A 1.4-kg (3.1-lb) 4-year-old sexually intact male pet guinea pig (Cavia porcellus) was brought to the William T. Pritchard Veterinary Medical Teaching Hospital at the University of California-Davis for an annual wellness examination. The owner reported that the guinea pig had been doing well at home with no notable changes in appetite or fecal production and no coughing, sneezing, or diarrhea. The initial physical examination revealed that the guinea pig was overweight (body condition score of 7 on a scale of 1 to 9 where 1 = very thin and 9 = morbidly obese). Masses in the left and right mammary glands noted during previous examinations were still present and thought to have increased in size. Another approximately 1.5-cm firm mass was palpated in the cranial portion of the abdomen. The remainder of the physical examination was unremarkable.
The guinea pig was sedated with dexmedetomidinea (0.03 mg/kg [0.01 mg/lb], IM) and midazolamb (0.3 mg/kg [0.1 mg/lb], IM) so that further diagnostic testing could be performed. A blood sample (approx 0.6 mL) was collected from a lateral saphenous vein for serum biochemical analysis. Results of that analysis were clinically unremarkable except for moderate hyperglycemia (glucose concentration, 388 mg/dL; reference interval,1 60 to 125 mg/dL) and hyponatremia (sodium concentration, 131 mEq/L; reference interval,1 146 to 152 mEq/L).
Survey whole-body radiographs were obtained and revealed mild periarticular mineralization of the hip and stifle joints, which had been present and appeared to be static, compared with that observed on radiographs obtained during a previous visit. Remarkable new radiographic changes included a fat-density mass effect in the inguinal region, mild hepatomegaly, and a 1.5-cm round soft tissue opacity located caudal and dorsal to the liver and separated from the liver by abdominal fat that was confluent with the stomach (Figure 1).
Right lateral radiographic image of the thorax, abdomen, and pelvis of a 4-year-old sexually intact male pet guinea pig (Cavia porcellus) with a 1.5-cm round soft tissue opacity (arrows) located caudal and dorsal to the liver and separated from the liver by abdominal fat that was confluent with the stomach. Other radiographic abnormalities include hepatomegaly, mild periarticular mineralization of the hip and stifle joints, and fat-opaque masses in the inguinal region.
Citation: Journal of the American Veterinary Medical Association 249, 12; 10.2460/javma.249.12.1415
Ultrasonographic examination of the abdomen was then performed. Remarkable findings included debris in the urinary bladder, small areas of increased echogenicity in the right kidney, and a well-defined soft tissue mass adjacent to or confluent with the stomach that measured 1.5 cm in diameter and had a diffusely hypoechoic appearance. An ultrasound-guided fine-needle aspirate of the abdominal mass was obtained by use of a 25-gauge 1.5-inch needle attached to a 3-mL syringe. Smears of the aspirate sample were made on microscope slides, allowed to air dry, and then stained with Wright-Giemsa stain. Cytologic evaluation of the smears revealed a purple background that contained scattered free-cell nuclei and erythrocytes; the few intact cells that were present were primarily spindle cells with elongated nuclei, finely stippled chromatin, and indistinct to occasionally prominent nucleoli and were often observed in poorly defined aggregates. The spindle cell population had mild to moderate anisokaryosis and included cells with small to moderate amounts of moderately basophilic cytoplasm, which occasionally had an indistinct border. Although no unequivocal evidence of malignancy was observed, the number of spindle cells observed in the absence of inflammation or other stimuli for fibroplasia was suggestive of a well-differentiated sarcoma; however, reactive fibroplasia could not be excluded. Histologic examination of an incisional or excisional biopsy specimen of the mass was recommended to obtain a definitive diagnosis.
An ultrasonographic examination of the heart was performed at the request of the owner because other guinea pigs in her collection had developed cardiac disease, and the results were within reference limits. The sedation was reversed with flumazenilc (0.02 mg/kg [0.009 mg/lb], IM) and atipamezoled (0.3 mg/kg, IM), and the guinea pig had an uneventful recovery. The guinea pig was discharged from the hospital the same day with the recommendation that it be brought back for a surgical biopsy of the mass so that a definitive diagnosis could be obtained.
The guinea pig was brought back to the hospital 9 days later for an exploratory laparotomy. The owner reported that the guinea pig had been doing well at home and no new health concerns had developed since the previous visit. Physical examination results were consistent with the findings of the previous visit. The following day, the guinea pig was sedated with oxymorphone hydrochloridee (0.1 mg/kg [0.045 mg/lb], IM), midazolamb (1 mg/kg [0.45 mg/lb], IM), and ketamine hydrochloridef (2.5 mg/kg [1.1 mg/lb], IM). Anesthesia was induced with isoflurane (concentration, 3%) administered with supplemental oxygen via a face mask. A 24-gauge catheter was aseptically placed in a cephalic vein. The guinea pig was intubated with a 2.0-mm uncuffed endotracheal tube. Anesthesia was maintained with isoflurane (concentration range, 1% to 3%) in oxygen.
The guinea pig was positioned in dorsal recumbency, and the ventral aspect of the abdomen was clipped and aseptically prepared for surgery in a routine manner. A standard ventral midline surgical approach was used as described.2 A self-retaining retractor was used to retract the body wall and maximize visibility of the abdominal contents. The liver had a diffusely coarse and heterogeneous appearance, and a biopsy specimen of the liver was obtained by use of the guillotine technique with 3-0 polydioxanone.g An approximately 1.5-cm mass protruded from the serosal surface of the stomach on the cranial aspect midway between the greater and lesser curvatures at a point approximately two-thirds caudal to the most orad aspect of the long axis of the stomach (Figure 2). The stomach was exteriorized and isolated from the rest of the abdomen by the use of laparotomy sponges that were moistened with sterile saline (0.9% NaCl) solution. Stay sutures were placed within the stomach wall to aid in its manipulation. A full-thickness partial gastrectomy was performed and the mass was excised with 1-cm margins of grossly and palpably normal gastric wall. The stomach wall was closed by the use of 3-0 polydioxanoneg in a full-thickness continuous suture pattern. The stomach wall was closed transverse to the long gastric axis, which effectively shortened the greater curvature but maintained the diameter of the gastric lumen. Sterile saline solution was infused into the gastric lumen with a 27-gauge needle attached to a 3-mL syringe to ensure that there was no leakage from the gastrectomy site (Figure 3). No other abnormalities were detected during the remainder of the abdominal exploration. The linea alba was apposed with 3-0 polydioxanoneg in a simple continuous suture pattern, and the subcuticular layer was closed with 4-0 poliglecaprone 25h in a continuous intradermal suture pattern.
Intraoperative photograph of a 1.5-cm mass protruding from the serosal surface of the stomach of the guinea pig of Figure 1. The mass was located on the cranial aspect of the stomach midway between the greater and lesser curvatures at a point approximately two-thirds caudal to the most orad aspect of the long axis of the stomach. Histologic evaluation of the mass revealed that it was a gastric leiomyoma. Cranial is to the left, caudal is to the right, and the tip of the DeBakey forceps indicates the pyloric sphincter.
Citation: Journal of the American Veterinary Medical Association 249, 12; 10.2460/javma.249.12.1415
Intraoperative photograph of the completely sutured gastrectomy site of the guinea pig of Figure 1 following excision of a 1.5-cm gastric leiomyoma. Sterile saline (0.9% NaCl) solution was infused into the gastric lumen with a 27-gauge needle attached to a 3-mL syringe to ensure that there was no leakage from the gastrectomy site.
Citation: Journal of the American Veterinary Medical Association 249, 12; 10.2460/javma.249.12.1415
The mammary gland masses were then removed. Briefly, an approximately 3.5-cm elliptical skin incision was made on both the left and right ventrolateral aspect of the caudal portion of the abdomen. Two masses were resected from the left mammary gland, and 1 small well-defined mass and another ill-defined mass along with the nipple were resected from the right mammary gland. On both sides, the subcutaneous tissue was closed with 4-0 poliglecaprone 25h in a simple interrupted suture pattern, and the subcuticular layer was closed with 4-0 poliglecaprone 25h in a continuous intradermal suture pattern.
The guinea pig had an uneventful recovery from anesthesia and was administered oxymorphonee (0.1 mg/kg, SC, q 4 h) for analgesia for the first 24 hours after surgery. Approximately 5 hours after the discontinuation of anesthesia, crackles were detected during lung auscultation, and administration of trimethoprimsulfamethoxazolei (30 mg/kg [13.6 mg/lb], PO, q 12 h) was initiated because of concerns about potential aspiration pneumonia. The lung crackles could also have been a consequence of the guinea pig having been maintained at a surgical plane of anesthesia while positioned in dorsal recumbency for a prolonged period.
While the guinea pig was hospitalized, analgesia was maintained with buprenorphine hydrochloridej (0.1 mg/kg, SC, q 8 h) for 2 days followed by tramadol hydrochloridek (10 mg/kg [4.5 mg/lb], PO, q 12 h) for 1 day. Assisted feeding was initiated within 4 hours after surgery; 9 g of herbivore recovery formulal mixed with water to create a 10- to 20-mL solution was fed to the guinea pig with a syringe approximately every 8 hours until the animal was sent home. The guinea pig was discharged from the hospital 4 days after surgery with instructions for continued administration of tramadol (10 mg/kg, PO, q 12 h) for 5 to 7 days and trimethoprim-sulfamethoxazole (30 mg/kg, PO, q 12 h) for 14 days.
The liver biopsy specimens, gastric mass, and mammary gland masses were evaluated histologically. Examination of the gastric mass revealed that the mass-associated gastric serosa was markedly elevated by a well-demarcated, nonencapsulated, bilobed, densely cellular neoplastic mass that projected away from the gastric lumen and slightly elevated the overlying tunica muscularis. The mass was composed of long, interlacing streams and bundles of spindle cells interspersed with mature adipocytes. Individual cells were spindloid with indistinct cell borders and eosinophilic cytoplasm. Nuclei were ovoid with finely stippled to vesicular chromatin and indistinct nucleoli. Anisocytosis and anisokaryosis were moderate, and 2 mitotic figures with occasional binucleation were counted in ten 400× fields (Figure 4). The serosal vessels at the base of the mass were markedly thickened with expansion of the tunica media. The overlying submucosa and muscular tunics were moderately edematous. The cellular and morphological features of the gastric mass were consistent with either a leiomyoma or a gastrointestinal stromal tumor. To distinguish between those 2 types of neoplasms, sections of the mass underwent immunohistochemical staining with antibodies against smooth muscle actin and c-kit (CD117). The neoplastic cells were diffusely immunoreactive to smooth muscle actin but not c-kit. On the basis of those results, a histologic diagnosis of leiomyoma was made. Histologic examination of the mass margins revealed that the tumor was completely, but narrowly, excised. Neoplastic cells extended to the sectional margins but were circumferentially bound by a thin layer of compressed stroma.
Photomicrograph of a section of the gastric leiomyoma excised from the guinea pig of Figure 1. The mass was characterized by the presence of streams and bundles of neo-plastic smooth muscle cells with abnormally elongated nuclei with blunted ends when in longitudinal section, eosinophilic cytoplasm, and nondistinct cell margins. This nonencapsulated tumor was immediately subjacent to the tunica muscularis of the stomach. H&E stain; bar = 50 μm.
Citation: Journal of the American Veterinary Medical Association 249, 12; 10.2460/javma.249.12.1415
Histologic abnormalities observed in the liver biopsy specimens were characterized by mild multifocal hepatic lipidosis and centrilobular hepatic glycogenesis with arterial wall hyalinosis and were considered incidental findings. The histologic diagnoses for the mammary gland masses included hamartoma, lipoma, and fibrolipoma.
The guinea pig underwent a recheck examination 19 months after surgery. An abdominal ultrasonographic examination was performed at that time, and results revealed no evidence of gastric mass recurrence.
Discussion
To our knowledge the present report was the first to describe a partial gastrectomy for resection of a gastric leiomyoma in a guinea pig. Leiomyomas are benign submucosal tumors that arise from smooth muscle cells and are generally small, localized, and well encapsulated.3 The mucosa over the tumor usually remains intact, and gastric ulceration and hemorrhage are uncommon.4 Leiomyomas most commonly develop in the gastrointestinal tract, but have also been identified in the spleen, liver, genitourinary tract, retroperitoneal space, and SC tissue.3 Gastric leiomyomas have been described in humans,5 horses,6 dogs,7 cats,8 and guinea pigs.9
Although primary neoplasms of the gastrointestinal tract are rare in guinea pigs, they are typically gastric leiomyomas that arise in the pyloric region.9 In dogs, gastric leiomyomas are most frequently located in the cardia of the stomach or at the gastroesophageal junction.4 Other types of gastric tumors described in guinea pigs include lipomas, fibromyomas and gastrointestinal stromal tumors.10,11 Uterine leiomyoma is another commonly reported neoplasm in guinea pigs.12 Leiomyomas are occasionally associated with paraneoplastic syndromes such as hypoglycemia, nephrogenic diabetes insipidus, and secondary erythrocytosis.3 The gastric leiomyoma in the guinea pig of this report was located in the body of the stomach orad to the pylorus and was not associated with any paraneoplastic syndrome.
Gastric neoplasia is fairly uncommon in other species. In dogs, gastric leiomyoma is the second most common gastric neoplasm, preceded only by gastric adenocarcinoma.4 In horses, gastric neoplasms account for only 1.5% of all neoplasms, of which squamous cell carcinoma is the most common.6 Although uncommon, gastric lymphoma is the most common gastric neoplasm in cats.13 Reports of gastric neoplasms in rodents are rare and include gastric adenocarcinoma in a chinchilla14 and hamster.15 Lymphoma has been reported in the gastric fundus of a rabbit,16 and there are multiple reports of pyloric adenocarcinoma in ferrets.17,18 Other differential diagnoses for gastric neoplasms include leiomyosarcoma, adenoma, and fibrosarcoma.19
In dogs, clinical signs associated with gastric leiomyoma range from none to weight loss, gastrointestinal hemorrhage, vomiting or regurgitation, gastric dilatation-volvulus, intussusception, and anorexia.7 The nature and severity of the clinical signs depend on the location and size of the tumor, and clinical signs may not be detected until the mass is large enough to cause a mechanical obstruction.4 The guinea pig of the present report did not have any clinical signs, and the gastric mass was an incidental finding detected during the animal's annual physical wellness examination.
In veterinary patients, hypoglycemia is a common paraneoplastic syndrome associated with leiomyoma.20 The mechanism by which smooth muscle tumors such as leiomyomas cause hypoglycemia is not clearly understood.20 The guinea pig of the present report had hyperglycemia rather than hypoglycemia, which was suspected to be a stress response associated with the physical examination. The guinea pig of this report also did not have microcytic anemia or hypoproteinemia, 2 other clinicopathologic abnormalities commonly observed in dogs with gastric leiomyomas.21
Evaluation of survey radiographs may assist in the diagnosis of gastric neoplasms, and contrast abdominal radiographs may reveal areas of ulceration or masses in the gastrointestinal tract.21 Abdominal ultrasonography typically provides the best opportunity for anatomic characterization of gastric neoplasms.19 Ultrasonographic features of smooth muscle tumors of the gastrointestinal tract can be used to distinguish them from other types of neoplasia.22 Leiomyomas typically have a smooth and homogeneous ultrasonographic appearance, although areas of central necrosis may appear as hypoechoic or anechoic cavities within the mass.22 Smooth muscle tumors of the gastrointestinal tract originate intramurally and either grow into the bowel lumen or bulge extraluminally from the serosa.22 In human patients, leiomyomas most commonly develop in the esophagus at the lower esophageal sphincter and only rarely develop in the stomach and intestines.7,19 During abdominal ultrasonographic examination of the guinea pig of this report, it appeared that the mass was either adjacent to or confluent with the stomach.
Fine-needle aspiration of gastric masses can be attempted, but cytologic evaluation of the resulting aspirates may be unrewarding because they frequently contain few cells.19 For the guinea pig of this report, cytologic evaluation of fine-needle aspirates of the gastric mass, although diagnostic, was suggestive of mesenchymal neoplasia and provided enough information to justify surgical biopsy of the mass. In dogs and horses, gastroscopy is commonly used to identify gastric masses and obtain biopsy specimens.6,19 Gastroscopy was not considered feasible for the guinea pig of this report because of its small size and the fact that the palatal ostium and presence of the endotracheal tube limited the space available in the caudal aspect of the oropharyngeal cavity for maneuvering the gastroscope. Additionally, because this particular mass did not involve the mucosa and bulged extraluminally into the peritoneal cavity, it is unlikely that gastroscopy would have aided in the successful visualization or biopsy of the mass. Laparoscopy, however, could have been considered as an alternative to laparotomy.
Histologic examination is necessary to achieve a definitive diagnosis of leiomyoma.7 Leiomyomas are composed of well-differentiated smooth muscle cells that have a strongly eosinophilic cytoplasm and have a low or moderate number of cells with small and elongated nuclei and little to no mitotic activity.7 Immunohistochemically, leiomyomas express α-smooth muscle actin and desmin but do not express c-kit.7 Gastric leiomyomas and gastrointestinal stromal tumors have similar characteristics, and the 2 types of tumors are differentiated on the basis of histologic features and cell differentiation markers.7 In human patients, most mesenchymal tumors of the gastrointestinal tract develop in the stomach and small intestine and are most commonly gastrointestinal stromal tumors that range from small benign tumors to aggressive sarcomas.7 Gastrointestinal stromal tumors express c-kit and CD34.7,19 The c-kit protein sequence is highly preserved among species, with 100% homology between dogs and humans.7 The percentage of c-kit homology between guinea pigs and dogs or humans is unknown.
The recommended treatment for gastric leiomyoma is surgical resection.19 Surgical techniques for abdominal surgery of guinea pigs follow the same recommendations established for dogs.2 An advantage of the surgical technique used for the guinea pig of the present report was that it allowed us to completely resect the tumor from the gastric wall. The biology and anatomy of guinea pigs are important considerations during abdominal surgery.23 It is important that the gastrointestinal tract is handled gently to reduce the risk of postoperative ileus.23 Additionally, the natural history of rodents such as guinea pigs substantially affects their ability to survive surgical procedures. In general, rodents are fearful prey species, and rodents exposed to stressful situations, such as surgery, have a high mortality rate, presumably because of excessive catecholamine release.24 For example, guinea pigs with gastric dilatation-volvulus frequently die or are euthanized because of severe systemic compromise before surgery can be attempted.23,25 In fact, we are aware of only 1 case report26 that describes the successful surgical treatment of gastric dilatation-volvulus in a guinea pig. The guinea pig of the present report recovered well following gastrectomy to remove a gastric leiomyoma. The prognosis for other species is good following complete surgical resection of gastric leiomyomas.4
In dogs and cats, postoperative complications associated with gastrointestinal surgery include gastric necrosis, gastric wound dehiscence, and intraoperative or postoperative gastric leakage resulting in peritonitis, ulceration at anastomotic sites, gastric outlet obstruction, and pancreatitis.27,28 Dogs typically have few long-term adverse effects following gastrectomy; however, lesions near the pylorus or that require resection of a large portion of the gastric wall may require complex surgical procedures that are more likely to result in complications.27 In dogs, resection of a substantial portion of the stomach can cause abnormally rapid gastric emptying resulting in bloating, vomiting, diarrhea, and signs of pain and vasomotor dysfunction such as tachycardia.27 Fortunately, the guinea pig of this report did not develop any such complications despite the fact that resection of the leiomyoma necessitated removal of a moderately large proportion of the gastric wall.
Guinea pigs, like rabbits, have substantial hind-gut fermentation, and it is imperative that those animals resume eating as soon as possible after abdominal surgery to prevent changes in the gut flora and hepatic lipidosis.29 Food consumption is extremely important for stimulation of gastrointestinal motility in the postoperative period.27,29 Therefore, in guinea pigs, maintaining adequate analgesia during the postoperative period is critically important to stimulate appetite and gastrointestinal motility and decrease pain and stress.29 The guinea pig of the present report was administered opioids for analgesia throughout the postoperative period, and assisted feeding was initiated within 4 hours after surgery to stimulate and maintain gastrointestinal motility.
The present report described the diagnosis and successful surgical resection of a gastric leiomyoma in a guinea pig. Gastric leiomyomas are rare in guinea pigs, as is abdominal surgery. For the guinea pig of this report, partial gastrectomy resulted in the complete removal of the tumor without any major postoperative complications. The information provided in this report might help direct the clinical management and treatment of guinea pigs with similar conditions.
Footnotes
Dexdomitor, Zoetis, Florham Park, NJ.
Midazolam injection, Hospira Inc, Lake Forest, Ill.
Flumazenil injection, West-Ward Pharmaceuticals, Eatontown, NJ.
Antisedan, Zoetis, Florham Park, NJ.
Oxymorphone hydrochloride injection, Endo Pharmaceuticals, Chadds Ford, Pa.
Ketamine hydrochloride injection, St Joseph, Mo.
3-0 PDS II, Ethicon Inc, Summerville, NJ.
4-0 Monocryl, Ethicon Inc, Summerville, NJ.
Sulfamethoxazole and trimethoprim oral suspension, Vintage Pharmaceuticals, Huntsville, Ala.
Buprenorphine hydrochloride, Hospira Inc, Lake Forest, Ill.
Tramadol hydrochloride suspension, Teva Pharmaceuticals, North Wales, Pa.
Critical Care Formula, Oxbow Animal Health, Murdock, Neb.
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