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