Uroliths form when minerals normally found in the urine crystallize and combine with sticky proteinaceous materials (referred to as an organic matrix) to form masses. These concretions can be singular or multiple and large (referred to as stones) or small and immense in number (referred to as sand). The disease that is associated with the formation of uroliths is referred to as urolithiasis.
Uroliths usually form in the bladder but some originate in the kidneys and then pass down the ureter and into the bladder where they can enlarge. While uroliths usually form in the bladder or kidney, they may be found at any location in the urinary tract. In some cases, guinea pigs with uroliths have concurrent bacterial infections while in others, uroliths occur in the absence of a detectable bacterial infection. The clinical changes that might occur depend on the size and location of the urolith and whether or not a bacterial infection is occurring.
Some guinea pigs with uroliths may be clinically normal with no recognizable changes in attitude or blood values. If a urolith is causing damage of the lining of the urinary tract or if a bacterial infection is present, then the guinea pig may be feverish, depressed and have an elevated white blood cell count. These clinical changes are similar to those associated with many bacterial, fungal or viral infections. Clinical changes from uroliths that restrict urine from properly entering or exiting the bladder include frequent urination, blood in the urine, urine dribbling, wet fur in the rear end and legs and urine dribbling. All of these clinical changes warrant immediate veterinary attention.
Inability to pass urine rapidly leads to coma and death and should be treated as an emergency. Seek veterinary care immediately if a guinea pig strains or vocalizes while trying to urinate, or if a normal amount of urine is not noted.
Other problems with the urinary tract that may cause similar clinical changes include cancer, abscesses and infections. In older males (called boars), accumulations of ejaculate can cause obstruction of the urethra causing clinical changes similar to those noted with uroliths.
The exact factors that determine which guinea pigs will or will not develop uroliths remain undetermined. In other species, a genetic predisposition, metabolic disorders, improper nutrition, inadequate water intake, obesity and bacterial infections have all been suggested to precipitate the formation of uroliths. Female guinea pigs older than three years are particularly likely to have inflammation of the bladder (called cystitis) and urolithiasis. Diabetes mellitus and the anatomy of the guinea pig urinary tract are also considered factors in the development of urolithiasis.
Most uroliths in guinea pigs are comprised of calcium carbonate and occasionally magnesium and ammonium. It is theorized that the formation of uroliths may predispose a guinea pig to bacterial infections of the urinary tract or conversely, bacterial infections of the urinary tract may precipitate formation of uroliths. Calcium oxalate uroliths have been reported in guinea pigs with bladder infections associated with Streptococcus sp. E. coli and Staphylococcus sp. infections have also been reported in guinea pigs with urolithiasis.