Currently, there is a great amount of interest in the condition referred to as Feline Urologic Syndrome (FUS). This is a general term referring to the development of kidney stones and the ensuing complications associated with these stones in cats.
While the composition of these stones varies between cats, the majority of the stones are composed of struvite crystals (usually a combination of magnesium and ammonium phosphate). Though a major concern among cat owners, the condition is present in only about one percent of the cat population. However, the reoccurrence rate is very high, with 70-75 percent of the cats with the syndrome having repeated episodes. Thus, once a cat has developed the condition, special care should be exercised to aid in the prevention of a reoccurrence.
The incidence and severity is more pronounced in male cats due to their longer and flexed urethra.
The condition is chronic in that the stones develop over a period of time. However, the outward signs usually exhibit themselves very rapidly. There are a variety of signs to look for. In mild cases, the cat may show signs of pain, such as restlessness or loud excessive meowing.
Other common indications are difficulty in urination, lack of urination or blood in the urine. In severe cases, where the stone may block the urethra, the urethra may rupture, resulting in massive bleeding. Treatment will vary from surgery to remove the stones in severe cases to providing special products, which dissolve the stones in milder cases.
Many causes of this syndrome have been suggested and studied. These include infection, alkaline urine, hormonal imbalances, low water intake, obesity, strict confinement, infrequent urination and diet. The complexity of the syndrome dictates that many factors are involved and there is no one specific cause. The low incidence would certainly suggest an inherited predestination for certain cats.
Several dietary factors have been suggested as promoting the development of stones, including ash (minerals, such as magnesium, and phosphorus) and nitrogen. Within the last few years, these dietary factors have been shown to be of minimal significance. The most recent research has demonstrated that the pH of the urine is of major importance in preventing the precipitation of stones. Dietary interests have therefore shifted to dietary methods of controlling the pH of the urine.
Ash includes all of the non-combustible materials of a product. The ash level does not indicate the composition of that ash, even though the ash composition may vary greatly between samples. The composition is known to be of greater importance than the level of ash. A diet high in ash due to high salt (NaCl) may actually be beneficial in preventing FUS by encouraging water consumption. Many of the components of ash are required by the cat, especially for bones and teeth.
Early studies conducted with added magnesium oxide demonstrated a link between magnesium and FUS development. In these studies, 0.37-0.40 percent magnesium was fed, resulting in a high incidence of FUS. From these studies, some workers proposed a "safe" level of 0.19 percent magnesium.
Later, it was determined that urinary pH is very important in the prevention of stone formation. If urinary pH is maintained below 6.6, stones are not able to develop. When the pH is above 7.1, crystallization of stones will occur spontaneously. Thus by supporting the production of an acid urine, kidney stones are less apt to occur. The earlier tests conducted with added magnesium oxide had raised the urinary pH to over 7. Magnesium oxide is a strong base. Thus, the change in urinary pH, not the magnesium, had stimulated the kidney stone development in these tests.
With this information, tests were redesigned to study the effects of the magnesium level without changing the pH of the urine. These tests involved diets with different sources of magnesium including magnesium oxide and magnesium chloride.
Magnesium chloride does not increase the urinary pH as magnesium oxide does. Cats fed the diet with added magnesium oxide quickly developed FUS. However, cats fed the diet with added magnesium chloride, or the control diet, did not develop FUS. These tests have demonstrated that ash and magnesium levels alone do not promote the development of FUS.
Diet can be important in helping to prevent the development of the syndrome. A diet which supports the production of an acid urine is recommended when feeding to prevent FUS.
There are several methods used to produce these types of diets. Special attention should be given to the mineral composition of the cat food since the interaction of minerals influences the animal's pH balance. The addition of organic acid to the food not only helps to reduce the pH of the urine, but also acts to enhance the palatability of the product.
Phosphoric acid is a major acid that is used and has the added benefit of supplying phosphorus, which helps to maintain mineral balance.
Acidifying agents have been used with various results. Animal proteins tend to support acid conditions more than proteins from vegetable sources, and therefore may be used as acidifiers.
Another consideration in the area of FUS prevention is cat management. Water intake is very critical in cats. Cats are normally poor drinkers. With reduced water intake, the concentration of solids in the urine is increased, which encourages the formation of stones. Water bowls must be kept clean, and fresh water must be provided on a regular schedule. Litter boxes must be clean and changed regularly.