The formation of calculi within the urinary tract is a relative common phenomenon, especially in guinea pigs over 2.5 years old. Stones may be present in the kidneys, ureters, bladder and urethra.
Factors that predispose to the formation of calculi are not totally clear but are thought to include urinary infections (ascending faecal bacteria, specifically Escherichia coli, Streptococcus pyogenes and Proteus mirabilis), an insufficient intake of water, dietary imbalances (excess of calcium) and maybe a genetic predisposition. Most stones are composed of calcium carbonate; other types are formed of calcium phosphate, calcium oxalate and magnesium phosphate.
The clinical signs of urinary tract calculi are haematuria, strangury, anorexia, curved posture, vocalisation during urination and pain at abdominal palpation. Stones present in the penile urethra may sometimes be detectable by palpation.
The diagnosis is made by radiographic examinations, which must always include two lateral projections and a ventrodorsal view (Figs. 1 and 2), in order to identify and locate the stones precisely. It is particularly important to distinguish between urethral and bladder stones. Ultrasonography is useful for assessing the possible presence of hydronephrosis and hydroureter. The identification and treatment of any infections are based on urine cultures, whereas blood tests are useful for assessing renal function before surgery.
Stones in the prepuce may be extracted manually; stones in the rest of the urinary tract are removed surgically although they often recur and the owner must be informed of this fact. Any stones in the ureters may be pushed into the bladder and removed from there.
In the presence of an infection therapy consists of administration of antibiotics, preferably after a bacterial culture of the urine has been performed. Medical therapy for the dissolution of stones is not successful; however, in the opinion of some authors it is useful for preventing relapses. The dose is 75 mg/kg every 12 hours. The diet must be modified by eliminating foods rich in calcium (alfalfa, taraxacum, aromatic plants) and oxalate (parsley, spinach, cabbage, salad, strawberries).