INTRODUCTION

Food items that constitute human diet (including animals) are contaminated when they get in contact with polluted environmental media-air, soil and water. Various researchers from different regions had attempted to assess the degree of pollution in various environmental media and at times related the results with the living organisms in the environment of study. Some physico-chemical characteristics and heavy metal prophiles of some Nigerian rivers, streams and waterways revealed high concentrations of some heavy metals (Asonye et al., 2007). In the evaluation of Cd and Zn in atmospheric deposit, soil, wheat and milk, Vidovic et al. (2005) observed that decreased Cd levels of 93% in atmospheric deposits resulted in decreased Cd concentrations of 17% in cattle feeds and 13% in milk and decreased Zn levels of 58% in atmospheric deposits resulted in decreased Zn concentrations of 30% in soil, 17% in cattle feeds and 17% in milk concluding that heavy metals from atmospheric deposits directly influence the level of heavy metals in other studied media. Industrial activities are major sources of heavy metal pollution and elevated levels of heavy metals in tissues of animals in a polluted environment may be attributed to grazing on contaminated pastures (Okada et al., 1997). Swarup et al. (2006) assessed the Pb burden and status of Cu, Co, Zn and Fe in the blood of goats reared around a primary lead-zinc smelter and it is concluded from the study that goats reared around a primary lead-zinc smelter had higher blood lead levels that also affected blood copper and cobalt concentrations in a dose-dependent manner. Liu (2003) reported higher levels of Pb and Cd in some tissues, including blood, in sheep and horses, correlating the values with those observed in soil, water, forage and feed in the vicinity of non-ferrous metal smelters and suggested that the disease of sheep and horses in that region was caused by Pb poisoning combined with Cd, as a result of heavy metal pollution by industrial activity. Geens et al. (2010) observed in the study of haemotological status of wintering great tits (Parus major) along a metal pollution gradient, that haemoglobin concentration, haematocrit, mean corpuscular volume and mean corpuscular haemoglobin were lower in great tits from the more polluted sites and as well significantly negatively correlated with blood Pb concentration. Changes in haematological parameters were observed in tilapia exposed to cadmium level of 5.5 ppm (Al-Attar, 2005). To evaluate the relationship between metal concentrations in soil, forage and animal tissues, Miranda et al. (2009) studied metal accumulation in cattle raised in serpentine-soil area and observed that high percentage of the animals showed tissue concentrations of Ni and Cu. Pourjafar et al. (2008) studied the profile in blood and hair from cattle around Isfahan oil industry in Iran and the cows closer to the industry showed higher blood and hair Pb levels. Ward and Savage (1994) recorded significantly elevated blood Pb and Cd levels in sheep grazing alongside motorway when compared with control. The results of investigation of tissue metal residues in cattle grazing on sludge-treated pasture suggested caution on prolonged grazing of cattle on pastures receiving heavy sludge applications, because significantly higher levels of fecal Cd were observed in samples collected from cattle immediately after grazing on the sludge-treated pastures when compared to the pre-exposure Cd levels in the same animal (Reddy et al., 1985). Analysing heavy metal contents in Egyptian meat, Abou-Arab (2001) recorded higher levels of Pb and Cd in tissues of some grazing animals-bovine, buffalo, elk, sheep and goat. In some instances, it is even the non-essential elements that are more importantly transferred through the food chain than essential elements (Rogival et al., 2007).

In the evaluation of blood Pb levels in lactating cows reared around polluted localities, with the aim of assessing the transfer of Pb into milk, Swarup et al. (2005) and Patra et al. (2008) reported significant correlation between blood and milk Pb concentrations. Possibility of metal transfer to animal’s milk was demonstrated by Assimakopoulos et al. (1995) by studying radiostrontium transfer to sheep’s milk as a result of soil ingestion and inferred that soil ingestion could be a main source of radiostrontium contamination in sheep and other free-grazing ruminants. Lorenzo et al. (1977) studied the equilibration of Pb between blood and milk of lactating rabbits and reported thus: The basal concentration of lead in milk of lactating rabbits is approximately 65% that of blood. After the intravenous injection of lead acetate, the lead concentration in blood peaked at 1 hour and thereafter declined rapidly, reaching a plateau within 5 days. In contrast, the lead concentration in milk continuously increased with time and by 7.5 days (maximum) exceeded that of blood 8-fold. The possibility that passage of Pb⁺⁺, like Ca^++, Sr⁺⁺ and other ions from blood to milk occurs against a concentration gradient is suggested.

Effects of toxicants on blood could indicate the health condition of animal under study (Dauwe et al., 2006). The level of Pb in blood has a significant correlation with the levels and/or metabolism of essential trace metals (Labbe, 1970; Miller et al., 1990; Singh et al., 1994). The interaction of Pb with some essential trace metals in the blood of anemic children from Lucknow, India, was studied by Ahmed et al. (2007) and their results indicated significant association between elevated blood Pb levels and the risk of anemia.

Mineral and trace metal concentrations studied in some diary products collected from Turkey indicated high levels (Mendil, 2006). There exist permissible levels of mineral and trace metals in body tissues, which, if exceeded, may constitute health hazards. Lead up to a level of 0.25 μg mL^-1 in blood is considered safe and above 0.35 μg mL^-1 is toxic for ruminants and concentration of 1 μg mL^-1 in blood is fatal for animals (Radostits et al., 2000; Swarup et al., 2006). The recommended tolerable levels of Pb and Cd for infants through baby foods are 3.57 and 0.8-1.0 μg kg^-1, respectively (Tripathi et al., 1999). Kazi et al. (2009) in their report on the assessment of toxic metals in raw and processed milk samples suggested continued monitoring of toxic elements in food and the environment. Blood being a major medium of transfer of heavy metals into milk as indicated in some of the literatures cited, it is necessary to constantly assess the levels of these metals in cow tissues, cow being a major source of milk in several countries. Hence, blood levels of Pb, Cd, Co, Zn, Cu and Fe in cows at slaughter at Awka abattoir, Nigeria, at three different seasons, were assessed in this investigation.