Use of Metal-Rich Slag as a Source of Grit and Its Effects on Pigeon Health and Fitness

Abstract

Uptake, utilization and physiological effects of slag as a source of grit were studied in laboratory pigeons provided with an ad libitum supply of a) industrial metal-rich slag b) pristine gravel or c) a combination of the above, over a one-year period. Although retention times undoubtedly differed, both grit types appeared to remain within the gizzard until near-completely abraded, thus resulting in near-maximal exposure of the bird to intrinsic metal loads present. Mortalities were confined to the slag-treated group (62%) and birds given choice (11%) with no deaths among gravel-treated controls. Surviving slag-treated birds indicated marked weight increases in both liver (2-fold) and kidney (~ 46%) whereas body fat scores and breast muscle mass were substantially reduced. Slag ingestion likewise affected marked hypertrophy of the parathyroid glands. Tissue metal assays indicated a striking accumulation of Fe (7-8 fold) in liver, and to a lesser extent in bone (3.2-fold) and kidney (1.5-fold) of slag-treated birds. Histologically, hepatocytes and invading clusters of macrophages appeared stuffed with numerous pigmented cytoplasmic granules – histochemically identified as predominantly stored Fe. These findings point to the development of an acute toxic Fe storage condition, not unlike avian hemosiderosis (iron storage disease), as the prime cause of morbidity and eventually death of these slag-treated birds. Long bones of slag-treated birds were substantially more pliable, of reduced strength, and characterized by significantly reduced Ca levels compared to control birds. Histological examination of keel and tibiotarsal bones revealed moderate to severe mineral resorption, with thinning of cortical layers in lamellar bone and reduced cross-strut formation in cancellous bone. Evidence of active woven bone formation at these foci was likewise confirmed, thus corroborating the marked hypertrophy of parathyroids and implied changes in parathyroid function. Given free choice, birds were basically non-selective, rather than exclusive, in their uptake of gravel versus slag. Given that slag was not avoided, and in light of its profound adverse physiological effects, concern for the ecological implications of the vast quantities of exposed slag within our environment and its potential health risks to wild bird populations is warranted.