What Causes Struvite Crystals in Cats?

Struvite Crystals
Struvite Crystals. Photo: Cuyahoga Falls Veterinary Clinic
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Firstly, what are struvite crystals? We often read about them in conjunction with feline urinary health problems. The Cuyahoga Falls Veterinary Clinic tell us that struvites come about when molecules become lumped together and precipitate out of the urine to form crystals. When these crystals come together too much they form stones in the bladder. Struvite crystals are about the size of a grain of salt.

Crystals or stones are also called uroliths. They are found in the urinary tract. Struvite is made up of magnesium and ammonium phosphate. There are several factors which influence the formation of struvite crystals in cats namely: bacterial infections, infrequent urination (perhaps caused by dirty litter box), reduced physical activity, reduced water intake (perhaps due to poor water quality or water is unavailable) or feeding a cat exclusively with dry cat food (I discuss this further below).

There is another type of crystal which is made up of calcium oxalate. Feline Lower Urinary Tract Disease (FLUTD) can be related to the presence of crystals or stones in the urinary tract. Heavily concentrated urine which contain sediment is not desirable and the cat may be predisposed to FLUTD.

The presence of struvite crystals can be a major reason why the urethra becomes plugged by a paste-like gritty, sandy material. Although, not all plugs are caused by the presence of struvite crystals. Sometimes they can be caused by mucus, blood and white cells.

Reduce water intake and diets that contain large amounts of magnesium and calcium can contribute to struvite crystals it is believed1. However…

Elizabeth M. Hodgkins, DVM, has an interesting section in her book, Your Cat, in which she asks the question “Why Do Cats Develop Urinary Tract Problems?”. She states that when cats are left to feed themselves, by which I mean they’re not fed by an owner, they do not suffer from major urinary tract health problems. She states that in the 1970s and 80s veterinarians inexplicably began to see a large number of cases of cystitis (which is a bladder inflammation and infection). They also encountered bladder crystals and stone formations together with urinary blockages in cats.

Blockages sometimes led to the death of the cat so this is a serious health issue. Critically, she says that experts applied poor thinking to finding a solution to the problem. Coinciding with the rise in urinary tract diseases was the increased popularity of dry cat food which is called “kibble” in the USA.

Scientists working for pet food manufacturers decided that magnesium in commercial cat food caused urinary tract inflammation. This simple conclusion arose out of the fact that the struvite crystals were, as mentioned, made up of a “magnesium salt”, meaning magnesium phosphate. They thought there was too much magnesium in cat food. Elizabeth Hodgkins says that the pH of the urine of cats fed on dry cat food is high meaning it is alkaline rather than acid and that magnesium crystals form in alkaline urine but not in acid urine.

As a consequence the pet food manufacturers removed the magnesium in the food and made it more acidic. This resulted in prescription foods to treat cats with urinary tract problems. These prescription diets were, I presume, unsuccessful in respect of crystals formed out of calcium oxalate. Therefore they were not always successful. In addition, cats on dry diets have more concentrated urine because they do not make up for the lack of water in their diet by drinking more water. This is due to the cat’s evolutionary origins living in arid environments.

As a consequence, Elizabeth Hodgkins DVM concludes that what the scientists at the pet food companies didn’t look at was a meat-based wet food diet, which is the best solution in treating urinary tract problems. The solution is superior to the prescription diets which are expensive to cat owners and which were not always successful and, as mentioned, might even cause other diseases.

It is worth noting that the urine of carnivores is acidic meaning below a pH of 7.4. Dry foods which have a high plant content cause an alkaline urine pH. When you add that fact to the additional fact that dry cat food provides almost no moisture in contrast to the natural died of a cat, a mouse, which contains 70% or thereabouts of water you can see were Elizabeth Hodgkins’s, argument is coming from.

The cat fed on exclusively dry cat food is usually somewhat dehydrated with very concentrated urine which together with an alkaline urine leads to urinary tract inflammation. Hodgkins’s ideas are not widely accepted but they make sense and I expect the dry cat food industry put vets under pressure to avoid rocking the boat.

Note: 1 Cat Owner’s Home Veterinary Handbook

5 thoughts on “What Causes Struvite Crystals in Cats?”

  1. Excellent article, Michael,

    When… when…. when will veterinarians get it that cats are obligate carnivores and need MEAT- not kibble- waterless, inappropriate food for cats- and dogs, in my opinion. Yes- these products are made for the convenience of the owner- certainly not for the health benefits of cats. It gets me so DAMNED angry that vets continue to prescribe RX DRY food even for cats with this condition- when they need hydration- not further water deprivation. It just gets me SO upset.

    I am so fortunate that I have a vet who carries the message to her patients that dry food is NOT appropriate for cats- I bless her each day for the huge amount of research she is doing in feline nutrition. She CARES. She carries raw food diets for sale in her food market- and high quality moist canned food for folks who don’t want to feed raw. Would that all vets would do the same- there is such a need for accurate nutritional information to be given to cat guardians.

    One day hopefully our cats will be fed correctly, and prolong their lives and give them the good health they so richly deserve. Thanks again for your post.

  2. Dry food has never been healthy for cats. It’s nothing more than convenience food for owners.

    Most of the nutritional education student vets receive is sponsored or taught at university by the pet food manufacturers. Vets are then offered financial incentives to stock and promote those brands in their clinics.

    Stress, obesity, lack of exercise are also possible triggers for feline urinary problems. It’s alleged that indoor-only cats are more likely to suffer from struvite crystals than cats with some access to the outdoors.

    I understand that one of the reasons UK vets have traditionally tended to recommend neutering males at around 6 months old, is to allow the cat’s urinary system time to fully develop. I don’t know if any studies have been carried out comparing early/late neutering to see if there are any differences in the number of cats affected by FLUTDs.

    • Most of the nutritional education student vets receive is sponsored or taught at university by the pet food manufacturers. Vets are then offered financial incentives to stock and promote those brands in their clinics.

      I am pleased you wrote that because I have always felt that that was happening. It surprises me that vets promote commercial products even when they should know that the product does not promote cat health and can harm.

      Thanks for the info about neutering at 6 months of age. I’ll look into that.

      • Michael, this article from 2010 was where I learned about the pet food manufacturers influence on the nutritional education of vets.


        The situation is exactly the same in the US and probably many other countries because these are global Companies.

        I remember asking my vet in Cyprus if he knew of anywhere I could buy premium quality cat food. When he suggested Hills Science Diet, I told him that I preferred not to feed my cats any dry food. Straight away he asked “Is that because of the risk of diabetes?” I politely explained that was just one of my reasons. Curiously, he didn’t attempt to defend dry food or persuade me to buy it from him.

        That’s why articles like this on PoC and Lisa Pierson’s web site are great, because they raise awareness about potential health problems from something as seemingly innocent as pet food.


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