Metal distribution in tissues of free-range chickens near a lead–zinc mine in Kabwe, Zambia
Concentrations of Pb, Cd, and other metals in tissues of 17 free-range and 32 commercial broiler chickens from the Kabwe mining town in Zambia were determined. Mean concentrations of Pb and Cd exceeded maximum levels for human consumption in some organs including muscle (Pb only) in free-range chickens, in contrast to low levels in broiler chickens. Human consumers in Kabwe could be exposed to Pb and Cd in free-range chickens. Environ. Toxicol. Chem. 2013;32:189–192. © 2012 SETAC
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The accumulation of toxic metals in the human food chain is recognized as a public health hazard worldwide. Metal pollution has been associated with anthropogenic activities, such as effluents and emissions from mines and smelters, that often contain elevated concentrations of toxic metals including Pb, Cd, Hg, and As 1. As such, widespread metal contamination has frequently been reported in regions with long histories of mining, where high concentrations of metals contaminate water, soils, and vegetation 2–6. In a review by Yabe et al. 7, contamination of food animals, fish, soil, water, and vegetables with Pb and Cd has reached unprecedented levels over the past decade in some parts of Africa. As a result, human exposure to toxic metals has become a major health risk. The recent Pb-poisoning disaster in Nigeria, where more than 160 people died––mostly children under the age of 5––and numerous others were left with long-term neurological impairment including blindness and deafness 8 (http://www.worstpolluted.org/files/FileUpload/files/2010/WWPP-Report-2010-Top-Six-Toxic-Threats-Web.pdf), indicates lack of awareness about metal toxicity as the affected families processed metals in their dwellings.
Kabwe, the provincial capital of Zambia's central province, has a long history of Pb and Zn mining (1902–1994). Because Cd was also produced as a by-product, dense fumes rich in Pb, Cd, and Zn were emitted from smelters and polluted the environment in the surrounding townships extensively 9. Nakayama et al. 10 recently recorded high concentrations of Pb, Cd, Cu, and Zn in soils from Kabwe and attributed this pollution to the Pb–Zn mine. In a follow-up study, Yabe et al. 11 recorded elevated concentrations of Pb and Cd in the liver and kidneys of cattle from farms in Kabwe. Because the distance of the farms where the cattle samples were collected varied, the extent of metal contamination in animals in the vicinity of the mine and the resultant health risk to human consumers could not be clearly established.
The present study investigated metal contamination in various tissues of free-range chickens reared in townships close to the Pb–Zn mine in Kabwe. For comparison, liver samples were also collected from commercial broiler chickens that were reared in Kabwe but were confined and fed commercial feed. Free-range chickens were selected for the study because they are reared by many people in the former mine townships as a source of income, they roam around the mine scavenging for food and are thus vulnerable to metal exposure, and they serve as an inexpensive source of protein to the local population.
MATERIALS AND METHODS
Sampling sites and sample collection
The town of Kabwe is located at approximately 28°26′E and 14°27′S. In August 2011, 17 free-range chickens were collected from townships surrounding the Pb–Zn mine, including Mutwe Wansofu, Makandanyama, and Kasanda. All the townships were close to the mine, and some households were within 1 kilometer of the mine. These townships were selected because soil samples in these areas are highly polluted with Pb (9–51,188 mg/kg), Cd (0.01–139 mg/kg), Zn (5–91,593), and Cu (2–5,727 mg/kg) 10. Live adult chickens were bought and transported to the laboratory at the Kabwe Provincial Veterinary Office for dissection after exsanguination. Liver, kidney, pectoral muscle, gizzard, heart, lung, and spleen samples were collected and kept frozen at the laboratory. Liver samples (n = 32) were also collected from broiler chickens reared in Kabwe. Samples were transported and analyzed for metal concentrations at the Graduate School of Veterinary Medicine, Hokkaido University, Sapporo, Japan.
Sample preparation and metal extraction
All laboratory materials and instruments used in metal extraction were washed in 2% nitric acid (HNO3) and oven-dried. Metals were extracted in chicken liver, kidneys, muscle, gizzard, heart, lung, and spleen using a microwave digestion system (Speedwave MWS-2; Berghof) according to the manufacturer's instruction. Briefly, 0.5 g of each sample was placed in a prewashed digestion bomb, and 5 ml of 60% nitric acid (Kanto Chemical) and 1 ml of 30% hydrogen peroxide (Kanto Chemical) were added. After digestion in the microwave for 52 min, up to a maximum temperature of 190°C, the samples were transferred into plastic tubes to which 100 µl of lanthanum chloride (atomic absorption spectrometry-grade, 100 g La/L solution; Wako Pure Chemicals) was added. The volume was then brought up to 10 ml with distilled deionized water.
The concentrations of metals in the digested samples were determined using a flame/flameless atomic absorption spectrophotometer (Z-2010; Hitachi High-Technologies) equipped with a Zeeman graphite furnace. The instrument was calibrated using standard solutions of respective metals to establish standard curves before metal analysis. Concentrations of Cu and Zn were determined using the flame method with acetylene gas, whereas concentrations of Pb, Cd, Ni, Cr, and Co were determined using a graphite furnace with argon gas. All chemicals and standard stock solutions were analytical reagent–grade (Wako Pure Chemicals), while the water was bidistilled and deionized (Milli-Q; Merck Millipore). Analytical quality control was performed using the DOLT-4 (dogfish liver, National Research Council of Canada) and DORM-3 (fish protein, National Research Council of Canada) certified reference material. Recovery rates (%) of all elements were acceptable; Pb (89–98), Cd (91–108), Ni (98–111), Cr (91–108), Cu (88–90), Zn (78–83), and Co (96–111). Method detection limits (µg/kg) of Pb, Cd, Ni, Cr, and Co were 1.93, 0.11, 2.95, 1.11, and 2.25, respectively. Method detection limits (mg/kg) of Cu and Zn were 0.456 and 0.148, respectively.
The data on metal concentrations were log-transformed to stabilize variances. Statistical analysis was performed using JMP, Version 9 (SAS Institute). The data are presented as mean and minimum to maximum values in milligrams per kilogram, wet weight. To analyze differences in the distribution of Pb and Cd in tissues of free-range chickens, analysis of variance (ANOVA) was done, followed by Tukey-Kramer's method. Student's t test was used to compare metal accumulation differences between the liver of broiler and local breed chickens. A p value <0.05 was considered to indicate statistical significance.
Metal concentrations in tissues of local breed chickens
Concentrations of Pb, Cd, Ni, Cr, Cu, Zn, and Co in the liver, kidneys, muscle, gizzard, heart, lung, and spleen of free-range chickens (n = 17), as well as liver of broiler chickens (n = 32) from Kabwe were determined (Table 1). Levels of Pb in the liver, kidneys, and lungs of the free-range chickens exceeded the maximum level of 0.5 mg/kg wet weight in offal for human consumption 12. In muscle, 59% of the sampled free-range chickens exceeded the 0.1-mg/kg maximum level for Pb in muscle for human consumption 13. Mean levels of Cd in the liver and kidneys of free-range chickens exceeded the maximum level of 1.0 mg/kg wet weight 12.
Table 1. Mean and extreme metal concentrations (mg/kg, wet wt) in various organs of free-range chickens (n = 17) and liver of broiler chickens (n = 32) reared near the Pb–Zn mine in Kabwe, Zambia
|Broiler chicken liver||0.056||0.002||0.031||0.064||56.8*||28.6||0.047|
As shown in Table 1, student's t test was used to compare the accumulation of metals in the liver of free-range and broiler chickens. Free-range chickens accumulated greater concentrations of Pb (p < 0.05) and Cd (p < 0.05) in the liver than broiler chickens. In contrast, liver tissue of broiler chickens accumulated greater levels of Cu (p < 0.05) than that of free-range chickens.
Pb and Cd distribution in tissues of local breed chickens
To analyze differences in the distribution of Pb and Cd in tissues of free-range chickens, ANOVA was conducted. Means of Pb accumulations in tissues were different (F ratio = 77.9). Similarly, means of Cd accumulations in tissues were different (F ratio = 175.7), with Prob > F (<0.0001) in both metals. The distributions of Pb and Cd in the tissues were then analyzed using Tukey-Kramer's method (data not shown). The mean concentrations of Pb in the liver, kidneys, and lung did not differ but were greater than levels in the other tissues (p < 0.05). Cadmium levels were greater (p < 0.05) in kidneys than in the liver. Concentrations of Cd in the other tissues showed differences, but values were generally lower than levels in the liver and kidneys.
The present study revealed the presence of large quantities of Pb and Cd in edible organs of free-range chickens reared near a Pb–Zn mine in Kabwe, Zambia. The accumulations of Pb and Cd (mean concentrations of 7.62 and 3.51 mg/kg, respectively) in kidneys could pose serious health concerns to the local human consumers. It is alarming given that all of the sampled chickens (n = 17) accumulated Pb in the liver, kidneys, and lungs in excess of 1.40 mg/kg. These Pb values exceed the maximum level of 0.5 mg/kg in offal for human consumption 12. In pectoral muscle, Pb levels (mean 0.228 mg/kg) exceeded the maximum level of 0.1 mg/kg 13 in 59% of the sampled chickens. Furthermore, 88% of the chickens exceeded the Cd maximum level of 1.0 mg/kg in kidneys 12. To the best of our knowledge, this could be the highest record of toxic metal contamination in offal and muscle of chickens under natural conditions. These findings imply that consumers in Kabwe are exposed to high concentrations of these toxic metals in muscle and offal of backyard chickens, which are allowed to roam and scavenge for food to reduce the cost of production and provide an inexpensive source of protein.
Despite high accumulation of Pb in the liver and kidneys, the sampled chickens overtly appeared healthy. This was in agreement with findings in experimental studies where adult hens showed tolerance to chronic Pb intoxication 14. However, consumption of contaminated chicken offal and muscle could pose significant health risks to humans, especially children, who are highly susceptible to Pb toxicity 15. This is important because neurological effects, such as a reduction in intelligence quotient, have been documented in children with blood Pb levels even lower than the 10-µg/dL threshold limit 16. Similarly, effects of Cd including renal toxicity, osteoporosis, and cancer of several organs have been reported in humans 17. Moreover, renal toxicity has been established as one of the causes of mortality among patients with itai-itai disease who were exposed to high concentrations of Cd in rice and drinking water in Japan 18. Given that Pb and Cd are both nephrotoxic 19, regulatory measures should be taken to protect human consumers in Kabwe from excessive or prolonged metal exposure. For this reason, studies are needed to investigate metal concentrations in humans in Kabwe, as these metals accumulate in the organs over time.
The present study also highlighted marked differences in the accumulation of toxic metals in liver tissues of free-range and broiler chickens from Kabwe. In contrast to contamination levels in free-range chickens, concentrations of Pb and Cd in the livers of broiler chickens were below the maximum level of 0.5 mg/kg 12, making them safe for human consumption. Although reared in the same town, this finding indicated different management practices as broiler chickens were kept indoors and fed commercial feed, whereas free-range chickens roamed and scavenged for food in the mine townships. Given these differences, the source of metal contamination in the free-range chickens was most likely contaminated soils as a study by Nakayama et al. 10 reported high concentrations of Pb and Cd in soils from the same areas. Findings in the present study were higher than reports by Yabe et al. 11 in offal of cattle from Kabwe. Therefore, offal from free-range chickens in Kabwe presents a much greater health risk to human consumers than cattle offal as the chickens are reared in townships in the vicinity of the mine.
The distribution of Pb in tissues of free-range chickens was greatest in the kidneys (7.62 mg/kg), followed by the liver (4.15), lung (3.34), spleen (0.878), muscle (0.228), gizzard (0.225), and heart (0.114). Apart from the higher concentration of Pb reported in the femoral bone (2.90 mg/kg, wet wt) of adult domestic breeding stock hens in Slovenia, the distribution pattern of Pb in kidneys (0.62), followed by the liver (0.3), spleen, and other tissues in the hens 20 was similar to the pattern in the present study. In chickens from Kabwe, high concentrations of Pb in lung tissue could be attributed to inhalation exposure as the backyard chickens scavenged for food from the Pb-polluted soils. Moreover, the high concentration of Pb in the spleen was not surprising given that Pb can accumulate in blood and affect the hematological system 15. Similarly, the tissue distribution of Cd in the present study was highest in the kidneys (3.51 mg/kg), followed by the liver (1.60) and spleen (0.114), with values <0.1 mg/kg in lung, gizzard, muscle, and heart tissues. A similar pattern was reported in hens by Doganoc 20, where Cd deposits were highest in the kidneys (2.63 mg/kg) and lowest in the heart. In chickens from Nigeria, the accumulations of Pb (0.22 mg/kg) and Cd (0.27 mg/kg) were higher in the liver than in the kidneys 21 but lower than the 0.5-mg/kg maximum level. Therefore, consumption of liver, kidney, spleen, and lung tissues from free-range chickens reared near the Pb–Zn mine in Kabwe should be completely avoided, especially by pregnant women and children. More still, allowing free-range chickens to scavenge for food near the mines should be strictly discouraged because high concentrations of Pb were found in the muscle.
The essential elements Ni, Cr, Cu, Zn, and Co were also analyzed. Mean levels of Cu in the tissues of free-range chickens were low (3.40 mg/kg) and lower than concentrations in broiler chickens (56.8 mg/kg) that were fed commercial feed. Surprisingly, concentrations of Zn were similar in both chicken types despite extremely high levels of Zn in Kabwe soils 9, 10. This could be attributed to efficient regulatory mechanisms of essential metals in the animal body. The low concentrations of Ni, Cr, and Co in the chicken tissues were similar to concentrations in the livers of chickens from Nigeria 20. As observed by Nakayama et al. 10 in Kabwe soils, concentrations of Ni, Cr, and Co in chicken tissues could also not be associated to the Pb–Zn mine.
Free-range chickens reared in Mutwe Wansofu, Kasanda, and Makandanyama townships near the Pb–Zn mine in Kabwe accumulate large quantities of Pb and Cd compared to commercial broiler chickens. The source of metal pollution is most likely the now closed mine site. Prolonged consumption of high concentrations of Pb and Cd in chicken offal may lead to accumulation of these metals in the human body and cause metal toxicity. There is a clear need to avoid consumption of contaminated chicken offal and muscle, as well as restrict chickens from roaming and scavenging for food near the mine.
The present study was supported in part by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan awarded to M. Ishizuka (24248056 and 24405004) and Y. Ikenaka (23710038), as well as a research fellowship from the Japan Society for the Promotion of Science grant-in-aid awarded to S. Nakayama (2200517701) and the JSPS Core to Core Program (AA Science Platforms). We also appreciate the assistance of M.S. Phiri (Kabwe District Veterinary Office).