By Ines Di Giacomo DVM
What are some different types of bladder stones in dogs? A dog's urine is overall a complex solution that contains waste products including mineral salts (calcium oxalate, magnesium ammonium phosphate) which should remain dissolved, but several factors can predispose urine to become oversaturated with crystallogenic substances (crystalluria) with consequent formation of stones if the crystals are aggregated with each other and are not excreted as they should. There are different types of bladder stones in dogs and their presence can cause a variety of problems.
The Formation of Stones
In the dog, the majority of stones are detected at the level of the urethra or bladder. The conditions that contribute to the precipitation (the process of solidparticlessettledout of a solution) of dissolved salts to crystals and the formation of stones, include a high concentration of these salts, their long persistence in the urinary tract (retention of salts and crystals), a urinary pH suitable for the precipitation, the presence of an aggregation core (around which crystallization can take place) and a decrease in urine of the factors capable of inhibiting it.
Predisposing factors include a diet rich in minerals and proteins which contribute to the production of over-saturated urine, urine that is over saturated and reduced number of urinations, which can also increase the possibility of formation of bladder stones.
Bladder stones in dogs usually are diagnosed based on medical history, evidence on physical examination, x-ray or ultrasound. In order to identify and treat properly any concomitant urinary infection, it is appropriate to perform a urine culture with relative antibiogram (the result of an antibiotic sensitivity test).
There are different types of bladder stones in dogs. Bladder stones are generally defined by the type of minerals that they contain. About half of the diagnosed stones are composed of struvite (magnesium ammonium phosphate), a third of calcium oxalate and a small part of urate. The aggregates of crystals represent almost the entire stone and the minimum part is composed of an organic matrix.
"The pH is an indicator of acidity that measures the concentration of hydrogen ions. pH = 7 neutral, pH < 7 Acid, pH > 7 alkaline. Our dogs tend to have urine with an acid pH."
Struvite Stones in Dogs
Struvite stones are the most common types of stone. The presence of a UTI (urinary tract infection) constitutes an important predisposing factor in the formation of these stones; pathogenic organisms generally responsible for these inflammatory processes belong to the genus Staphylococcus and Proteus, bacteria that contain the enzyme urease, capable of splitting urea into ammonia and carbon dioxide.
This reaction decreases the concentrations of hydrogen ions in the urine; follows its alkalization and a decrease of the solubility of struvite. High concentrations of ammonia can also harm the glycosaminoglycans that prevent adhering of bacteria to the urinary tract mucosa.
Struvite stones are more common in female dogs because of their predisposition for UTI (urinary tract infection). Furthermore, the formation of stones can be facilitated by the presence of urine with a high pH, often caused by the use of certain medications, certain diets or renal tubular disorders. Struvite stones can occur in dogs of all breeds and often appear smooth or blunt, with facets or a pyramid shape.
Did you know? Struvite stones are radiodense, which means that they can be observed well in an X-ray.
Calcium Oxalate Stones in Dogs
The increase of calcium concentration in urine (hypercalciuria) is among the factors that cause the formation of these stones. This hypercalciuria is most frequently the result of food ingestion and is associated with an increase in intestinal absorption of this mineral.
Prolonged therapy with certain medications (glucocorticoids, furosemide) or addition of calcium and/or sodium chloride in food can cause hypercalciuria in dogs.
Obesity seems to increase the risk of incidence. Approximately 70 percent of cases was found in male dogs of all breeds (except Dalmatians). The greater prevalence among males may be due to increased production of oxalate, which is mediated by testosterone.
These stones are rarely formed in association with a UTI and are formed more frequently in older subjects (mean age 8 to 12 years). An acid pH favors the formation of these stones. Calcium oxalate stones are radiodense and recurrences are common.
Urate/Ammonium Urate Stones
These stones are commonly found in middle-aged Dalmatians (especially in Dalmatian males). They are composed of uric acid ammonium.
In the Dalmatian breed there is a decrease in the hepatic metabolism of uric acid as well as a decrease in the reabsorption of this substance in the renal proximal tubules.
The ammonium uric acid stones can also be formed in any dog suffering from liver failure (for example, from hepatic cirrhosis or PSS, portosystemic shunt, see definition below), as a result of an increase in renal excretion of ammonium urate.
The crystallization of uric acid is facilitated by acidic urine, while alkaline urine seems to favor the precipitation of ammonium urate. Urate stones are radiolucent stones, meaning that they are not visible in a radiograph.
"PSS, portosystemic shunt: liver shunts are an anomaly of blood flow to the liver. The blood instead of flowing normally through the liver, bypasses it."
The Problems with Stones
The presence of crystals or stones can damage the epithelium of the dog's urinary tract and trigger an inflammatory process potentially causing hematuria (blood in urine), pollakiuria (urination in small amounts frequently), dysuria (difficulty urinating) and stranguria (straining to urinate).
Also, they may predispose the animal to the onset of a urinary tract infection (UTI) and if they are located in the ureters or urethra, they can cause an interruption of the urine flow.
The clinical symptoms associated with the presence of bladder stones depends on their number, the types of stones present and their location in the urinary tract. Most of the stones are located in the bladder; therefore will result in clinical signs of cystitis. The spiky surface stones cause a significant irritative phenomena of the bladder mucosa, contrary to what happens if the stone is solitary and has a smooth surface.
In males dogs, the smaller stones can get stuck in the urethra, causing a partial or complete obstruction of the duct, with clinical signs of bladder overdistension, dysuria/stranguria, depression of the sensorium, anorexia, vomiting. Stones that pass from the bladder into usually get stuck at level of the dog's baculum.
Tip: Make sure that our dog always has fresh water (not cold!) available and has the possibility to urinate frequently. Be alert for symptoms in your dogs that may suggest the presence of bladder stones and, if in doubt, consult with your veterinarian.
As seen, there are different types of bladder stone in dogs! If you suspect your dog has bladder stones, please see your vet for proper diagnosis and treatment.
- “Medicina interna del cane e del gatto”, C.G. Couto, R.W. Nelson, 5° ed., Edra-EV, 2015
- Joel Mills - Own work, Picture of Struvite (magnesium ammonium phosphate) stones removed from a dog's urinary bladder. CC BY-SA 3.0
- Joel Mills - Own work, Photo of calcium oxalate stones found in the urinary bladder of a dog CC BY 2.5
- Joel Mills Own work Picture of Urate stones removed from a dog's urinary bladder. CC BY-SA 3.0
About the Author
Ines Di Giacomo DVM graduated from the Faculty of Veterinary of the University of Teramo, in Abruzzo Italy. She's interested in ultrasound diagnostics so decided to follow a specialist so to learn more about this topic.
In the meanwhile, she has been attending a veterinary clinic to put into practice what she has learned during her years of study. Since her graduation, Ines has attended numerous seminars to improve her knowledge in different areas of veterinary medicine such as radiology, ultrasound, reproduction, dermatology.
Ines would love to expand her knowledge in parasitology, anethesiology, and surgery but the road is still long. She says "Although the university is over, you never stop learning!"