J Vet Intern Med 2004;18:346–349
Dietary Risk Factors and Colonic pH and Mineral Concentrations in Horses with Enterolithiasis Diana M. Hassel, Peter C. Rakestraw, Ian A. Gardner, Sharon J. Spier, and Jack R. Snyder A prospective, unmatched case control study was performed to identify dietary and environmental risk factors for enterolithiasis in horses in California and to determine whether colonic ingesta analyses differed between horses with and without enteroliths. Forty-three horses with enterolithiasis were compared with 19 horses with surgical colic attributable to nonstrangulating obstruction of the colon without enteroliths. Colonic ingesta samples were collected at surgery from horses with enteroliths and control horses. Colonic pH and colonic concentrations of magnesium, phosphorus, sulfur, sodium, calcium, potassium, and nitrogen were measured. Questionnaires were distributed to owners to determine diet and management practices. Student’s t-test and Mann-Whitney tests were used to evaluate differences in pH, dry matter content, percent nitrogen, and mineral content. Associations between dietary and management risk factors and enterolith occurrence were quantified by odds ratios. Mean pH of colonic contents from horses with enterolithiasis was significantly higher than for control horses. Horses with enterolithiasis had significantly lower percent dry matter in colonic fecal samples and higher mean mineral concentrations than controls. On the basis of reported feeding and management practices, horses with enterolithiasis were fed a significantly higher proportion of alfalfa in their diet and were less likely to have daily access to pasture grass than horses without enteroliths. Results suggest that decreasing alfalfa consumption and allowing daily access to pasture grazing might reduce the risk of enterolithiasis. Dietary modifications promoting acidification of colonic contents and dilution of minerals might be beneficial as preventive measures for enterolithiasis in horses. Key words: Alfalfa hay; Colic; Colon mineral content; Enteroliths.
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nterolithiasis remains an important and common form of colic in several geographic regions throughout the world, yet little is known about disease pathogenesis. Excessive levels of dietary magnesium, nitrogen, and phosphorus have been hypothesized to contribute to enterolith formation because struvite is the predominant component of enteroliths.1,2 Environmental factors, breed, intestinal pH, and the presence of a nidus have been documented as potential elements involved in enterolith formation.3,4 Earlier studies indicated that in vivo dissolution of enteroliths can occur and that alterations in intestinal pH might play an integral role in enterolith formation and dissolution.5,6 Studies evaluating dietary management and mineral analysis of colonic contents in horses with enteroliths are lacking and are essential to the understanding of enterolithiasis in horses. We hypothesized that horses with enteroliths consume higher proportions of alfalfa in their diet and have higher colonic pH and colonic mineral concentrations than horses without enteroliths with nonstrangulating obstructive disease of the colon. Thus, the objectives of this study were to confirm previous reports of increased colonic pH, to determine whether there are differences in colonic mineral content (nitrogen, magnesium, phosphorus, sulfur, calcium, sodium, and potassium) between surgical colic patients with From the Comparative Gastroenterology Laboratory, Department of Surgical and Radiological Sciences (Hassel, Rakestraw, Snyder), and the Department of Medicine and Epidemiology (Gardner, Spier), School of Veterinary Medicine, University of California, Davis, CA. Dr Rakestraw is presently affiliated with the College of Veterinary Medicine, Texas A&M University, College Station, TX. Presented in part at the 6th International Equine Colic Research Symposium, Athens, GA, 1998. Reprint requests: Diana M. Hassel, DVM, 2730 Concord Avenue, Davis, CA 95616-6102; e-mail:
[email protected]. Submitted May 15, 2003; Revised November 3, 2003; Accepted December 12, 2003. Copyright q 2004 by the American College of Veterinary Internal Medicine 0891-6640/04/1803-0014/$3.00/0
and without enteroliths, and to determine whether such differences were related to dietary or management risk factors.
Materials and Methods Case Selection Horses admitted between August 1996 and April 1998 that had a confirmed surgical diagnosis of nonstrangulating colonic obstruction with (experimental group) or without (control group) enteroliths were included. Horses with colic were sequentially enrolled in the study. Because development of obstructive enterolithiasis is rare in horses ,2 years old, horses ,2 years of age were excluded from the study. Colonic obstruction without strangulation was determined by evaluation of the type of obstruction and gross examination of the intestine during surgery. Horses with evidence of serosal discoloration or diffuse edema of the ascending or descending colon were excluded between both groups to allow a comparison of horses with similar obstructive processes and to minimize physiologic abnormalities pertaining to the obstructive process. In addition, horses with strangulating or nonstrangulating disorders of the small intestine, cecum, stomach, or other visceral organ were excluded. All procedures were approved by the Institutional Animal Care and Use Committee of the University of California-Davis.
Methods Samples of ascending colonic ingesta were obtained by pelvic flexure enterotomy. Immediately after collection, colonic fluid pH was measured with a pH meter.a Wet samples of colonic ingesta were stored at 2708C for further analysis. Dry matter content (mechanical convection oven at 1008C for 16 hours); total nitrogen concentrationb; and a mineral panelc including magnesium, phosphorus, sulfur, sodium, calcium, and potassium concentrations were determined. Questionnaires were distributed to owners at or near the time of admission to determine current diet, feeding, and management practices. Questionnaire data were obtained by personal or telephone interview by the same individual to ensure consistency in data collection. Information extracted from medical records and from owner questionnaires included age, breed, and sex of horse; dietary history, including proportions of various hays and supplements in the diet; duration of feeding of the described diet; source of water supply; and management practices, including level of confinement, type of bedding, frequency of feeding, and frequency of exposure to pasture grazing.
Risk Factors for Enterolithiasis in Horses
Data Analysis Odds ratios (ORs) and 95% confidence intervals were used to measure the strength of association between dietary and management risk factors and the occurrence of enteroliths. Proportion of alfalfa in the diet was dichotomized to a high (.70%) and low (#70%) category to distinguish between those horses receiving $75% of their diet as alfalfa hay versus those receiving #50% of their diet as alfalfa. These cutoffs were chosen because values of 50, 75, and 100% were commonly reported. Breed was categorized as Arabian or Other. Logistic regression was used to model associations between risk factors with significant ORs in univariate analysis following standard recommendations.7 Briefly, forward and backward variable selection were used, with P , .1 to enter and P . .15 to remove. Once the final model was selected, 2-way interactions between main effects were evaluated. The overall goodness of fit of the selected model was assessed with the Hosmer-Lemeshow (HL) statistic. Percent dry matter; pH; and wet and dry matter content of nitrogen, magnesium, phosphorus, sulfur, calcium, sodium, and potassium in the ascending colon of horses with and without enteroliths were compared by Student’s t-test and MannWhitney tests for normal and nonnormal data, respectively. The level of significance was set at P , .05. d
Results During the study period, samples were collected from 43 horses with enterolithiasis and 19 horses with nonstrangulating obstructive disease of the large intestine without enteroliths. Causes of nonstrangulating obstruction of the large intestine within the control horses were colonic displacement (n 5 5) and intraluminal obstruction of the ascending or descending colon by feed (n 5 3), sand (n 5 3) or fecoliths (n 5 8). Horses with enterolithiasis were fed a significantly higher (P , .001) mean (6SEM) proportion of alfalfa in their diet (91.9% 6 2.6%) compared with controls (62.1% 6 7.7%). In univariate analysis, horses on a diet of .70% alfalfa were at greater risk for enterolithiasis (OR 5 13.0; 95% CI, 3.5–48.7) than horses on a low-alfalfa (#70%) diet. In addition, horses with enteroliths were about one-tenth as likely (OR 5 0.11; 95% CI, 0.03–0.45) to have daily access to pasture grass than horses without enteroliths. Horses with enterolithiasis and those without enteroliths were about equally likely (P . .05) to have been of the Arabian breed, to have been fed a daily bran or grain supplement, or to have had access to well water as their primary water supply (data not shown). Both risk factors from the univariate analysis remained significant in the logistic regression model, with adjusted odds ratios of 10.8 (95% CI, 2.6–44.0) for the .70% alfalfa diet and 0.15 (95% CI, 0.03–0.72) for daily access to pasture. Model fit was good (HL goodness of fit statistic 5 0.884). Mean concentrations of magnesium, phosphorus, sulfur, sodium, calcium, and potassium in colonic contents of horses being fed .70% alfalfa were 1.5–2 times higher than of horses on low-alfalfa diets (#70%), regardless of the presence or absence of enteroliths. Horses on diets consisting of .70% alfalfa without daily access to pasture had mineral concentrations 1.7–3.2 times greater than that in horses on low-alfalfa diets (#70%) with daily access to pasture. Mean pH of colonic contents for horses with enteroliths (pH 7.32 6 0.07) was higher (P 5 .004) than for control horses (pH 6.93 6 0.13). Percent colonic dry matter was lower (P , .01) in horses with enteroliths (13.6% 6 0.7%)
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Table 1. Mean dry matter–corrected concentrations (ppm) of calcium, magnesium, phosphorus, potassium, sodium, sulfur, and percent total nitrogen in colonic ingesta of horses with enterolithiasis and horses with nonstrangulating colonic obstruction without enterolithiasis. Mean (SEM) Variable Calcium Magnesium Phosphorus Potassium Sodium Sulfur Total nitrogen (%) a
Cases 51,801 35,489 76,000 84,196 156,000 17,250 1.53
(9,171) (6,512) (16,354) (12,896) (21,968) (2,243) (0.11)a
Controls 23,849 13,620 23,677 37,701 66,084 11,001 0.91
(3,337) (2,996) (4,176) (8,533) (15,217) (2,530) (0.44)a
P value ,.005 ,.005 ,.005 ,.005 ,.005 ,.05 .02
Median value (SEM).
than in control horses (20.9% 6 2.1%). After correcting for differences in dry matter content, horses with enterolithiasis had higher (P , .005) mean concentrations of magnesium, phosphorus, sulfur, sodium, calcium, and potassium in colonic ingesta samples compared with control horses (Table 1). Percent nitrogen was also higher (P 5 .02) in horses with enteroliths.
Discussion Enterolithiasis remains the most frequent cause of surgical colic at the University of California, Davis, and the goals of this study were to identify risk factors for occurrence of the disease and improve our understanding of disease pathogenesis. Identification of increased alkalinity in the colonic contents of horses with enteroliths compared with a control population is consistent with previous reports.3,5,6 This result also correlates with findings in canine and feline struvite uroliths that develop in association with alkaline conditions in the urinary tract.8–11 A number of factors can contribute to the detectable difference in alkalinity between horses with and without enteroliths. Diet, bacterial flora, undetermined genetic factors or innate deficiencies affecting colonic luminal ion exchange mechanisms, or buffering capacity and pH of the water supply all have the potential to play a role in colonic pH regulation. Previous studies have shown that the addition of magnesium oxide to the diet increased the pH in the large intestine, and a decrease in hay : grain also resulted in decreased colonic pH.5 A reduction in percent colonic dry matter was observed in horses with enterolithiasis compared with controls. This increased colonic fluid content in horses with enterolithiasis was observed concurrently with higher mineral content of all minerals evaluated. This environment might prove ideal for struvite precipitation by creation of a supersaturated solution of minerals in an alkaline environment. Identification of factors that contribute to this environment, such as diet, environmental factors, genetic factors affecting colonic transport of water and ions, or other unknown mechanisms, could provide the key to understanding the pathogenesis of this disease process. Considering the nature of this study, the observed effects of increased mineral content and re-
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duced dry matter might represent an effect of the presence of the enteroliths rather than a predisposing cause. It is possible that the disease process itself results in alteration of the ingesta or results in changes in the colonic wall that subsequently modulate intestinal content. The study of animals with a proven susceptibility to enterolithiasis that have undergone surgical removal of enteroliths might help differentiate cause from effect. In this study, alfalfa was identified as a risk factor for enterolithiasis. However, alfalfa cannot be solely responsible for the formation of enteroliths, because most horses fed an alfalfa diet do not develop enteroliths. Alfalfa has both high magnesium and protein content that can result in higher levels of ammonia nitrogen in the ascending colon of horses during digestion, as well as providing magnesium for the precipitation of struvite.4 Alfalfa products might also contribute to alkalinization of colonic contents because alfalfa has a strong buffering capacity in ruminants.5,12 In addition to the direct effects of diet on the colonic luminal environment, indirect effects via colonic ion exchange mechanisms also could play a role. Lack of exposure to daily pasture grazing was an additional risk factor observed in this study. Cohen et al13 identified increased risk for enterolithiasis in horses fed alfalfa hay and under stall confinement. The findings of this study are in agreement with their observations. Several potential factors associated with time at pasture, such as activity level, differences in patterns of ingestion, and changes in colonic contents as a result of ingestion of grass, should be considered when evaluating the effect of pasture access on disease development. Daily access to pasture could be associated with increased activity. Exercise has been demonstrated to reduce retention time for particulate digesta and to increase digestibility of dry matter in equids.14,15 Perhaps a certain frequency of intestinal hypomotility within the ampulla coli of the right dorsal colon is a prerequisite for precipitation of struvite around a nidus. In addition to its potential for altering colonic motility, ingestion of grass might have a dilutional effect on consumed alfalfa. This is supported by the findings here of higher colonic mineral concentrations in horses without daily access to pasture grazing. The higher fiber content of grasses compared with alfalfa might also result in increased short-chain fatty acid production from microbial colonic fermentation, potentially reducing colonic pH. An association between the feeding of bran and the development of enteroliths was not observed, although it has been proposed by others.4 Wheat bran has a relatively high content of phosphorus, magnesium, and protein, and a recent report describes an association of high serum concentration of phosphorus and enterolithiasis.13 Despite the higher serum phosphorus concentrations, only 1 of 26 horses with enterolithiasis had been fed bran. In this study, significant differences in serum phosphorus concentration were not found between controls and horses with enterolithiasis. The relationship between phosphorus content in the water supply and enterolithiasis has not been evaluated and requires further investigation. Arabians and Arabian crosses were slightly overrepresented compared with controls, but the difference was not statistically significant (OR 5 1.8, P 5 .3). This finding is
consistent with prior reports that identified a predisposition for the Arabian breed.4,13,16 We categorized breed as either Arabian or Other because of the small sample size and the variety of other breeds represented. American Miniatures have been identified at increased risk for both enterolithiasis and fecolith obstruction.13,17,18 In this study, there was 1 American Miniature within the experimental group and 2 within the control group, both with fecolith impactions. These horses were also only 2 years old and therefore could represent animals at increased risk for enterolithiasis. Selection of an older cutoff age as an exclusion criterion for the study would have increased the likelihood of identifying horses with the potential for developing enteroliths and reduced the risk of misclassification errors. Inclusion of animals within the control group that have the potential to develop enterolithiasis later in life would tend to dilute observed differences between the 2 groups, lending further credibility to these findings. In this study, we attempted to evaluate alterations in the colonic environment present in horses with an increased susceptibility for enterolithiasis. However, the differences observed might be reflective of differences in the diet rather than true physiologic differences between the 2 groups of horses. In the comparison of effects of a high-alfalfa diet (.70%) to a low-alfalfa diet (#70%), when disease was not accounted for, increased colonic mineral concentration was detected in horses on the high-alfalfa diet, indicating that diet alone plays an important role in the establishment of the colonic milieu. In other species, down-regulation of colonic sodium/hydrogen exchange can be achieved with increased mucosal ammonia concentration and reduced luminal short-chain fatty acid concentration, both of which can be expected on a high-alfalfa diet compared with a grass or oat hay diet.19,20 This could contribute to the observations of higher colonic pH and reduced sodium and water absorption from the colonic lumen, conditions consistent with the observed environment in horses with enterolithiasis in this study. Experimental studies incorporating strict controls of diet and environment are necessary to determine whether the observed differences in pH and colonic mineral content between experimental and control populations are secondary to variations in diet, factors associated with the colic episode itself, such as effects of treatment with solutions administered PO or prolonged colonic distention, or true physiologic distinctions between the groups of individuals as a result of altered genetic expression. The strong breed predilections and high prevalence in siblings is suggestive of a heritable component to the disease.16 Alternatively, the observed differences might reflect an effect of the presence of the enteroliths within the colonic lumen, rather than a contributing cause to the disease. Results of this study suggest that dietary modifications promoting acidification of colonic contents and dilution of minerals can be beneficial as preventive measures in horses at risk for development of enterolithiasis. Reducing colonic pH can be accomplished by increasing the grain : hay ratio in the diet, by supplementation with apple cider vinegar, and potentially by alternative dietary supplements that can alter colonic bacterial fermentation. However, caution must be exercised to maintain colonic pH within a narrow opti-
Risk Factors for Enterolithiasis in Horses
mal range to avoid the potential for severe gastrointestinal adverse effects. Diets high in alfalfa (.70%) and limited access to pasture were significant risk factors for enterolithiasis in horses. Further studies are necessary to identify the magnitude of effect of various diets on pH and mineral content of colonic ingesta in horses.
Footnotes pH/mV Meter, model M-320, Corning Inc, Fountain Valley, CA Nitrogen Analyzer, model FP428, Leco Corp, St Joseph, MI c Model 3520 sequential ICP, Applied Research Laboratories, Dearborn, MI d Epi-Info, version 6.0, Centers for Disease Control and Prevention (CDC), Atlanta, GA a
b
Acknowledgments Supported by the Center for Equine Health with funds provided by the Oak Tree Racing Association and the State of California pari-mutuel fund and contributions by private donors. The authors thank Dr Francis D. Galey and the California Veterinary Diagnostic Laboratory for technical assistance and Dr John R. Pascoe for manuscript contributions.
References 1. Blue MG, Wittkopp RW. Clinical and structural features of equine enteroliths. J Am Vet Med Assoc 1981;179:79–82. 2. Hassel DM, Schiffman PS, Snyder JR. Petrographic and geochemic evaluation of equine enteroliths. Am J Vet Res 2001;62:350– 358. 3. Murray RC, Constantinesu GM, Green EM. Equine enterolithiasis. Compend Contin Educ Pract Vet 1992;14:1104–1113. 4. Lloyd K, Hintz HF, Wheat JD, et al. Enteroliths in horses. Cornell Vet 1987;77:172–186. 5. Hintz HF, Lowe JE, Livesay-Wilkins P, et al. Studies on equine enterolithiasis. Proc AAEP 1988;24:53–59.
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6. Hintz HF, Hernandez T, Soderholm V, et al. Effect of vinegar supplementation on pH of colonic fluid. Proceedings Equine Nutrition Physiology Symposium 1989:116–118. 7. Hosmer DW, Lemeshow S. Applied Logistic Regression, 2nd ed. New York, NY: Wiley and Sons; 2000:373 pp. 8. Buffington CA, Rogers QR, Morris JG, et al. Feline struvite urolithiasis: Magnesium effect depends on urinary pH. Feline Pract 1985; 15:29–33. 9. Domingo-Neumann RA, Ruby AL, Ling GV, et al. Ultrastructure of selected struvite-containing urinary calculi from dogs. Am J Vet Res 1996;57:1274–1287. 10. Neumann RD, Ruby AL, Ling GV, et al. Ultrastructure of selected struvite-containing urinary calculi from cats. Am J Vet Res 1996;57:12–24. 11. Osborne CA, Sanna JJ, Unger LK, et al. Analyzing the mineral composition of uroliths from dogs, cats, horses, cattle, sheep, goats, and pigs. Vet Med 1989;84:750–764. 12. Fadel JG. In vitro buffering capacity changes of seven commodities under controlled moisture and heating conditions. J Dairy Sci 1992;75:1287–1295. 13. Cohen ND, Vontur CA, Rakestraw PC. Risk factors for enterolithiasis among horses in Texas. J Am Vet Med Assoc 2000;216: 1787–1794. 14. Orton RK, Humes ID, Leng RA. Effects of exercise and level of dietary protein on digestive function in horses. Equine Vet J 1985; 17:386–390. 15. Pearson RA, Merritt JB. Intake, digestion and gastrointestinal transit time in resting donkeys and ponies and exercised donkeys given ad libitum hay and straw diets. Equine Vet J 1991;23:339–343. 16. Hassel DM, Langer DL, Snyder JR, et al. Evaluation of enterolithiasis in equids: 900 cases (1973–1996). J Am Vet Med Assoc 1999;214:233–237. 17. Ragle CA, Snyder JR, Meagher DM, et al. Surgical treatment of colic in American Miniature horses: 15 cases (1980–1987). J Am Vet Med Assoc 1992;201:329–331. 18. McClure JT, Kobluk C, Voller K, et al. Fecalith impaction in four miniature foals. J Am Vet Med Assoc 1992;200:205–207. 19. Cermak R, Lawnitzak C, Scharrer E. Influence of ammonia on sodium absorption in rat proximal colon. Eur J Physiol 2000;440:619– 626. 20. Argenzio RA, Southworth M, Lowe JE, et al. Interrelationship of Na, HCO3, and volatile fatty acid transport by equine large intestine. Am J Physiol 1977;233:E469–E478.
