Histologic, neurologic, and immunologic effects of methylmercury in captive great egrets

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/k g, wet weight) in fish. Low dosed birds were given methylmercury at concent rations comparable to current exposure of wild birds in the Everglades (Flo rida, USA). When compared with controls, low dosed birds had lower packed c ell volumes, dingy feathers, increased lymphocytic cuffing in a skin test, increased bone marrow cellularity, decreased bursal wall thickness, decreas ed thymic lobule size, fewer lymphoid aggregates in lung, increased perivas cular edema in lung, and decreased phagocytized carbon in lung. High dosed birds became severely ataxic and had severe hematologic, neurologic, and hi stologic changes. The most severe lesions were in immune and nervous system tissues. By comparing responses in captive and wild birds, we found that s ublethal effects of mercury were detected at lower levels in captive than i n wild birds, probably due to the reduced sources of variation characterist ic of the highly controlled laboratory study. Conversely, thresholds for mo re severe changes (death, disease) occurred at lower concentrations in wild birds than in captive birds, probably because wild birds were exposed to m ultiple stressors. Thus caution should be used in applying lowest observed effect levels between captive and wild studies.