Histologic, Neurologic, and Immunologic Effects of Methylmercury in Captive Great Egrets

Abstract

Captive great egret (Ardea albus) nestlings were maintained as controls or were dosed with methylmercury chloride at low (0.5), and high doses (5 mg/kg, wet weight) in fish. Low dosed birds were given methylmercury at concentrations comparable to current exposure of wild birds in the Everglades (Florida, USA). When compared with controls, low dosed birds had lower packed cell volumes, dingy feathers, increased lymphocytic cuffing in a skin test, increased bone marrow cellularity, decreased bursal wall thickness, decreased thymic lobule size, fewer lymphoid aggregates in lung, increased perivascular edema in lung, and decreased phagocytized carbon in lung. High dosed birds became severely ataxic and had severe hematologic, neurologic, and histologic changes. The most severe lesions were in immune and nervous system tissues. By comparing responses in captive and wild birds, we found that sublethal effects of mercury were detected at lower levels in captive than in wild birds, probably due to the reduced sources of variation characteristic of the highly controlled laboratory study. Conversely, thresholds for more severe changes (death, disease) occurred at lower concentrations in wild birds than in captive birds, probably because wild birds were exposed to multiple stressors. Thus caution should be used in applying lowest observed effect levels between captive and wild studies.

Patterns and Interpretation of Mercury Exposure in Freshwater Avian Communities in Northeastern North America

“…Particular emphasis has been placed on larger species for which trophic status is potentially greatest; such species include the common loon (Meyer et al, 1995Burgess et al, 1998a, b;Evers et al, , 2003Scheuhammer et al, 1998bScheuhammer et al, , 2001Fevold et al, 2003;Burgess and Hobson, in press;Champoux et al, in press), bald eagle (Grier, 1974;Wiemeyer et al, 1984;Frenzel and Anthony, 1989;Bowerman et al, 1994;Anthony et al, 1999;Bowerman et al, 2002), osprey (Cahill et al, 1998;DesGranges et al, 1998), wading birds (Gariboldi et al, 1998;Bouton et al, 1999;Spalding et al, 2000b;Henny et al, 2002), and seabirds (Braune, 1987;Burger and Gochfeld, 1995;Furness, 1995, 1997;Braune et al, 2001). Other foodweb pathways important for MeHg transfer are generally of lesser concern because trophic status of the endpoint species is generally lower than piscivores.…”

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

“…After 9 weeks of exposure, appetite and mass declined signi fi cantly in both dosed groups (Spalding et al 2000a ) . Adverse effects, related to immune function, were observed in individuals fed the lesser dose, whereas individuals fed the greater dose exhibited adverse effects on tissues related to immune and nerve functions (Spalding et al 2000b ) . In a study of the biochemical effects of MeHg (Hoffman et al 2005 ) , only activities of the enzyme aspartate aminotransferase (AST) in blood plasma, and thiobarbituric acid-reactive substances (TBARS) in liver of individuals fed the lesser dose were signi fi cantly greater than those of the control group.…”

“…Several different effects, including behavior (Bouton et al 1999 ) , survival, growth and accumulation in tissues (Spalding et al 2000a ) , histology, neurology, and immunology (Spalding et al 2000b ) from exposure to MeHg, were addressed in three sladies, in which juvenile great egrets were exposed to two dietary doses of 0.5 or 5 m g MeHg/g (wwt) for 12 weeks. Severe ataxia was observed in individuals fed the greater dose, whereas the lesser dose produced effects on activity, a tendency to seek shade, and motivation to hunt prey (Bouton et al 1999 ) .…”

Avian Resources of the Northern Gulf of Mexico

“…The toxic effects of methylmercury, particularly reproductive and neuro-behavioral deficits, have been demonstrated in the laboratory (Heinz 1979;Spalding et al 2000b) and in the field (Frederick et al 1999). Mercury levels in eggs from some Great Egrets in the Everglades exceeded effects levels found in the laboratory (Rumbold et al 2001).…”