Effects of dietary methylmercury on liver and kidney histology in the neotropical fish Hoplias malabaricus

Abstract

Methylmercury is a potent toxic present in Amazonian fish species due to gold mining activities. In the present work, we investigated the morphological effects of methylmercury in liver and kidney of Hoplias malabaricus feeding contaminated prey fish over 70 days. Two groups of nine mature fish (tested and control) were acclimatized for four weeks to laboratory conditions and then the tested group fed prey fish previously contaminated at an additional level of 0.075 μg MeHg g⁻¹ at 5-day intervals and over 14 successive intervals whereas control group fed uncontaminated fish. H. malabaricus specimens were then dissected for chemical and morphological analyses. The low and realistic level of MeHg in the prey fish induced a low increase of total mercury in liver (1.8-fold) and muscle (2.2-fold). The biomagnification factor (Hg in predator/Hg in prey) reached 142 in liver and 21 in muscle and was indicative of a relatively fast contamination of internal organs by dietary exposure. The liver of exposed individuals presented leukocyte infiltration, increased number of melano-macrophage centers, necrotic areas and lesions in Disse's space. Evident disorder and chaos in cytoskeleton organization suggest a strong toxic effect in hepatocytes, such as organelles positioning and movement, vesicles traffic and secretion. Head kidney showed large necrosis areas, increased number of melano-macrophages centers, phagocytic areas, intercellular space among parenquimal cells and atypical cells. Injuries and damages to tissues suggest too slow defense mechanisms to immobilize or eliminate ingested methylmercury, demonstrating that the sensitivity of fish cells to methylmercury exposure is higher than it has been previously described in the literature.

Introduction

Human and animal exposures to mercury are mostly caused by wet and dry precipitations generated by atmospheric releases of this heavy metal by natural sources such as forest fires and human activities such as coke burning in power plants (Anderson et al., 2004). High levels of total mercury in muscle of carnivorous fish species from Amazonian waters are mainly caused by gold mining and have been previously reported exceeding maximum levels allowed by Brazilian and international legislation (WHO, 1990), a safety limit for human consumption (Pfeiffer et al., 1989, Pfeiffer et al., 1991; Malm et al., 1995; Souza Lima et al., 2000; Santos et al., 2002). According to Santos et al. (2002), aquatic predators located at the top of the trophic chain represent an important link between mercury pollution and human health.

The adverse effects of methylmercury (MeHg) were first recognized in the early 1970s following the pollution of Minamata Bay, Japan (Tsubaki and Takahashi, 1977) and shortly thereafter a massive human poising that occurred in Iraq (Bakir and Damluji, 1973). This organometallic compound is a highly lipophilic environmental contaminant derived from inorganic mercury by bacterial activity which easily crosses the blood barrier. The primary route of exposure for fish is through ingestion of contaminated food (Oliveira Ribeiro et al., 1996, Oliveira Ribeiro et al., 1999; Limke et al., 2004; Drevnick et al., 2006). One of the major issues complicating the risk assessment of MeHg is its large number of potential molecular targets within the body due to SH reactive groups in several proteins and membrane constituents (Vassallo et al., 1996).

In north of Brazil and particularly in the Amazonian region where mercury is still used for gold mining activities (Pfeiffer et al., 1989; Bidone et al., 1997), the species Hoplias malabaricus is largely used in the daily human consumption (Grandjean et al., 1999). MeHg is easily biomagnified along the food chain in aquatic ecosystems and top predator carnivorous species become the main route of mercury uptake by human populations causing neurotoxic effects (Lebel et al., 1998; Aschner, 2002).

Fish tissues are sensitive indicators of aquatic pollution and have a high mercury bioaccumulation capacity for both organic and inorganic forms (Gochefeld, 2003). Although many reports on mercury distribution and speciation, accumulation and effects in non-tropical freshwater fish species are available, data on mercury in tropical fish and its toxic effects on fish tissues and organs are scarce (Oliveira Ribeiro et al., 2002). Although damages have been observed in the gill arches, liver, kidney, blood parameters, olfactory epithelium and nervous system (Julshammn et al., 1982; Choi, 1989; Baatrup 1991; Oliveira Ribeiro et al., 1995, Oliveira Ribeiro et al., 1996, Oliveira Ribeiro et al., 2002, Oliveira Ribeiro et al., 2006), there is little available information related to the response of fish exposed to MeHg from its diet, especially for tropical fish species, and only recently Drevnick et al. (2006) reported the effects of MeHg on Pimephales promelas ovarian follicular after dietary exposure.

The exposure to chemical contaminants can induce a number of lesions and injuries to different fish organs (Oliveira Ribeiro et al., 2006) but liver and kidney represent important target organs suitable for histopathological examination in searching for damages to tissues and cells (Rabitto et al., 2005; Oliveira Ribeiro et al., 2006). As an example, melano-macrophages centers (MMCs) have been described in teleost fish as a discrete aggregation of closely stacked cells, or as large cells containing heterogeneous inclusions (Wolke et al., 1985; Herraez and Zapata, 1986) and have been used in liver and kidney as biomarkers for fish health or in diagnostic studies of field contamination (Leknes, 2001). MMCs are found in the dermis, hypodermis, kidney, spleen, liver and small groups of cells along the blood and lymph vessels in fishes (Agius and Roberts, 1981), removing by phagocytosis foreign particles or products from cell degradation (Wolke et al., 1985).

In the present work, morphological features were examined in liver and head kidney of H. malabaricus after exposure to sub-chronic dietary doses of MeHg over a period of 10 weeks. This paper is first to report lesions and damages in target organs after an experimental dietary sub-chronic exposure to MeHg in Brazilian tropical species. Our experimental approach closely mimicked the contamination conditions encountered in natural freshwater tropical ecosystems, increasing our biological knowledge about this tropical species and the toxicity of MeHg in fishes inhabiting Amazon polluted regions.