History

An ovoid, well-defined mass was palpated in the caudal portion of the abdomen of a 2-month-old Sprague-Dawley outbred male rat that was part of a biomedical research project involving surgical placement of an intramedullary pin in the femur. The rat had no other abnormal clinical signs. It was the judgment of the laboratory and clinical personnel that the change would not interfere with research study and that the rat was not in pain or distress. After the rat was included in the study, the research protocol necessitated that a dorsoventral radiographic view of the pelvis, caudal portion of the abdomen, and hind limbs be obtained immediately after placement of the pin and at 14 days after surgery (Figure 1). At the end of the study, the rat was euthanized and the abdominal space was examined.

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Dorsoventral radiographic view of a 2-month-old Sprague-Dawley outbred male rat included in a biomedical research project involving surgical placement of an intramedullary pin in a femur. The radiographic image was obtained 14 days after surgery.

Citation: Journal of the American Veterinary Medical Association 231, 1; 10.2460/javma.231.1.41

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Radiographic Findings and Interpretation

Round to irregularly shaped mineral opaque bodies surrounded by soft tissue opacity are arranged in a well-demarcated pattern that conforms to the shape of the bladder (Figure 2). The radiographic findings are consistent with multiple, numerous cystic calculi. Both radiolucent and radiopaque calculi can be seen in animals. Calcium oxalate, magnesium phosphate, triple phosphate, and silica calculi are radiopaque, whereas urate and cystine calculi are radiolucent. The radiographic findings narrowed the possible composition to radiopaque types of calculi.

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Same dorsoventral radiographic view as in Figure 1. Round to irregularly shaped mineral opaque bodies surrounded by soft tissue opacity are arranged in a well-demarcated pattern that conforms to the shape of the bladder (arrow). The radiographic findings are consistent with multiple, numerous cystic calculi.

Citation: Journal of the American Veterinary Medical Association 231, 1; 10.2460/javma.231.1.41

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Comments

Necropsy revealed a distended urinary bladder filled with calculi 1 to 3 mm in size. Laboratory analysis of the calculi revealed that they were composed of magnesium ammonium phosphate. Although this type of calculi has been reported in rats, this is, to the authors' knowledge, the first radiographic report. A histologic diagnosis of chronic hyperplastic cystitis was made from an assessment of tissue from this rat's urinary bladder. The lack of clinical signs is not uncommon in animals with cystic calculi that have an unobstructed urinary tract.

Calculi in the urinary bladder have been reported in Sprague-Dawley–derived and other types of rats.¹ A link between urinary calculi and neoplasia has been described in ACI/N² and BN/Bi/Rij³ rats. Factors such as dietary minerals and urinary pH have been studied to determine their role in the formation of uroliths.⁴ One report⁵ describes cases of urolithiasis in rats that were fed a diet that contained the racemic (dl) bitartrate form of choline instead of the neutral (l) tartaric acid isomer. Laboratory diets for rodents were modified to increase the calcium-to-phosphorus ratio (from 0.75:1 to 1.3:1) approximately 14 years ago to reduce the formation of renal calculi and calcium deposits in female rats.⁶ A recent report⁷ described a vesical calculus-induced obstructive dystocia in a Sprague-Dawley rat. Radiographic evidence of calculi in the bladder of the rat of this report could prove useful for those who use diagnostic imaging for pet rats or rats used in biomedical research that may not have clinical signs of vesicular calculi.