Trace elements in struvite equine enteroliths: Concentration, speciation and influence of diet

Equine enteroliths ∼1.5 cm in diameter were collected from an Arabian horse in Louisville, Kentucky, United States. Scanning electron microscopy (SEM) and light microscope imaging of a sectioned enterolith showed two distinct regions of concentric growth outward from the central nidus, a small pebble. After initial growth, acidic colonic fluids permeated the stone inducing recrystallization and alteration of crystals closest to the nidus. A second growth event, when mineral crystallization was again favorable, produced an outer region of unaltered crystals at the rim. The mineral was identified as struvite (MgNH₄PO₄∙6H₂O) by powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. Elemental analysis confirmed concentrations of P, Mg and N consistent with the struvite composition, and detected trace elements Fe (1050–1860 mg kg⁻¹), Mn (262–280 mg kg⁻¹) and Zn (197–238 mg kg⁻¹). All elements were traced to dietary sources, with the Fe:Mn:Zn ratio of the enterolith consistent with that of the horse feed. X-ray absorption fine structure (XAFS) spectroscopy at the Zn K-edge revealed distorted ZnO₄ tetrahedra located between crystallographic planes in the struvite structure forming bidentate linkages to struvite phosphate groups. Emplacement of Zn in structural cavities likely occurs during struvite crystallization. Trace elements and organic impurities increase susceptibility of the enterolith to heat-induced decomposition relative to pure struvite, which could be a consideration for treatment. Results reveal enterolith growth processes, composition and mechanisms of trace metal accumulation that can inform management and prevention of equine enteroliths.