Abstract

Enterolithiasis has been reported in domestic horses in California and has been documented in several non-domestic equids at the San Diego Wild Animal Park (WAP). A laparotomy was performed on a pregnant 4.5-year-old Somali wild ass (SWA, Equus africanus somalicus) with acute onset of severe abdominal pain. An enterolith, 10 cm in diameter, was removed from the transverse colon. Postoperatively, the animal experienced decreased appetite, decreased fecal output, dehydration, brief diarrhea, and a stillbirth. Following passage of the stillborn foal, the SWA improved rapidly and was released to the exhibit 60 days after surgery. Enterolithiasis has been documented in approximately 15 non-domestic equids at the WAP, and preventive measures such as dietary changes, water modifications, and radiographic screening have been implemented.

Introduction

Equine enteroliths are most common in California² and the southwestern U.S., including Texas.¹ Equine enteroliths are usually comprised of struvite (magnesium ammonium phosphate) crystals deposited in concentric layers around a non-digestible nidus.³ In a review of 900 equine patients treated for enterolithiasis at the University of California Davis Veterinary Teaching Hospital,² enteroliths were present and associated with disease in 15% of all horses admitted for colic; Arabians and Arabian crosses appeared over-represented. Stones were found most often in the right dorsal colon, the descending colon, and the transverse colon, and were both solitary and found in large numbers. While no dietary predisposition prior to admission was identified, recurrence of stones occurred significantly less frequently in horses that were fed less alfalfa hay. With a high magnesium component, alfalfa is theorized to increase magnesium content in the colon, as well as to alkalinize colonic contents, thereby favoring the formation of struvite. Thus far, an adequate experimental model for enterolith formation has not been developed, and much information regarding etiology, prevention, and treatment remains unknown.

Enteroliths have also been reported in non-domestic perissodactylids. Struvite enteroliths were successfully removed surgically in two female Grant’s zebras (Equus burchelli boehmi) showing signs of colic in northern California after enterolithiasis was demonstrated at necropsy in other zebras of the same herd.⁴ Enteroliths from South American tapirs (Tapirus terrestris) and a Malayan tapir (Tapirus indicus) were analyzed and were found to be primarily composed of vivianite and newberyite.⁵

In 1989, at the San Diego Wild Animal Park (WAP), an enterolith was surgically removed from an eastern kiang (Equus kiang holdereri) that was demonstrating signs of colic. Approximately 15 more cases of enterolithiasis have been identified between 1996 and 2003, predominantly in kiangs, but also in Somali wild asses (SWA, Equus africanus somalicus), Przewalski’s horses (Equus przewalksii), a Persian onager (Equus hemionus onager), and a Grant’s zebra.

Case Report

A 252-kg, 4.5-year-old female SWA presented with acute onset of abdominal discomfort in November 2002. She had no significant prior medical history. Anesthesia was induced with etorphine hydrochloride (0.02 mg/kg, IM) and acepromazine maleate (0.04 mg/kg, IM) via remote injection. Glycerol guaiacolate was administered IV to effect to facilitate intubation with a 26 mm endotracheal tube. Isoflurane and oxygen were administered, and the patient was placed on a mechanical ventilator for the two hour procedure. An IV catheter was placed in the right jugular vein, and the patient was positioned and aseptically prepared for a ventral midline laparotomy. Abdominal exploratory revealed an enterolith, approximately 10 cm in diameter, in the transverse colon. Two enterotomies were performed to remove fecal material and the enterolith. The enterotomies were closed in two layers with 00-polydioxinone (PDS®) suture using a simple continuous pattern followed by a continuous Cushing’s pattern. A viable third trimester foal was palpated in the uterus. No other abnormalities were observed. The abdominal wall was closed in three layers. The abdominal fascia was apposed using #2 polyglactin 910 (Vicryl®) in an interrupted cruciate pattern. The subcutaneous tissue was apposed using 0-polyglactin 910 and the skin edges were apposed using stainless steel staples. Intraoperative treatments consisted of a balanced electrolyte solution (lactated Ringer;s solution [LRS], 8 liters, IV), flunixin meglumine (1.6 mg/kg, IV), ampicillin sodium (16 mg/kg, IV), and gentamicin sulfate (8 mg/kg, IV). Anesthesia was antagonized with diprenorphine (0.04 mg/kg, IV) and naltrexone (1 mg/kg, IV). The SWA appeared quiet and responsive the following morning (day two). Postoperative phenylbutazone (6 mg/kg, PO) was prescribed once daily for five days.

Food was withheld for the initial 24 hours post-surgery, but water was offered free choice. The SWA was placed on a standard WAP post-enterotomy dietary schedule (Table 1), to return to a regular diet by day 16 post-op. On day four, she appeared moderately depressed, and had failed to eat or defecate since prior to surgery. Flunixin meglumine (1.2 mg/kg, IM) was administered, with no improvement noted following treatment. On day five, she was anesthetized with etorphine hydrochloride (0.02 mg/kg, IM) and detomidine hydrochloride (0.04 mg/kg, IM) by remote injection. Propofol was administered in three separate 500 mg IV boluses to maintain anesthesia. On examination, mucus membranes were bright red and the SWA appeared 5% dehydrated. Fetid diarrhea was noted in rectum. Three fecal cultures were negative for Salmonella sp., and the diarrhea resolved quickly over the next two days. Abdominal ultrasonographic examination revealed a strong fetal heartbeat. An IV catheter was placed in the right jugular vein, 11 liters of a balanced electrolyte solution (LRS) were administered over 40 minutes, then administered at a rate of 750 ml/h overnight. Flunixin meglumine (1 mg/kg, IV) was administered twice daily for two days. On day six, the SWA had eaten a small amount of pellets. On day seven, she appeared brighter and more alert. On day eight, she was observed eating pellets and hay, and the catheter was removed under manual restraint. On day nine, labor was observed mid-day, and a stillborn foal was delivered. Following the stillbirth, the SWA improved steadily, with increasing appetite and fecal production noted.

Table 1. Standard postoperative management of non-domestic equids at the San Diego Wild Animal Park following abdominal surgery to remove enteroliths

The SWA was anesthetized again on day 45 for a hoof trim and recheck of the suture line. Anesthesia was induced with etorphine hydrochloride (0.023 mg/kg, IM) and detomidine hydrochloride (0.06 mg/kg, IM), and maintained with propofol (1 mg/kg, IV), and inhalation anesthesia using isoflurane and oxygen. Other than a firm swelling at the ventral abdominal suture site, no abnormalities were noted. Staples were removed, and anesthesia was antagonized with naltrexone (1.45 mg/kg, IV) and yohimbine (0.1 mg/kg, IV). The SWA was released from the hospital to a holding pen on day 46 and was released back to the exhibit on day 60. No problems have been noted since release, and the SWA recently gave birth to a healthy female foal.

Discussion

The acute onset of severe abdominal discomfort in the SWA in this report is consistent with the majority of enterolith-associated colics observed in non-domestic equids at the WAP. With the history at the WAP, rapid surgical intervention is often selected, in place of diagnostics such as a complete blood cell count, a serum biochemistry panel, abdominocentesis, or abdominal radiographs. However, elective radiographic examination has become the cornerstone of the preventive screening program developed to manage enterolithiasis in non-domestic equids at the WAP. Between December 2002 and January 2004, seven SWA and ten kiangs were screened for the presence of enteroliths, using a four-quadrant approach described for domestic horses (Figure 1).⁶ View 3 visualizes the right dorsal colon/ampulla coli, transverse colon, and proximal small colon and is the most beneficial view in domestic horses.⁶ Unlike diagnostic abdominal radiographic studies in domestic horses, the studies performed at the WAP on non-domestic equids are done with the animals in lateral recumbency during general anesthesia, making stone location and accurate anatomic identification more challenging. In 2003, four asymptomatic kiangs (including a female that had enteroliths surgically removed in 1989) were determined radiographically to have enteroliths large enough to potentially cause full or partial obstructions and underwent elective laparotomies without complications. In several asymptomatic animals where radiographic findings were considered equivocal, repeat radiographs were performed six months later, to evaluate size and movement of suspected enteroliths. Adequate hospital facilities and availability of veterinary and support staff are paramount to being able to implement and adhere to an aggressive preventive health program such as this one. Having recently completed screening all of the equids at the WAP that are considered “at-risk”, WAP veterinary services is considering repeating radiographs for each animal within two to three years, depending on development of clinical cases.

Figure 1

Standard four-quadrant radiographic technique utilized by San Diego Wild Animal Park to screen non-domestic equids for presence of enteroliths. Adapted from domestic horse literature(6)

The enterolith removed from the SWA in this case report was a solitary stone located in the transverse colon, one of the three most common locations reported in domestic horses.^2,6 Analysis of previous stones recovered from WAP equids has identified struvite, or magnesium ammonium phosphate, as the predominant component, identical to domestic equid enteroliths. Possible causes of enterolithiasis in WAP equids are similar to those proposed for domestic horses and include dietary contributions, nidus ingestion, mineral content in drinking or ground water, and breed or species predisposition. It has been reported that California may far exceed recommended magnesium requirements in both alfalfa and water.² Dietary changes were made at the WAP in November 2002, and modification of the watering system for these animals is currently underway.

Post-operative complications in enterolith-associated colic surgery are reportedly low, compared to other forms of intestinal surgery in domestic horses, and include postoperative diarrhea, incisional infection, incisional hernia formation, and positive fecal Salmonella sp. culture.² Decreased appetite, decreased fecal output, diarrhea, and mild dehydration were considered mild to moderate postoperative complications in this case. Stillbirth of a previously healthy third trimester foal was also a complication, and may have been related to maternal stress, dehydration, anesthetic or therapeutic agents, or other unknown etiologies. Gross and histologic examination of the placenta and foal was unremarkable; however, tissue cultures were not performed. Fortunately, this event did not appear to affect the female’s long-term reproductive health, as she gave birth to a healthy female foal in March 2004, one year and four months following her stillbirth.

Literature Cited

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2.  Hassel DM, Langer DL, Snyder JR, Drake CM, Goodell ML, Wyle A. Evaluation of enterolithiasis in equids: 900 cases (1973–1996). JAVMA. 1999;214:233–237.

3.  Hassel DM, Schiffman PS, Snyder JR. Petrographic and geochemic evaluation of equine enteroliths. Am J Vet Res. 2001:62:350–358.

4.  McDuffee LA, Dart AJ, Schiffman P, Parrot JJ. Enterolithiasis in two zebras. JAVMA. 1994;204:430–432.

5.  Murphy MR, Masters JM, Moore DM, Glass HD, Hughes RE, Crissey SD. Tapir (Tapirus) enteroliths. Zoo Bio. 1997;16 427–433.

6.  Yarbrough TB, Langer DL, Snyder JR, Gardner IA, O’Brien TR. Abdominal radiography for diagnosis of enterolithiasis in horses: 141 cases (1990–1992). JAVMA. 1994;205:592–595.