Cadmium and lead concentrations in livers and kidneys of cattle slaughtered at Grootfontein abattoir in Namibia

Abstract

The aim of this study was to determine the levels of cadmium (Cd) and lead (Pb) in livers and kidneys of cattle slaughtered at Grootfontein abattoir in Namibia. The study design was based on the epidemiological principles to detect a single animal whose kidneys or liver contained Cd or Pb residues. The Grootfontein area of Namibia has extensive base-metal ore reserves, which were and are still extracted and processed in localities used as livestock pastures. Namibia is also an arid country which predominantly uses borehole water for livestock and human consumption. These underground water bodies share the same space as base-metal ores. The anthropogenic activities in this area under the existing geological and hydrogeological circumstances offer opportunities for Cd and Pb to enter the food chain. Entry of Cd and Pb in the food chain leads to bioaccumulation in cattle kidneys and livers to concentrations above Codex Alimentarius Commission (CAC) standards, creating a possible public health risk. The CAC withdrew the maximum limit (ML) of 1mg kg-1 Cd in bovine kidneys and liver, but it has a provisional tolerable monthly intake (PTMI) of 0.025mg kg-1 human body weight. This CAC PTMI translates to a total exposure of 1.5mg Cd for a 60kg body weight person. The CAC ML for Pb in bovine offal is 0.5mg kg-1, while its provisional tolerable weekly intake (PTWI) of 0.025mg kg-1 human body weight is under review. This investigation intended to establish if Cd and Pb in the livers and kidneys of cattle slaughtered in the study area exceeded CAC human exposure limits. Liver and kidney specimens were collected from 31 randomly sampled mature cattle (estimated over five years old based on incisor teeth examined post slaughter). The specimens were analysed at a local mine laboratory, which was the only facility available and capable of performing the tests. They were digested using wet-ashing (the oxidation procedure). All liver digestates were analysed, while one kidney analyte was insufficient. Cd and Pb were measured using flame atomic absorption spectroscopy (FAAS). The detection limit (DL), which was the minimum metal concentration FAAS could measure was 0.2mg kg-1 for Cd and 1.1mg kg-1 for Pb. The laboratory could not refine the Pb DL which was more than twice the CAC ML. All livers had Cd concentrations below 0.2mg kg-1. One discarded kidney specimen was assigned a concentration below DL for analysis purposes. The Cd concentrations in 12 kidney specimens were below 0.2mg kg-1, between 0.288 and 1.221mg kg-1 in 16 and above 1.5mg kg-1 (2.6 - 3.64mg kg-1) in 3 specimens. The mean renal Cd concentration for the population (0.71±0.96mg kg-1) was statistically lower than 1.5mg kg-1 (p <0.05). Cd was therefore shown to be a chemical hazard for consumers of kidneys and a potential environmental hazard in the study area. Pb was negative in all of the 31 liver specimens, while in all the 30 kidney specimen digestates it was detected at concentrations below 1.1mg kg,-1. This result confirmed the presence of Pb as a potential chemical hazard found in bovine kidneys. However, a more sensitive analytical method was required to assess Pb food chain and public health hazard parameters in the study area. An epidemiological investigation of the study area using geographical information systems (GIS) to explore geographical factors that could have influenced exposure to Cd and Pb was done. While proximity to operational and decommissioned mining ventures appeared to result in higher mean renal Cd concentrations, the influence was not statistically significant. Feedlot rearing also appeared to cause higher mean renal Cd concentrations but the impact was also not statistically significant. It was concluded that Cd and Pb were chemical environmental contaminants which enter the animal and human food chain in the study area. A kilogram of bovine kidney-meat from approximately one in ten cattle (9.7%) carried more than 1.5mg Cd, exceeding the recommended CAC total dietary exposure for a 60-kg man. A mathematical model was used to estimate the risk of cattle with renal Cd concentration exceeding 1.5mg kg,-1, the expected number of cattle affected and the weight of meat entering the food chain. The estimated cattle population affected monthly was 5.95%, with a range of between 5 and 11 (mean = 8±4) cattle. These cattle were estimated to yield 8 to 18 (mean 13±6) kg bovine kidney-meat per month. A WHO standard-weight man who consumes a kilogram of kidney meat in this category in a month risks exposure to Cd doses beyond the CAC PTMI. The epidemiological triad of interactions between the host (cattle), agent (Cd and Pb) and the environment (proximity to mines) was used in order to suggest risk mitigation options. Recommendations from this study included advocacy on Cd and Pb in the food chain and developing partnerships with mining entities so that risk mitigation and communication can be better coordinated. Consumers are advised to reduce their monthly intake of kidney meat as this organ tissue has higher levels that those in other bovine organs and tissues. The cattle in the study were mature breeding animals (>5 years) and it is likely that this factor increased the risk of higher levels in kidneys as temporal determinants for bioaccumulation of Cd are important. The kidneys of younger animals would thus be less likely to contain significant Cd levels. Lastly, use of mathematical modelling, to translate research findings into quantitative estimates useful for public health safety programs, is recommended.