Geochemical Study of an Equine Enterolith, Medina County, Ohio

An intestinal concretion (enterolith) was recovered during necropsy of a horse from Medina County, Ohio. The object is approximately spherical in shape and measures 9 X 8 X 7 cm in diameter. It is composed of struvite (NH4MgPO4 • 6H2O). The monohydrate dittmarite was not found. In addition to Mg and P, significant concentrations (^500 ppm) of Ca, K, and Fe and lesser amounts (^500 ppm) of Na, Mn, Rb and Sr were measured. The concretion is concentrically zoned with concentrations of Ca, Sr, Fe and Na decreasing from the inside out whereas those of K and Rb increase. The concentrations of Mg and Mn appear to be constant within analytical uncertainty. The cause for the chemical zonation of the enterolith is not known. The Rb/Sr ratio of the struvite is high compared to that of calcium phosphate and ranges from 4.9 to 20.7. The Sr/Sr ratio is 0.7101 ± 0.0003 which leads to the interpretation that most of the Sr available to plants in the soil of Medina County was derived from particles of Devonian limestone (Sr/Sr = 0.7083) and lesser amounts from feldspar (Sr/Sr = 0.7141) and clay (Sr/Sr = 0.7345). OHIOJ . SCI. 83 (1): 54-59, 1983 INTRODUCTION and Wittkopp 1981). Most enteroliths are Intestinal concretions called enteroliths composed of struvite (NH4MgPO4 • 6H2O) are frequently found in horses but only a n d m a y contain fragments of undigested rarely in other animals. They occur in the ' h a i r a n d mucus. In addition, several stomach-like dilatation of the large colon chemical elements including Ca, Na, K either singly or in clusters and are usually a n d F e a s w e l 1 & carbonate and chloride spherical in shape. They range from peai o n s h a v e b e e n reported. (Nieberle and size to objects weighing up to 11 kg (Blue Cohrs 1967). Equine enteroliths form by precipitation of insoluble chemical compounds around a Manuscript received 14 December 1981 and in foreign object in the colon, but the specific revised form 19 March 1982 (#81-50). causes for their formation are not known. t ti l ti ll t lit tl i t l l i t i l . i t t li il t ti t l l it i l i l t ll i l i . i t j t i i t l anuscript received 14 ece ber 1981 and in . c se f str ite ( 4 4 • 2 ) and ay contain fra ents of i este food, hair and ucus. In addition, several c e ical ele e ts i cl i a, a, and Fe as well as carbonate and chloride ions have been reported. ( ieberle and rs ). i t r lit s f r r i it ti f i s l l i l s r foreign ject i t e c l , t t e s ecific OhioJ. Sci. EQUINE ENTEROLITH 55 Blue and Wittkopp (1981) reported the presence of small chert pebbles in the centers of equine enteroliths recovered during necropsy or laparotomy from 11 horses near Davis, CA. Additional enteroliths were found in pastures where they had been excreted by clinically healthy horses. They identified both monohydrate and hexahydrate forms of NH4MgPO4 in X-ray diffraction patterns of concentric layers within several enteroliths. They also reported that the chemical composition of an enterolith removed from the small colon of a 7-year-old Arabian stallion consisted mainly of Mg (12%) and P (18%) with lesser amounts of Ca (0. 18%), Mn (375 ppm), Fe (25 ppm), Zn (9 ppm), Cu (8.5 ppm), and Ni (2 ppm). The drinking water from the farm on which the Arabian stallion was raised was found to contain an unusually high concentration of Mg (53 ppm) compared to Ca (25 ppm), Na (23 ppm) and K (8 ppm). A similar composition of water was observed near Davis, CA, where equine enteroliths are found with abnormal frequency (Blue and Wittkopp 1981). A summary of 30 case histories of equine enteroliths was published by Blue (1979), and a general review of the occurrence of minerals in the plant and animal kingdoms was presented by Lowenstam (1981). The objective of this study was to determine whether the chemical composition of an equine enterolith varies systematically as a function of distance from the center, thereby presumably recording changes in the composition of the intestinal fluid. METHODS AND MATERIALS We analyzed an enterolith recovered during necropsy of a horse from Medina County, Ohio. The enterolith (fig. 1) is approximately spherical in shape with 3 flattened faces suggesting it may have been part of a cluster of 4 stones. It is approximately 9 X 8 X 7 cm in diameter and has a density of 1.72 g/cm. The surface has a tan to light brown color that changes to bluish gray after drying and exposure to air. A slice, approximately 10 mm thick, was sawed from the center of the enterolith and was divided into 3 samples shown as A (inner), B and C (outer) in fig. 1. The center of this enterolith contains a fragment of metallic iron that was identified by X-ray fluorescence. The mineral matter occurs in concentric layers about this nucleus and consists both of thin laminations and radiating crystals. Fragments of undigested food (hay or straw?) and horse hair occur in the central region around the iron nucleus. The zones composed of radiating crystals appear to be porous and contain fragments of organic matter whereas the laminated portions are cryptocrystalline and dense. The laminations are marked by dark-colored bands and average about 0.5 mm in width. Concentrations of Mg and Ca were determined by atomic absorption spectrometry, Na and K by flame emission spectrometry, and Sr, Rb, Fe and Mn were measured by X-ray fluorescence spectrometry. Calibrations for Mg, Ca, Na, and K were based on standard solutions whereas the determinations of Sr, Rb, Fe and Mn were obtained by comparison with rock standards of the U.S. Geological Survey (Flanagan 1973). The isotopic composition of Sr, expressed by the Sr/Sr ratio, was measured by standard methods using a solid-source mass spectrometer (Faure and Powell 1972, Faure 1977). The Eimer and Amend SrCO3 isotope standard was analyzed in duplicate and yielded an average Sr/Sr ratio of 0.7081 ± 0.0002. This value is in agreement with results obtained by other analysts.