Uroliths in Pets: What are they and why do they matter?

“Uroliths” is the medical term given to large collections of crystals that mineralise and form anywhere in the urinary tract. They can include kidney stones and bladder stones, as well as smaller stones lodged in the ureters or urethra.

Most crystals that are in the urine are dissolved, and do not cause major problems in small amounts. However, uroliths form when the microscopic crystals precipitate out of solution and group together, forming a solid concrete-like stone. The most common types of crystal are magnesium ammonium phosphate hexahydrate (known as ‘struvite’), calcium oxalate, urate, cysteine and silicate, and large uroliths may be made up of layers of several different types.

Who gets urolithiasis?

Urolithiasis is the presence of stones in the urinary tract. Any animal, including humans, can get urolithiasis, but some animals are more prone to it than others.

Struvite: A urine infection can cause growth of struvite stones as the bacteria change the pH of the urine, allowing the stones to precipitate and form. Females are more likely to get urine infections and therefore also more likely to get the stones. Miniature Schnauzers, Miniature Poodles and Bichon Frise are all predisposed to developing struvite uroliths.

Calcium oxalate: These are a lot more common in cats, causing about 60% of stones found in the urinary tract of cats. However, they occur in dogs too and account for 35% of stones in dogs. Again, Schnauzers and Miniature Poodles are more affected than most, although the Yorkshire Terrier and the Shih Tzu also join their ranks.

Urate stones: Urate stones are the most common stone formed by Dalmatians as they have an abnormal liver metabolism. Although this is gene-related, all Dalmatians should potentially be on a special diet to reduce this problem as it is a widespread defect in the breed. Any animal with a liver shunt can also produce urate stones.

Cysteine stones: Fairly rare, these are usually found in young male Dachshunds.

Silicate stones: These are more common in large dog breeds such as German Shepherds and Labrador Retrievers. They have been found to be related to dietary intake of corn gluten and soy beans.

What are the signs of urolithiasis?

Dogs and cats with urolithiasis may struggle to pass urine, have painful or slow streams, urinate more frequently, pass bloody urine, or stop passing urine altogether. Any animal that seems in pain whilst passing urine should be seen by a vet as soon as possible, especially if no urine is coming out at all.

How will my vet diagnose urolithiasis?

It is really helpful to take a fresh urine sample to the vet if you think your dog has a problem. Dogs, unlike us, cannot usually urinate on command and if they refuse to perform at the vets then the delay getting a sample can be really frustrating. It’s far better to collect one ‘just in case’ and take it along with you than find you need one and you can’t get one. Along with looking at the urine and performing some tests on it, the vet will do a full clinical exam (including a prostate exam for boys), and may suggest blood tests, x-rays and ultrasound.

Dogs and cats that present ‘obstructed’ i.e unable to pass urine due to a stone will need to be sedated or anaesthetised so that a urinary catheter can be passed to relieve the obstruction before damage to the kidneys occurs. This is usually done before any major investigation into the cause of the obstruction, as time is of the essence.

What next?

Well that depends on the type of urolith your pet has. Without sending the stone off to a specialist lab it is impossible to know for sure, but considering the breed, age, size and symptoms can give a clue. The pH of the urine can help, and results of ultrasound or x-ray can give us clues too.

Small uroliths of particular types can be dissolved, and sometimes a dietary change is all that is needed. The aim is to change the pH of the urine, thereby re-dissolving the crystals and allowing them to pass without complaint.

Referral centres can use specialist ultrasound or laser under general anaesthetic to blast the stones into smaller pieces, allowing them to be passed in the urine. This is relatively non-invasive technology as the camera and laser can be passed through the urinary catheter, but is a fairly new field and not available to everybody.

Larger uroliths, as measured on ultrasound or x-ray, are often removed surgically. Although dissolving them can work, they may be made up of several layers each needing a different dietary approach. There is also the risk that they dissolve to a certain size then move and get stuck in the urinary tract requiring emergency surgery. The urolith surgery involves opening the bladder, scooping out the stone(s), flushing and cleaning, then sewing the bladder back up again. Pets are normally placed on fluids intended to dilute the urine and flush everything through, and are usually hospitalised for 24-48 hours for pain relief and observations. They need regular toilet trips so that no undue pressure is placed on the bladder sutures and are usually released from hospital after several toilet breaks to ensure everything is working as it should.

Will the uroliths come back?

Well, that depends on the cause of the uroliths. Dogs with struvite uroliths caused by an infection can often go the rest of their lives without another problem. Dogs with a genetic predisposition are much more likely to get the problem again, and Dalmatians are particularly bad repeat offenders, even if placed on a more suitable diet.

The bottom line

Although uroliths are uncommon, any animal with signs of cystitis should be checked for signs of crystals in the urine in order to rule out uroliths as an underlying, and potentially troubling, cause.