Superphosphate poisoning of sheep: the role of fluoride

Earlier work confirmed that the fundamental lesion of superphosphate poisoning is an acute toxic tubular nephritis in which both phosphate and fluoride may play a part but their respective roles could not be determined. In this study, sheep poisoned by sodium fluoride (NaF) were compared with sheep poisoned by superphosphate containing approximately 1.5% fluoride. The LD50 of NaF was in the range 100 to 300 mg/kg (45 to 135 mg F/kg). This range is of the same order as the amount of fluoride in a toxic dose of superphosphate (70 to 90 mg F/kg). A lethal dose of NaF caused severe depression, salivation, hyperpnoea, blindness, ataxia and incoordination. Death ensued three to 52 hours after dosing. Acute necrotizing rumino-reticulitis and abomasitis and necrosis of epithelial cells in the proximal convoluted tubules of the kidney were the characteristic lesions of NaF toxicity. Superphosphate poisoning took a more protracted course with depression and diarrhoea as the predominant clinical signs until the terminal coma. As with NaF, the notable lesions were in the gastrointestinal tract and kidneys, but were less severe. Although there were differences in the clinical and pathological manifestations of the two forms of poisoning, the comparable toxic dose of NaF and of the fluoride in a toxic dose of superphosphate, and the similar target organs involved, support the view that fluoride plays a dominant role in the pathogenesis of superphosphate poisoning. It is probable that phosphate plays a contributory role but the nature of the interaction of fluoride and phosphate remains to be established.