INTRODUCTION

Environmental pollutants are released into the air, soil and surface water as a result of unguided dumping, deliberate application and accidental discharge of industrial and agricultural effluents (Ogbuagu et al., 2005). Industrial and urban activities generate metal wastes which pollute soils, leading to the accumulation of toxic metals in the soils, transferred to plants and consequently into animals (Swarup et al., 2006). Application of pig and poultry manure which contain Cu and Zn at higher concentrations on agricultural lands as fertilizer could also result in metal pollution of agricultural lands (Poulsen, 1998) which may have an eventual effect on the grazing cattle. Through the polluted water sources, the aquatic animals and agricultural livestock have been infected with pathogenic microorganisms and as well accumulate chemical pollutants. The heavy metals and organic pollutants could have a detrimental effect on aquatic and farmland animals (Van de Merwe et al., 2010). If agricultural lands are close to highways, aerial deposition of metal-containing particulates from automobile exhausts could occur and eventual uptake by crops (Garcia and Millan, 1998). Medvedev (1995) inferred some indications of industrial pollution in the forest ecosystems by determining the concentrations of cadmium, lead and sulphur in the tissues of wild, forest reindeer from North-West Russia. In the determination of the levels of some heavy metals in urban run-off sediments in Ilorin and Lagos, Nigeria, by Adekola et al. (2002), very high concentrations of Zn, Fe and Cd were found in the urban sediment from these cities. Miranda et al. (2005) studied the effects of moderate pollution on toxic and trace metal levels in calves from a polluted area of northern Spain; Cd and Pb contents in the liver were moderately and significantly higher in calves from industrialized area. In the findings of Atayese et al. (2008) for Lagos environment, Amaranthus grown along major highways accumulated Pb and Cd at concentrations above the normal limit for plants, suggesting pollution by aerial deposition. Asonye et al. (2007) reported the concentrations of Pb, Cr, Cd, Fe, Zn, Mn and Cu in water samples of rivers, streams and waterways in southern Nigeria, exceeding the guidelines of WHO. Kpee et al. (2009) evaluated seasonal variation of Cd, Ni, Cu and Pb pollution in catfish, sediment and water samples from Ipo stream in Ikwere district of Rivers state, Nigeria. In this study, we made attempt to evaluate metal pollution by studying the levels in the liver of cattle at different seasons.

Cattle are raised in the northern part of Nigeria; while some graze from northern part through to southern part, others are transported to every other part of the country for human consumption as meat. The cattle are allowed to feed on grasses in the surroundings and drink water from any nearby streams and stagnant water. It has been an established fact that non-essential elements could be transferred through food chains (Rogival et al., 2007; Ma et al., 2007). It is therefore possible that this eating habit of cattle may lead to bioaccumulation of heavy metals in the liver where several detoxifications take place.

Sometimes, liver is used as a constituent of animal feed. And due to its good iron content, human consumption of animal liver has increased over the years in this part of the world. The importance of meat and milk derived from cattle as sources of protein for humans brings about the need to ensure that these protein sources do not contain harmful substances capable of causing health problems to humans. Exposure to Pb may result in human breast lesions (Siddiqui et al., 2006). Accumulation of higher concentrations of heavy metals could predispose animals to illness (Harper et al., 2007) and toxic heavy metals could cause alterations of immune system in diverse animals (Lapierre et al., 1999). Therefore, the global environmental health concern calls for research to determine the pollutant levels, especially metals, in aquatic and terrestrial animals, since humans depend mainly on such animals for their protein. Exposure to pollutants runs through the food chain and humans at the receiving end stand at risk (Abbas et al., 2006). Humans are exposed to metal pollutants in surface water by consumption of contaminated aquatic organisms (Mackey, 1991). These among others, have prompted this investigation of metals concentrations in the liver of cattle. Pollutant metals of environmental concern include Pb. Cd, Zn, Co, Cu and Fe; hence, these are studied in this investigation. This study aims at assessing the level of liver accumulation of the heavy metals in different seasons; the most and the least bioaccumulated metals and the season of highest bioaccumulation which may give a clue to any variation in seasonal heavy metal pollution.

MATERIALS AND METHODS

Sample collection: Liver samples were collected from Awka abattoir from five different cows at slaughter during the three different seasons of the year: on-set of rainy season (April-July, 2004), peak of rainy season (July-October, 2004) and dry season (January-April, 2005); the samples were stored in refrigerator, waiting the time of extraction and analysis of the metals.

Digestion and analysis: Ten gram of each liver sample was placed in conical flasks; 5 mL of phosphoric acid were added, heated on a heating mantle for about an hour, until heated to dryness; 100 mL of distilled water were added and thoroughly shaken. It was filtered into a 100 mL standard flask and the filtrate was made up to mark with distilled water. Aliquots of this were analyzed for Pb, Cd, Co, Zn, Cu and Fe using atomic absorption spectrophotometer, model Shimadzu AA-6800.

Statistical analysis: Correlations were made between the concentrations of one metal and the other to establish any possible relationships in the accumulation of the metals in the kidney, using the RSQ worksheet function.