Diseases that affect the urinary tract of guinea pigs are bladder stones/ bladder infections, Renal Disease, and Urolithiasis. When a guinea pig is suffering from a urinary tract infection, symptoms may include: - Weight loss. - Lethargy. - Lack of appetite. - Blood in the urine.  - Straining or crying when peeing. - Usually, an X-ray is required along with samples of the affected pet’s urine to diagnose before treatment commences.