Sub-lethal impacts of lead poisoning on blood biochemistry, immune function and delta-aminolevulinic acid dehydratase (d-ALAD) activity in Cape (Gyps coprotheres) and white-backed (G. africanus) vulture chicks

Abstract

Although the prevalence of lead poisoning in southern Africa's Gyps vultures is now well-established, its finer physiological effects on these endangered species remain poorly characterised. We evaluated the sub-lethal impact of acute lead exposure on Cape and White-backed Vulture chicks from two breeding colonies in South Africa, by analysing its possible effects on key blood biochemistry parameters, immune function, packed cell volume and δ-aminolevulinic acid dehydratase (δ-ALAD) activity. All 37 White-backed Vulture nestlings sampled displayed elevated lead levels (>10 μg/dL), and seven had blood [Pb] >100 μg/dL. Eight of 28 Cape Vulture nestlings sampled had blood [Pb] exceeding background exposure, with one showing blood [Pb] >100 μg/dL. Delta-aminolevulinic acid dehydratase (δ-ALAD) activity was significantly and negatively related to blood [Pb] in nestlings from both species, with 50% inhibition of the enzyme predicted to occur at blood [Pb] = 52.8 μg/dL (White-backed Vulture) and 18.8 μg/dL (Cape Vulture). Although no significant relationship was found between % packed cell volume (PCV) and blood [Pb], the relatively lower mean PCV of 32.9% in White-backed Vulture chicks, combined with normal serum protein values, is likely indicative of depression or haemolytic anaemia. The leukogram was consistent in both species, although the presence of immature heterophils suggested an inflammatory response in White-backed Vulture chicks with blood [Pb] >100 μg/dL. Values for cholesterol, triglycerides, total serum protein, albumin, globulin, albumin/globulin ratio, alanine aminotransferase (ALT) and gamma-glutamyl transferase (GGT) were consistent with values previously reported. Calcium and phosphorus concentrations suggested no adverse effects on bone metabolism. A significant decrease in urea: uric acid (U:UA) ratio at blood [Pb] >100 μg/dL in White-backed Vulture chicks, brought about by a decrease in urea production, raises the possibility of hepatic abnormality. These results suggest that δ-ALAD activity may serve as a sensitive biomarker of lead toxicity in both species, while highlighting the need to better understand the significant variability in sensitivity that is observed, even between closely related members of the same genus. HIGHLIGHTS • A significant proportion of Cape and White-backed Vulture nestlings sampled experienced elevated blood lead levels. • Delta-aminolevulinic dehydratase (δ-ALAD) activity were significantly and negatively correlated to blood lead concentration in both species. • Cape and White-backed Vultures are predicted to experience 50% depression of the δ-ALAD enzyme at blood [Pb] = 18.8 and 52.8 μg/dL, respectively. • δ-ALAD activity may serve as a sensitive biomarker in both species. • In White-backed Vultures blood [Pb] > 100 μg/dL suggest hepatic abnormality. • Lead's ability to disrupt haem synthesis and cause hepatic injury may compromise young birds' survival as newly-fledged juveniles.