or 1.5 m g MeHg/g (wwt) (Kenow et al 2003 ) , nor behavioral effects at doses of 0.08, 0.4, or 1.2 m g MeHg/g (wwt) (Kenow et al 2010 ) .…”

“…Neither adverse effects on growth or survival occurred in loons that were exposed to three doses, 0.1, 0.5, or 1.5 m g MeHg/g (wwt) (Kenow et al 2003 ) , nor behavioral effects at doses of 0.08, 0.4, or 1.2 m g MeHg/g (wwt) (Kenow et al 2010 ) . However, adverse effects on immune function (Kenow et al 2007a ) and effects related to oxidative stress and altered glutathione metabolism (Kenow et al 2008 ) occurred at 0.4 and 1.2 m g MeHg/g (wwt). Using a bm of 4.67 kg for common loon adults (Barr 1996 ;Dunning 1993 ) , a food intake rate of 0.839 kg/day was derived from the allometric equation for carnivorous birds (Nagy 2001 ) .…”

“…When this value was used to derive a TRV, and a UF L of 1.0, a UF S of 1.0 and a UF A of 3.0 were considered; the resulting avian dietary-based TRV would be 4.7 ng MeHg/g (bm)/day, which is similar to the value derived from the MeHg toxicity to white ibis. The UF L was set to 1.0 because the common loons study provided a NOAEL rather than a LOAEL (Kenow et al 2007a ) . According to pharmacokinetic studies of absorption and elimination of MeHg in common loon chicks (Fournier et al 2002 ) , the result of a study of common loons, with a duration of 15 weeks, was accepted as being a chronic exposure study.…”

“…According to pharmacokinetic studies of absorption and elimination of MeHg in common loon chicks (Fournier et al 2002 ) , the result of a study of common loons, with a duration of 15 weeks, was accepted as being a chronic exposure study. Thus, a UF S of >1.0 was deemed to be unnecessary (Kenow et al 2007a ) . Although the study by Kenow et al ( 2007a ) did not provide information on the most ecologically relevant endpoint, which is reproduction, the common loon is a piscivorous wading bird and is the second most sensitive avian species to MeHg.…”

“…Thus, a UF S of >1.0 was deemed to be unnecessary (Kenow et al 2007a ) . Although the study by Kenow et al ( 2007a ) did not provide information on the most ecologically relevant endpoint, which is reproduction, the common loon is a piscivorous wading bird and is the second most sensitive avian species to MeHg. Applying a UF A of 10 is likely to be overly conservative.…”