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Struvite crystals in dogs
It seems that every time I mention struvite, another patient comes in to my office with this diagnosis, so I have started to document the case histories that I receive. One of the most common conditions in my practice that I think would benefit from a discussion of the etiology is struvite crystals in dogs.
Struvite is a naturally occurring mineral composed of magnesium, potassium and HPO43-. It is the second most common cause of gout in dogs, after calcium oxalate. Struvite has been known to occur in normal dogs, but is almost always a secondary condition. It is most commonly encountered in older dogs, with no gender predisposition.
The most commonly seen clinical signs with struvite are a painful joint, and possibly a mass and/or lameness of a joint. In dogs with struvite, the hindlimbs are usually affected, with one to four joint involvement depending on the severity of the disease.
Historically, struvite was originally thought to be a result of urinary tract stones, but in recent years, it has been found to be a common problem in dogs with idiopathic hypercalciuria. That means that dogs with struvite have high urinary calcium excretion, but they do not have stones in their urinary tract. This finding is not as simple as it sounds, and this is why I often say that we just don't know what causes struvite to develop.
What is idiopathic hypercalciuria?
There is no single cause for idiopathic hypercalciuria. However, a number of breeds have been shown to have a much higher incidence of idiopathic hypercalciuria. For a list of breeds with an increased risk for idiopathic hypercalciuria, click here. When a dog is diagnosed with idiopathic hypercalciuria, then there is no effective treatment for that dog.
In humans, idiopathic hypercalciuria is a fairly common condition. This is likely because we eat too much calcium! In fact, many people who are diagnosed with this condition usually have an increased consumption of milk and milk products.
Is struvite an infectious disease?
Sometimes, dogs that develop struvite are also diagnosed with a bacterial urinary tract infection. This is not common, but it has been noted. I will discuss that more in detail in my next blog post.
There are two types of struvite: monohydrate (magnesium ammonium phosphate, MgNH4PO4.H2O) and anhydrous (magnesium ammonium phosphate, MgNH4PO4). There are also other, more exotic forms of struvite that are found in laboratory animals. Struvite often appears as tiny, white, spheroid or flaky crystals within the urine. On occasion, this stone forms within a cyst.
How are struvite stones different than struvite crystals?
The most significant difference between struvite crystals and struvite stones is that the stones are a calcium salt of magnesium and ammonia. This differs from struvite crystals which are composed of magnesium and ammonia. This is important because the former is a disease of the bladder and not the kidneys. Struvite stones are also a urolithiasis, a stone in the urinary tract. Struvite stones in cats are more commonly seen than struvite stones in dogs.
Other things to note are that struvite crystals usually form under acidic conditions and that struvite stones are usually calcified. This is because they are composed of calcium and phosphate. This is a bit misleading. There are some types of struvite crystals which are magnesium-free. Even though these have the chemical formula AMP, they are not struvite stones. This is why patients and veterinarians get confused about the difference between struvite stones and struvite crystals.
What is the difference between struvite and struvite stones?
There are three main differences between struvite and struvite stones. Struvite stones are composed of magnesium and ammonia. Struvite stones occur commonly in cats. Struvite stones are often calcified and white or greenish-white. These can be found on the kidneys, the bladder, and the ureters. Struvite stones can be associated with cysts or renal calculi. Struvite stones are usually composed of small, round to rod-shaped struvite crystals. Struvite crystals have a prism-like or needle-like appearance.
What are struvite stones that are green or pinkish?
Some crystals of struvite are green. These green stones may be a result of cystine or toluidine or may be the result of a reaction of the body to some disease states. Such a reaction would be associated with urea, urine, or blood. Green struvite stones may be brownish or creamy, depending upon the amount of calcium and other elements that are present. Some of the calcium present in green struvite stones may be in the form of magnesium phosphate.
What are struvite stones in people?
Struvite stones that are seen in people are actually struvite crystals found in the pancreas or gallbladder. These stones are white or cream-colored and are located in the upper abdomen. Such stones are less commonly seen in people. Their cause is similar to that in other animals. A similar type of struvite stone, also known as phosphoaminic acid or APA, is sometimes seen in people with primary or secondary hyperparathyroidism. It is thought that phosphoaminic acid crystals are related to the formation of the stones in these people. There have been some reports of a relationship between APA stones and the presence of calcium in the urine in people with primary or secondary hyperparathyroidism. More studies are needed to prove this possible link. It is unclear whether people with APA stones should undergo surgery to remove the parathyroid glands in order to prevent the development of struvite stones.
What are struvite stones in domestic animals?
Struvite stones are found in the gallbladders of dogs, cats, cats, pigs, cows, and horses. They are most commonly seen in cats, and they are most commonly diagnosed in adult cats. Struvite stones are seen in the gallbladders of dogs, as well. They are also occasionally seen in cows and sheep.
Struvite stones are common in horses, and they are seen in about 80 percent of adult horses. They are most often found in young horses, and they are associated with colic. The stones may result from infection or dietary factors. They may also result from gallbladder or biliary tract blockage.