Abstract: The increasing level of pollution in both aquatic and terrestrial environment with which some animals which form part of human diet are constantly in contact has been a global concern, minding the contamination of aquatic and terrestrial organisms especially in countries which are prone to severe environmental pollution due to the lack of appropriate waste management system and technologies. In Nigeria as well as several other countries, meat from cattle is the most common hence, the need to assess the level of heavy metals in cow muscle tissue. The levels of Pb, Cd, Co, Zn, Cu and Fe were determined in the muscle tissue of 15 cattle at slaughter during 3 different seasons. The samples were digested and analyzed with atomic absorption spectrophotometer. The values ranged from ND-14.80 mg kg^-1 at the on-set of rainy season; 0.01-3.53 mg kg^-1 at the peak of rainy season and 0.005-3.90 mg kg^-1 during the dry season.

INTRODUCTION

The increasing levels of environmental pollutions by toxic metals from various sources have generated a great concern on the impact on human health. Humans are prone to several routes of exposure and hence the need to evaluate the levels in human diet which is one of the easiest routes of exposure. While the occurrence of toxic metals in some aquatic organisms which form human diet have been of great concern in that they could accumulate the metals at a level exceeding the proportion that occur in the environment, bioaccumulation by animals raised for human consumption has been dreaded as a great risk to humans (Hashmi et al., 2002). Urban run-off sediments in some parts of Nigeria, Particularly Lagos where environmental pollution takes various forms have been evaluated for the levels of heavy metals and Zn, Fe and Cd were found in very high concentrations (Adekola et al., 2002) while water samples of 72 rivers, streams and waterways in southern Nigeria analyzed for Pb, Cr, Cd, Fe, Zn, Mn and Cu were found to contain higher levels of these pollutants at concentrations exceeding the guidelines of WHO (Asonye et al., 2007).

Humans consuming vegetables grown in contaminated soils and animals raised in such area stand at a risk (Sedki et al., 1995). Plants growing around major highways are prone to contamination by aerial deposition of metal-containing particulate matter from automobile exhaust this is exemplified by some studies carried out by Atayese et al. (2008) on heavy metal contamination of Amaranthus grown along major highways in Lagos, Nigeria. The effects of moderate pollution on toxic and trace metal levels in calves from a polluted area of northern Spain were studied by Miranda et al. (2005) and the results indicated contribution of anthropogenic pollution to toxic metal residues in cattle in an industrialized area of Asturias. The results of metal levels in body tissues, forage and fecal pellets of elk living near the ore smelters at Sudbury, Ontario, exceeded the WHO guidelines and Canadian regulatory standards, implying significant health risk for human consumption (Parker and Hamr, 2001). In the determination of heavy metal contents in Egyptian meat, Abou-Arab (2001) observed that the Pb, Cd, Zn, Cu, Mn and Fe contents in muscle, liver, kidney, heart and spleen in industrial areas were higher than in the same organs for rural areas. Bovines grazing on the municipal wastewater spreading field of Marrakech City (Morocco) were found to be seriously contaminated by toxic metals (Sedki et al., 2003). In the evaluation of metal accumulation in cattle raised in a serpentine-soil area, Miranda et al. (2009) observed that tissue accumulation in animals was related to concentrations of the metals in soils and forage. Concentrations of some heavy metals in animal tissues were correlated with the heavy metal content in the soil (Lopez-Alonso et al., 2002). Apart from being in contact with polluted soil environment and grazing on contaminated plants, cattle could as well be exposed to heavy metals through contaminated feeds (Miranda et al., 2005). In some instances, high concentrations of Cu and Zn are added to pig and poultry feeds; application of pig and poultry manures as fertilizers may then result in pollution of agricultural lands by these metals (Poulsen, 1998) and uptake by plants; these then pose risks to grazing cattle.

Diet and season have been identified as factors determining the transfer of metals from the surrounding environment to terrestrial animals (Hunter et al., 1987). Also, there have been indications of industrial pollution in the forest ecosystems (Medvedev, 1995). In all tissues analyzed in gray whale carcasses from the Northern Pacific Mexican Coast, Fe, Cu, Zn and Mn were present in the highest concentrations (Mendez et al., 2002).

Some essential elements, though necessary for life and are particularly involved in some metabolic processes, if taken in excess could be toxic (Spears et al., 1986). Concentration of a metal may affect the level of others in animal tissues (Blanco-Penedo et al., 2006). Exposure of humans to some heavy metals have indicated risk factors for breast lesions (Siddiqui et al., 2006). In Nigeria as well as several other countries, meat from cow is the most common and the part mostly processed for consumption is the muscle.

Some heavy metals are present mainly in muscle tissue (Storelli and Marcotrigiano, 2003). Though caution is exercised to ensure that a very healthy cow is processed for consumption, a cow with high levels of toxic metals may not show any obvious symptoms of illness and thereby posses hidden potential health risk to humans. Food chain contamination has been a common route of exposure to heavy metals for humans (Ferner, 2001). This study assesses the levels of Pb, Cd, Co, Zn, Cu and Fe in the muscle tissue of cow at slaughter during three different seasons.

MATERIALS AND METHODS

Portions of muscle tissue from fifteen cows at slaughter were collected from Awka abattoir, five cattle in each of 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 collected in contaminant-free sample bags and preserved in refrigerator pending the time of analysis.

About 10.0 g of each muscle sample was placed in conical flasks, 5 mL of phosphoric were added it was then heated on a heating mantle for about 1 h, until heated to dryness; 100 mLs 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 analysed for Pb, Cd, Co, Zn, Cu and Fe using atomic absorption spectrophotometer, model Shimadzu AA-6800 (Nwude et al., 2010).

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 muscle tissue using the RSQ worksheet function.

RESULTS AND DISCUSSION

The results of the determination of levels of heavy metals in cow muscle tissue during the three seasons are shown in Table 1. At the on-set of rainy season (April-July), the values ranged from ND-4.80 mg kg^-1 at the peak of rainy season (July-October), 0.01-3.53 mg kg^-1 during the dry season (January-April), 0.005-3.90 mg kg^-1. Figure 1 shows the average concentration in April-July, the values ranged from 0.03-7.95 mg kg^-1 in July-October (Fig. 2), 0.01-3.25 mg kg^-1 and in January-April (Fig. 3), 0.01-3.8 mg kg^-1.

Table 1:	Levels (mg kg-1) of heavy metals in cow muscle tissue in April-July (2004), July-October (2004) and January-April (2005)

Fig. 1:	Average concentration of metals in cow tissue in April-July

Fig. 2:	Average concentration of metals in July-October

Figure 4 shows the average concentration in the three seasons, the values ranging from 0.02-5.00 mg kg^-1. Levels of heavy metals in muscle tissues of various animals to compare with the results in this study are shown in Table 2 while the results of correlations between levels of the various metals are shown in Table 3.

At the on-set of rainy season, Zn level was the highest followed by Pb while the least was obtained for Cd. The same pattern was observed at the peak of rainy season.

During the dry season, the level of Zn was followed by that of Cu. In the aggregate of the metal levels for the three seasons, the accumulation of Zn in the cow muscle tissue was the highest followed by Pb and the least being Cd.

The varying levels of corresponding metals in the different seasons indicate the effect of season as a factor influencing the accumulation of heavy metals by cattle.

Fig. 3:	Average concentration of metals in January-April

Fig. 4:	Average concentration of metals in the three seasons

The leading levels of Zn and Pb in the muscle tissue during the different seasons may indicate inherent potential in cattle to accumulate Zn and Pb in the muscle tissue or an indication of the degree of environmental pollution by these metals.

The level of Pb observed in this study was ND-4.98 mg kg^-1; literatures are not available on the level of Pb in cattle muscle tissue; the available literatures cover the levels in sheep, goat, calves, elk and fur seal: 0.081±0.03 mg kg^-1 for sheep and 0.084±0.04 mg kg^-1 for goat (Abou-Arab, 2001); ND-150 μg kg^-1 calves (Miranda et al., 2005) and 1.09±0.06 mg kg elk (Parker and Hamr, 2001).

The results fairly compare with the references. While the range of Cd levels (0.004-0.1 mg kg^-1) is slightly lower than that observed by Sedki et al. (1995) (0.25-1.0 mg kg^-1) which may imply higher level of pollution in the region of study by Sedki et al. (2003) the values are comparable to those observed in sheep (0.02±0.01 mg kg^-1), goat (0.041±0.03), calves (ND-20.7 μg kg^-1), elk (0.17±0.02 mg kg^-1) and fur seal (0.04-0.36 μg g^-1) by Abou-Arab (2001), Miranda et al. (2009), Parker and Hamr (2001) and Noda et al. (1995), respectively. Report on Co levels in animal tissues is available only for elk (Parker and Hamr, 2001), the value being 0.54±0.05 mg kg^-1 and the results obtained in this study ranged from 0.20-1.10 mg kg^-1. The results compare well with the reference.

Table 2:	Levels (mg kg-1) of metals in the muscle tissue of various animals
aSedki et al. (2003); bAbou-Arab (2001); cMiranda et al. (2005); dParker and Hamr (2001); eNoda et al. (1995); * (μg Kg-1); **(μg g-1); ND: Not Determined

Table 3:	Results of correlations between levels of the metals in cow muscle tissue

The values for the levels of Zn in this study are closer to that observed for fur seal by Noda et al. (1995) disparity exists with those observed by the others; higher level of pollution may be inferred for the higher levels observed by the other and while the levels of Cu in this study agree with the references for Fe, disparity exists. The results of correlations show little or no relationships between the levels of the various metals as observed at the on-set of rainy season.

Some significant relationships exist between the levels of Pb and Cd and between Co and Zn at the peak of rainy season between Co and Zn, Zn and Cu and Cu and Fe during the dry season.

CONCLUSION

The levels of heavy metals in cow muscle tissue as observed in this study vary with season. The references selected to compare with the results in this study are those of studies carried out in some polluted regions. Hence, where the results of this study agree with those references, pollution may be inferred.