Histopathology of the fish Corydoras paleatus contaminated with sublethal levels of organophosphorus in water and food
Introduction
Folidol 600 is a commercial pesticide that contains 600 g/L of the organophosphate (OP) methyl parathion. It is widely used as insecticide in food storage shelters (Rabindra et al., 1988) and in agriculture. It has been also used in fish-farming tanks to eliminate aquatic larval stages of insects. All these procedures may cause different levels of environmental contamination often affecting nontarget organisms (Silva et al., 1993; Fanta, 1997b; Rodrigues and Fanta, 1998).
Many pesticides are not effective, and exhibit low toxicity, before they are metabolically transformed into active toxic substances (Loomis and Hayes, 1996). Methyl parathion is one such product, becoming highly toxic when transformed into paraoxon, by oxygenation at the endoplasm reticule of hepatocytes (DuBois et al., 1949; Loomis and Hayes, 1996). Consequently, the liver is the organ that suffers serious morphological alterations in fish exposed to pesticides (Anees, 1978; Bender, 1969; Gill et al., 1988; Rodrigues and Fanta, 1998). Retention of the OP in the liver for days or months after intoxication opposes the usual opinion that such pesticides are quickly degraded in nature (Ansari and Kumar (1987a), Ansari and Kumar (1987b); Murty, 1986). In reality, the oxygenated analog of malathion that is quickly hydrolyzed in mammals, becoming inactive, is hydrolyzed very slowly in fish (Areechon and Plumb, 1990). Repeated pollution of streams and lakes by OP pesticides and the persistence of these products in the environment long enough to be concentrated in organisms that fish feed on may cause their contamination even at some distance from the source of pollution (Bender, 1969).
The majority of research done with organophosphorus pesticides is based on their lethal effects. However, for an early warning about the harmfulness of these compounds, knowledge of the sublethal effects and their ecological implications is important. Diagnosis and prediction of physiological consequences of sublethal contamination can be obtained through histopathology (Anees, 1978; Bender, 1969; Fanta, 1997b; Rodrigues and Fanta, 1998; Rudolph and Boje, 1987; Silva et al., 1993).
C. paleatus was chosen as test organism because it is a very common fish in rivers or lakes of southern and southeastern Brazil, as well as northern Argentina (Gomez, 1992; Santos, 1981), where intense agricultural and fish culture activities are carried out and where the use of Folidol 600 is ordinary practice. It is also small and easy to keep in the laboratory. Active during the day, C. paleatus are opportunistic feeders with a preference for dipteran larvae and nonplanktonic microcrustacea, indicating that this species feeds close to the bottom (Aranha et al., 1993; Kramer and McClure, 1981). Consequently, fish like C. paleatus may absorb OP pesticides not only from water, but also from food.
This study was focused on acute tests with the aim of evaluating sublethal effects of the OP methyl parathion in those tissues that are responsible for its absorption, respectively, the gills (from water), intestine (from food), and liver, which is mainly responsible for the metabolism of toxic substances in general, including xenobiotics. Another purpose was to define which absorption route would possibly be more dangerous for the fish, impairing sooner or more intensely the function of some vital tissues, with physiological and behavioral consequences. Reduction in the state of health of each individual would suggest the ecological consequences and interference with the local food chain, caused by levels of contamination that did not kill the fish in the short term, usually being considered safe.
Section snippets
Materials and methods
Tests were conducted in the laboratory with the adult freshwater fish C. paleatus (Callychthidae) obtained from rivers of southern Brazil, in a region without any agricultural activity, and transported to the laboratory for acclimation. They were maintained in 100-L storage tanks with filtered, dechlorinated, and constantly aerated tap water (mean temperature of 24.5°C, photoperiod 8L/16D), fed daily with flocculated food (Alcon), 1 h after the start of the light period.
Healthy individuals (n
Control gills
Gill arches of C. paleatus contain primary lamellae, each one with respiratory lamellae (secondary lamellae) lined up along both of its sides, as is usual for teleosts. Primary lamellae are covered by stratified squamous epithelium. The surface of the respiratory lamellae is covered with simple squamous epithelium and blood spaces are limited by pillar cells separated from the epithelium by a thick basal membrane (Fig. 1D). Usually, one slightly elliptical and nucleated erythrocyte is seen
Discussion
The commercial product Folidol 600, which is routinely employed in agriculture, contains the organophosphorus methyl parathion as the active substance. It is considered of moderate toxicity because the CL50 is below 10 ppm. However, methyl parathion was responsible, since 1984, for the major number of intoxications in the State of Paraná (Brazil) (Nieweglowski, 1992).
This product enters water bodies as a consequence of rains and leaching from the soil (Kumar and Ansari, 1984), or because it is
Acknowledgements
We thank CNPq for a Productivity in Research Stipend to E. Fanta (Process No. 300831/93-5), and master stipends to S. Freiberger and F.S. Rios; CAPES for master stipends to A.C.C. Vianna and S. Romão; and FUNPAR for financial help.
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