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 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.
