We can’t really talk about bladder stones without mentioning urine pH. The urine pH that we expect to find in most adult dogs is about 7.0. This can vary widely and still be normal. In many cases, we test the pH with a strip that urine is dropped onto. These strips are useful screening tools, but not very accurate when it comes to pH. The pH may be as much as 0.5 higher or lower than what is indicated on the strip. So, if the strip says 7.5, the actual pH could be 7.0 or even 8.0.
The timing of collection of a urine sample should also be considered. After a meal, the urine pH rises. This is called the alkaline tide. Urine that is allowed to sit with also have a pH alteration and crystal formation. It is ideal to have a urine sample analyzed immediately after it is collected for best results.
Crystals in the urine may be an incidental finding, especially if the urine sat for more than 20 minutes before it was examined (or if it were refrigerated). Struvite crystals are normally present in canine urine, in relatively low numbers. If there are high numbers of crystals or if there are stones present, then this is almost always a result of a bladder infection, with a type of bacteria that produces an enzyme called urease. The urease alters the urine pH, and with the bacteria and white blood cells in the bladder, makes the perfect environment for stone formation.
Treating struvite crystals is not necessary, unless the dog is exhibiting symptoms of a problem or has an obvious infection. A urine culture and sensitivity should always be performed to determine firstly if an infection is present and secondly which antiobiotic is most appropriate to treat the infection.
If we diagnose an infection, we sometimes change the diet while we are treating with antibiotics to speed up the process of dissolving the stones. Very large stones may need to be surgically removed because of the time required to dissolve them with medical therapy. The bladder will remain irritated in the presence of the stone and infections will tend to recur. Another potential problem with a very large stone is that it may have multiple different compositions. So, the stone may not be completely a struvite stone, and won’t dissolve with medical therapy alone.
If there are large numbers of smaller stones, surgery may also be indicated, especially in male dogs. Smaller stones may exit the bladder through the urethra but become lodged at the base of the os penis, a small bone in the dog’s penis. When the stone becomes lodged, urine may be unable to leave the bladder. Urinary outflow obstructions are medical emergencies that must be resolved immediately.
Calcium Oxalate Stones
Calcium oxalate stones are another type of stone commonly found in dogs. This type of stone is not related to infections, and is found in certain breeds of dogs and older dogs of any breed. The breeds that are known to suffer from calcium oxalate stones more frequently than other breeds include Miniature Schnauzer, Bichon Frise, Shih Tzu, Lhasa Apso, and Yorkshire Terrier. This does not mean that every dog of these breeds will be stone formers. However, these breeds are more commonly diagnosed with calcium oxalate stones than other breeds. There may be a genetic cause, explaining the higher incidence in the above breeds. In some cases, there is a metabolic problem, causing an inability to properly regulate calcium levels in the blood. In the majority of the cases, we don’t identify a specific cause.
Calcium oxalate stones cannot be dissolved with medical treatment. They must be surgically removed or sometimes pulsed out of the bladder with a procedure called urohydropulsion or even more rarely dissolved by a procedure called lithotripsy.
Dietary and medical therapy may be recommended to help prevent recurrence of these stones.
A less common type of stone is the urate stone. These are most commonly seen in male Dalmatians. Dalmatians (all) excrete uric acid in their urine and this uric acid can precipitate into stones. Although all Dalmatians excrete this compound, only some actually become stone formers. Some studies have shown that this “stone forming” group of dogs can be limited by selective breeding, because this trait is passed down with great frequency. Diet and medical therapy help prevent stone formation in susceptible dogs.
Cystine stones are another less common type of stone that can be found in dogs. Cystine is an amino acid, present in animal protein sources (such as chicken or lamb). Some dogs have a metabolic error that causes them to excrete excess cystine in the urine, leading to stone formation. These stones can be dissolved with diet and medical therapy, and can be prevented by keeping the urine alkaline (above 7.5).