Chapter 30. Dietary Management of Urolithiasis in Cats and Dogs
Lower urinary tract disease is a common disorder in dogs and cats. It is estimated to occur in approximately 0.6% of the owned cat population and is diagnosed in between 3% and 13% of cats presented for veterinary care. 1 In dogs, lower urinary tract disease is diagnosed in approximately 3% of dogs admitted to veterinary colleges in North America. 2 Urolithiasis is a specific type of lower urinary tract disease characterized by the presence of urinary crystals (crystalluria) or macroscopic concretions (uroliths or calculi) within the bladder or lower urinary tract, as well as associated clinical signs. Urethral plugs often contain varying proportions of mineral matter and so are classified with urolithiasis. In cats, urolithiasis is now considered to be one manifestation of a collection of lower urinary tract disorders collectively referred to as FLUTD (feline lower urinary tract disease).
Urolithiasis is associated with a set of diverse risk factors and can be caused by several different types of mineral aggregates. Dogs show breed predilections for certain types of urolith formation and are more susceptible to infection-induced urolithiasis than cats are. 3 In both species, identification of the mineral composition of uroliths is important because dietary treatment or management must be directed toward the specific type of urolith present. This chapter reviews the types of uroliths found in dogs and cats, historical shifts in canine and feline urolith composition and location, risk factors for their development, and dietary management to treat urolithiasis and to prevent recurrence.
INCIDENCE
Urolithiasis is typically a disease of adult animals. In cats, it is rarely seen in animals younger than 1 year old, and the majority of cats are first diagnosed when they are between 2 and 6 years old. 4 In dogs, the mean age at time of diagnosis is between 6 and 7 years. 5 More than 80% of uroliths in both dogs and cats are composed of magnesium ammonium phosphate (struvite) or calcium oxalate. However, a major difference between struvite urolithiasis in cats and dogs is that most struvite uroliths in cats are not associated with urinary tract infection (sterile struvite), while urinary tract infection is common in dogs with struvite urolithiasis. Other, less frequently seen mineral composites include ammonium urate, xanthine, cystine, calcium phosphate, silica, and dried, solidified blood (cats).
More than 80% of uroliths in both dogs and cats are composed of magnesium ammonium phosphate (struvite) or calcium oxalate. However, a major difference between struvite urolithiasis in cats and dogs is that most struvite uroliths in cats are not associated with urinary tract infection (sterile struvite), while urinary tract infection is common in dogs with struvite urolithiasis.
Urolithiasis is typically a disease of adult animals. In cats, it is rarely seen in animals younger than 1 year old, and the majority of cats are first diagnosed when they are between 2 and 6 years old. Cats less than 4 years old are more likely to develop struvite uroliths, while cats older than 7 years old are at greater risk for developing calcium oxalate uroliths. In dogs, the mean age at time of diagnosis is between 6 and 7 years. Struvite, urate, and cystine calculi are associated with younger adults, and oxalate calculi are associated with older adults.
Initial studies of urolithiasis in cats found that the majority of cases were caused by struvite and a very small percentage was caused by calcium oxalate. However, by the early 1990s the percentages of struvite- and calcium oxalate–containing calculi observed in cats were approximately equal, and the prevalence of calcium oxalate uroliths continued to gradually increase as the prevalence of struvite uroliths decreased. 6 This reciprocal trend continued until 2002, after which the proportions of the two types of calculi again became approximately equal. A similar trend has been reported in dogs. 7. and 8. During the period that the incidence of feline calcium oxalate urolithiasis increased, there was a concomitant increase in cat foods formulated to decrease urine pH with the intention of preventing struvite urolithiasis. This change in formulation presumably reduced the risk of struvite urolithiasis in the subpopulation of cats that were susceptible to this type of urolith. Conversely, feeding an acidifying diet to healthy cats may have increased the risk for developing calcium oxalate calculi. As recognition of this trend became widespread, foods were formulated to produce more moderate changes in urine pH, correcting the shift that may have temporarily favored calcium oxalate formation. 6
In both species, age of onset of urolithiasis is related to the type of urolith present. For example, struvite, urate, and cystine calculi are associated with younger dogs (mean age 4.25 to 5.92 years), and oxalate, silica, and brushite calculi are associated with older dogs (mean age 7.04 to 8.71 years). 5 In dogs younger than 1 year old, the most common urolith is infection-induced struvite. 9 Similarly, cats less than 4 years old are more likely to develop struvite uroliths, while cats older than 7 years are at greater risk for developing calcium oxalate uroliths. 6 Sex predispositions are also observed. Female cats have a higher prevalence of struvite urolithiasis than male cats, and more than 70% of cases of calcium oxalate urolithiasis are seen in male cats. 10. and 11. Studies with dogs have shown a similar relationship between sex and mineral prevalence. 12 Struvite-, urate-, or calcium phosphorus (apatite)–containing calculi are more common in female dogs, while oxalate-, cystine-, and silica-containing stones are seen more often in males.
There is a clear genetic influence on the development of urolithiasis in both dogs and cats. Early studies reported that, compared with domestic shorthair cats, Siamese had a decreased risk and Persians had an increased risk of developing FLUTD. 10. and 13. More recently, studies of calcium oxalate and struvite uroliths in cats revealed that British shorthair, exotic shorthair, foreign shorthair, Havana Brown, Himalayan, Persian, Ragdoll, and Scottish Fold cat breeds show a higher risk of calcium oxalate uroliths and that Chartreux, domestic shorthair, foreign shorthair, Himalayan, oriental shorthair, and Ragdoll cat breeds have a higher risk for struvite uroliths. 11 It has been speculated that breed characteristics such as low activity and a tendency toward obesity may be influential factors. Breed predilections for urolithiasis in dogs may be more pronounced, but there is variability among study results. Calcium oxalate uroliths are reportedly more common in the Pomeranian, Miniature and Toy Poodle, Miniature and Standard Schnauzer, Lhasa Apso, Maltese, Yorkshire Terrier, Cairn Terrier, Shih Tzu, Bichon Frise, and Samoyed breeds. 5.7. and 14. Struvite uroliths are reported in the Bichon Frise, Shih Tzu, Lhaso Apso, Yorkshire Terrier, Dachshund, Miniature Schnauzer, Poodle, Pekingese, Pug, Welsh Corgi, Beagle, Cocker Spaniel, Springer Spaniel, and Labrador Retriever. 5.14. and 15. Conversely, Dachshunds, English Bulldogs, Newfoundland, Scottish Deerhound, Rottweiler, and Chihuahuas appear to be at increased risk for the development of cystine-containing calculi. 5. and 14. Finally, urate-containing calculi are most often seen in Dalmatians, English Bulldogs, and Miniature Schnauzers. Further complicating the situation, some canine urinary calculi have multiple layers composed of a variety of elements. 5
Uroliths in dogs and cats have historically been located almost exclusively in the bladder and urethra. However a recent study reported a 10-fold increase in feline kidney and ureter uroliths (nephroliths and ureteroliths, respectively) during the 20-year period between 1980 and 1999. 16 Another study reported that only 10 feline uroliths submitted to the Minnesota Urolith Center for analysis were from the upper urinary tract in 1990, while 139 cases submitted in the year 2000 were located in the upper urinary tract. 17 The authors noted that this change occurred during a period in which the prevalence of chronic renal failure in cats was also increasing. Survey radiographic evaluation of the cats in this study revealed that 41% of the cats with chronic renal failure had evidence of nephroliths or ureteroliths. 17 However, none of the cats had shown clinical signs attributable to upper urinary tract stones, suggesting that these stones are often not detected. Acute ureteral obstruction by uroliths is an emerging clinical syndrome in cats that is not completely understood and warrants additional study. 18 The majority of upper urinary tract uroliths contain calcium in cats. 17 In contrast, in dogs, nephroliths and ureteroliths may contain struvite, calcium oxalate, or other urolith types. 19 Traditionally, many theories to explain calculi formation have focused on the urinary environment and saturation with particular components such as calcium and oxalate. However, the increased prevalence of upper urinary tract stones suggests that other factors may be involved. One theory proposed in humans, but not yet investigated in pets, is that an initial, primary calcium stone-forming event begins with injury in the vascular bed at the tip of the renal papilla, which subsequently leads to the formation of calcium oxalate–containing uroliths in the kidneys. 20. and 21.
CLINICAL SIGNS
Clinical signs of urolithiasis in dogs and cats are nonspecific and depend on the location, size, and number of crystals or uroliths present within the urinary tract. Uroliths may be found in the bladder, urethra, kidneys, or ureters. Although uroliths can be up to several millimeters in diameter or even larger, most range in size from microscopic to the size of a grain of sand.
Initial clinical signs of lower urinary tract disease include frequent urination, dribbling of urine, and urination in inappropriate places (Box 30-1). Hematuria and a strong odor of ammonia in the urine are often observed. Pet owners may report additional signs of dysuria, such as prolonged squatting or straining following urination (often confused with constipation) and frequent licking of the urogenital region. These signs are frequently the only signs that owners report to veterinarians. The most accurate method of diagnosis is double-contrast cystography, although survey radiography and ultrasonography are also useful. 22 Urinalysis is an important component in the diagnosis of all urinary disorders and provides additional information. However, because urinary crystals (crystalluria) may be present or absent in animals with urolithiasis, and because uroliths can be present without crystals, crystalluria by itself is not diagnostic. 23 In addition, normal urine can contain crystals, especially if it is concentrated or has been refrigerated prior to analysis. When uroliths are found and removed, definitive diagnosis is made by chemical analysis of the type of urolith that is present.
BOX 30-1
Frequent urination
Urination in inappropriate places
Prolonged squatting or straining following urination
Hematuria
Licking of urogenital region
Dribbling of urine
Depression
Anorexia
Vomiting and diarrhea
Dehydration
In some cases, partial or total urethral obstruction may develop. When obstruction occurs, a variable mixture of mineral components and a proteinaceous colloidal matrix forms a plug that molds itself to the shape of the urethral lumen. 24 Although this can occur in any dog or cat, it is most commonly reported in male cats, presumably because they have a longer and narrower urethra with a sudden narrowing at the bulbourethral glands as the urethra enters the penis. 25. and 26. If obstruction is complete, uremia develops rapidly and is characterized by abdominal pain, depression, anorexia, dehydration, vomiting, and diarrhea. Increased pressure in the urinary tract can cause renal ischemia, ultimately resulting in permanent renal damage. In severe cases the distended bladder may rupture, causing a transitory relief of signs, followed rapidly by the development of peritonitis and death. Uremia alone leads to coma and death within 2 to 4 days, so partial or total obstruction is always a medical emergency (see Box 30-1).
STRUVITE UROLITHIASIS IN CATS
Early studies reported that more than 95% of uroliths in cats were composed of struvite. 27. and 28. Because struvite crystals were found to be the most prevalent cause of urolithiasis in cats, research during the early 1980s focused on preventing these crystals from forming in the urine and on the development of effective dietary management for cats with struvite urolithiasis. Although it now appears that a substantial proportion of cases may have other causes, prevention of the formation of struvite crystals is still an important and effective protocol for the management of urolithiasis in many cats.
Evidence indicates that three distinct types of struvite urolithiasis occur in cats. These are: (1) sterile struvite uroliths, (2) infection-induced struvite uroliths, and (3) urethral plugs containing a variable quantity of struvite crystals. Treatment and dietary management is directed at promoting the dissolution of struvite uroliths and treating urinary tract infection and inflammation if these are involved.
Struvite Formation
Several conditions are necessary for the formation of struvite crystals or uroliths in the urinary tract. 29 First, a sufficient concentration of the composite minerals magnesium, ammonium, and phosphate must be present. In addition, these minerals must remain in the tract for an adequate period to allow crystallization to occur. Therefore the production of concentrated urine and small volumes of urine are important contributing factors. Finally, a pH that is favorable for crystal precipitation must exist within the urinary tract environment. Struvite is soluble when urine pH is below 6.6, and struvite crystals form when urine pH is 7.0 and above. 30 The solubility of struvite crystals also depends on the products of Mg 2+, NH 4+, and PO 43−, called the struvite activity product (SAP) as described by the following: SAP = ([Mg 2+] × [NH 4+] × [PO 43−]). 31 The SAP also increases concomitantly with urine pH. As urine pH increases above 6.8, the SAP begins to increase exponentially.
Sterile struvite urolithiasis in cats is associated with the previous factors and the absence of a detectable urinary tract infection. However, while the presence of alkaline urine is necessary for the initial formation of struvite crystals, studies of cats with sterile struvite urolithiasis have found that the urine of affected cats is not consistently alkaline. For example, a group of 20 cats with naturally occurring sterile struvite uroliths had a mean urine pH of 6.9 ± 0.4 at the time of diagnosis. 32 Practitioners must be cautioned that the production of neutral or acidic urine upon presentation should not be interpreted as precluding struvite as the underlying cause of urolithiasis.
Infection-induced struvite urolithiasis is less common in cats than dogs, but it still represents an important form of disease, occurring most commonly in cats <1 year of age and >10 years of age. 33 Infection with urease-producing bacterial species (especially Staphylococcus) accompanied by signs of urolithiasis and the presence of struvite in the urinary tract are necessary for diagnosis. These microbes release the enzyme urease. Urease hydrolyzes urea to ammonia, causing increased concentrations of ammonia and phosphate ion, two components of struvite. The increased ammonia ion further contributes to urine alkalinization. Abnormalities in local host defense mechanisms, such as a perineal urethrostomy, and the quantity of urea that is found in the cat’s urine, may predispose a cat to infection-induced urolithiasis. 34. and 35. However, because most cats are innately resistant to bacterial urinary tract infection, infection-induced struvite urolithiasis is encountered less commonly than sterile struvite. Antimicrobial therapy, preferably based on culture and sensitivity, is essential when treating cats with infection-induced struvite to prevent recurrence.
Several conditions are necessary for the formation of struvite uroliths in cats. These include a sufficient concentration of the composite minerals magnesium, ammonium, and phosphate, the production of concentrated urine, and urine pH that is 7.0 or greater. However, while the presence of alkaline urine is necessary for the initial formation of struvite crystals, not all cats will present with urine that is consistently alkaline.
Dietary Risk Factors
Diet and feeding practices represent important risk factors for struvite urolithiasis in cats (Box 30-2). These include the food’s urine-acidifying and urine-protein-excretion properties, level of magnesium and sodium, digestibility, caloric density, and water content. 36 The cat’s feeding schedule may also be important. More than any of the other risk factors involved, these are elements of a cat’s life over which pet owners have some control and that can be modified during the treatment and long-term management of struvite urolithiasis.
BOX 30-2
Urine-acidifying properties (production of an alkaline urine)
Urine-protein-excretion properties
High magnesium content
High phosphorous content
High chloride content
High calcium content
Low moisture content
Low digestibility and caloric density
Feeding regimen (meal-feeding)
Low water intake and balance
As discussed previously, one of the factors necessary for the formation of struvite in urine is the presence of sufficient concentrations of its three composite minerals, magnesium, ammonium, and phosphate. Feline urine always contains high amounts of ammonium because of the cat’s high protein requirement and intake. Urine phosphate in healthy cats is also usually high enough for struvite formation, regardless of dietary phosphorus intake. The concentration of urine magnesium, on the other hand, is normally quite low and is directly affected by diet. 37 Thus, early investigations of feline struvite urolithiasis focused on dietary magnesium as a potential causal agent. The manipulation of dietary magnesium levels to produce or prevent phosphate urolithiasis had previously been well documented in rats and sheep. 38. and 39. This work was used to suggest a role of this mineral in the etiology of urolithiasis in domestic cats. One of the first studies showed that urethral obstruction and cystoliths could be induced in adult male cats when they were fed a diet containing either 0.75% or 1.0% magnesium and 1.6% phosphate. 40 In this study, the obstructing uroliths were composed primarily of magnesium and phosphate. Subsequent work showed that high levels of dietary phosphorus were not necessary for urolith development, but they did increase the risk for urolith formation when dietary magnesium was also high. 41 However, if magnesium intake was low, the incidence of urolith formation was low, regardless of the level of phosphorus. In a later study by the same group, cats were fed diets containing 0.75%, 0.38%, or 0.08% magnesium on a dry-matter basis (DMB). Seventy-six percent of the cats that were fed 0.75% magnesium and 70% of the cats that were fed 0.38% magnesium developed urolithiasis and obstructed within 1 year or less, whereas none of the cats fed 0.08% magnesium obstructed. 37 Similarly, when random-source and specific–pathogen-free cats were fed diets containing either high magnesium or high magnesium and high phosphorus levels, urethral obstruction was induced. 42 The obstructing material was identifiable as struvite by radiographic crystallography in one of the seven cats.
These studies demonstrated the relationship between increasing magnesium in the diet and an increased rate of urolith formation and urethral obstruction in cats. However, the significance of these data to the role of dietary magnesium in naturally occurring feline struvite urolithiasis is questionable because the levels of dietary magnesium used in these studies were all substantially higher than those normally found in commercial cat foods. The domestic cat requires only 0.016% available magnesium for growth and maintenance. 43 The Association of American Feed Control Officials’ (AAFCO’s) Nutrient Profiles requires cat foods to contain a minimum of 0.04% magnesium. 44 Most commercial cat foods contain slightly higher than this amount but still less than 0.1%. Although the magnesium in naturally occurring ingredients is not 100% available, these levels supply cats with their magnesium requirement. The amount of magnesium in cat foods is higher than the cat’s minimum requirement for magnesium, but it is still substantially lower than the levels used in the early experimental studies to induce struvite formation (0.4% to 1.0%).
A second problem with data from early studies involved the composition of experimentally produced uroliths. The struvite found in naturally occurring cases is composed of three minerals: magnesium, ammonium, and phosphate. However, experimentally induced uroliths in some studies were actually made up of magnesium phosphate, with no detectable ammonium. 40. and 41. The composition of urethral plugs that caused obstruction in cats fed experimental diets was also different from the composition of urethral plugs of cats with spontaneous disease. Although the experimentally induced plugs were composed almost exclusively of struvite crystal aggregations, urethral plugs found in spontaneous disease are most often composed of a mucogelatinous protein matrix that contains varying amounts of minerals (usually struvite), sloughed tissue, blood, and inflammatory cells. 45.46. and 47.
The most important confounding factor of these studies involved the form of magnesium added to the experimental diets. A group of investigators examined the effects of two different forms of supplemental dietary magnesium on the urine pH of adult cats. 48 The data showed that the addition of 0.45% magnesium chloride to a basal diet resulted in a significant lowering of urine pH. In contrast, when the cats were fed the same basal diet supplemented with 0.45% magnesium oxide, a significantly higher pH was produced. In a free-choice feeding regimen, mean urine pH in cats fed the basal diet was 6.9, and urine pH values in cats fed the magnesium chloride– and magnesium oxide–supplemented diets were 5.7 and 7.7, respectively. When urine samples were examined microscopically, crystal formation was observed in cats fed the basal diet and the magnesium oxide–containing diet, but not in cats fed the magnesium chloride–containing diet. Therefore the form of magnesium included in the diet influenced urine pH and the formation of crystals. The observation that high levels of magnesium result in increased struvite formation may have been confounded by the effect of magnesium chloride versus magnesium oxide on urine pH. It can be concluded that similarities exist between early studies of experimentally induced struvite urolithiasis and naturally occurring disease, but the presence of significant differences and confounding factors show that magnesium intake is not singularly responsible for the natural development of struvite urolithiasis. Subsequent studies showed that dietary magnesium is less significant as a dietary risk factor for struvite urolithiasis than are urine pH, urine volume, and water balance.
As discussed previously, struvite crystals form in feline urine with a pH of 7.0 or greater and are soluble at a pH of 6.6 or less. Normal, healthy cats typically have acidic urine with a pH between 6.0 and 6.5, except after meals. 37 In all animals, the consumption of a meal results in a rise in urine pH within 4 hours. This effect, called the postprandial alkaline tide, is caused by renal compensation for the loss of gastric acids that are secreted during digestion of the meal. To compensate for the loss of acid and to maintain normal pH in body fluids, the kidneys excrete alkaline ions, resulting in an increased urine pH. The magnitude of the alkaline tide is directly proportional to the size of the meal and to the acidifying or alkalinizing components within the meal. Depending on the nature of the diet and the size of the meal, the postprandial alkaline tide in cats can result in a urine pH as high as 8.0. 49
Many studies have demonstrated the importance of urine pH in the formation of struvite crystals in cats. 48.49.50.51. and 52. One study examined the effects of feeding a canned diet, a dry diet, or a dry diet supplemented with a urine acidifier (1.6% ammonium chloride) on urine pH and struvite formation in adult male cats. 50 Urine pH was highest in cats fed the dry diet (mean = 7.55). The addition of ammonium chloride reduced urine pH to 5.97. The canned diet resulted in urine with a mean pH of 5.82. The most significant findings of this study concerned urine struvite formation. Struvite crystals were present in 78% of the cats fed the dry diet but only 9% of the cats fed the dry diet plus ammonium chloride. Intakes of dry matter (DM), magnesium, and other minerals were the same for cats fed each of the dry diets. None of the cats fed the canned diet developed urinary struvite crystals. In addition, when urine samples from all cats were adjusted to a pH of 7.0 using 0.5 molar (M) sodium hydroxide, 46% of the cats fed the canned diet and all of the cats fed the ammonium chloride–supplemented dry diet showed typical struvite formation. These results show that at similar levels of energy, DM, and magnesium intake, the most important factor affecting feline struvite formation is urine pH.
Regardless of the level of magnesium intake by a cat, the dietary manipulation of urine pH consistently affects struvite formation. When a dry diet containing a high level of magnesium (0.37%) was fed to adult male cats, the addition of 1.5% ammonium chloride resulted in a urine pH of 6.0 or less. 51 Cats fed the diet without supplemental ammonium chloride produced urine with a pH of 7.3. Of the nonsupplemented cats, 7 of the 12 formed struvite uroliths and obstructed on two occasions, while only 2 of the cats fed the acidifying diet obstructed on a single occasion. When the diets of the seven obstructed cats were supplemented with ammonium chloride, they experienced no further episodes of struvite urolith formation or obstruction. Radiographic examination prior to supplementation revealed visible uroliths, which dissolved after 3 months of consuming the acidifying diet. Similar results have been reported when diets containing levels of magnesium commonly found in commercial pet foods were fed. When adult cats were fed a purified diet containing only 0.045% magnesium, struvite formed, and the cats showed clinical signs of urolithiasis when the diet produced an alkaline urine. 47 However, if ammonium chloride was added as an acidifying agent, clinical signs disappeared within 4 days and did not recur while the acidifying diet was fed.
The domestic cat is a carnivorous mammal. Compared with an omnivorous or herbivorous diet, a carnivorous diet has the effect of increasing net acid excretion and decreasing urine pH. 53. and 54. This urine-acidifying effect is primarily a result of the high level of sulfur-containing amino acids found in meats. Oxidation of these amino acids results in the excretion of sulfate in the urine and a concomitant decrease in urine pH. 55 In addition, a diet that contains a high proportion of meat is lower in potassium salts than a diet containing high levels of cereal grains, which have been shown to produce an alkaline urine when metabolized. 56. and 57. Similarly, foods that contain high levels of digestible carbohydrate have been shown to increase urinary pH. 58 Therefore the inclusion of high levels of cereal grains and low levels of meat products in some commercial cat foods may be a contributing factor to the development of struvite urolithiasis. For example, the struvite-producing, commercial dry diet that was used in one of the previously discussed studies contained 46% cereal grains, primarily in the form of wheat meal. 50
Although a certain amount of cereal is necessary for the extrusion and expansion process of dry foods, high levels of these ingredients may contribute to the production of alkaline urine. Conversely, the inclusion of large amounts of meat products in cat foods usually contributes to the production of more acidic urine. Furthermore, increasing meat-source protein may be preferable to using ammonium chloride supplementation to reduce urine pH. 59 Although supplementation with urine acidifiers such as DL-methionine and ammonium chloride decreased urine pH, supplementation did not reduce the concentration of urine organic fraction, which may serve as the matrix for struvite urolith formation. 60 Increasing dietary protein also increases urea diuresis, which can contribute favorably to increased urine volume. 61 However, increased protein may not be needed provided a food already promotes acidic urine. A case-control study designed to identify dietary factors associated with decreased risk of struvite uroliths reported that foods formulated to include higher fat, lower protein, lower potassium, and increased urine-acidifying potential could potentially minimize formation of struvite uroliths in cats. 36
As pet food manufacturers search for ingredients to include in cat foods that will naturally produce acidic urine, each ingredient must be separately evaluated for its effect on urine pH. For example, one study compared the urine-acidifying effects of corn gluten meal, poultry meal, and meat and bone meal when diets containing these ingredients were fed to cats. 62 Of the ingredients tested, corn gluten meal had the strongest acidifying effect on urine. Unlike most plant protein sources, corn gluten meal contains higher concentrations of sulfur-containing amino acids than either poultry meal or meat and bone meal. Corn gluten meal is unusual in that it is a cereal protein that produces acidic urine when fed to cats. A study comparing meat meal with corn gluten meal as a protein source for dry cat food found that meat meal was similar to corn gluten meal in its effects upon urinary pH, SAP, number of struvite crystals in urine, and other measures of struvite urolith risk. 63 In addition, DM digestibility and nitrogen utilization of the meat meal was significantly higher when compared with DM digestibility and nitrogen utilization of corn gluten meal. Another study by the same group compared fish meal with corn gluten meal as protein sources in cat foods and reported that the two protein sources did not differ significantly in either dry-matter digestibility or nitrogen utilization. 64 However, the fish meal protein source contributed to lower urine pH and SAP values. Conversely, when meat meal, chicken meal and corn gluten meal were compared, chicken meal had moderate digestibility and nitrogen utilization, but its relatively high calcium and phosphorous content led to increased urinary pH. 65
Water Balance and Urine Volume
Decreased urine volume may be an important risk factor for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to struvite formation. It has been suggested that dry cat foods contribute to decreased fluid intake and urine volume. An early study showed that cats fed a dry cat food had decreased total water intakes when compared with cats consuming similar energy levels from canned food. 66 Cats did increase voluntary water intake when fed the dry food but not in sufficient amounts to fully compensate for the lower moisture content of the food. In another experiment, adult cats were fed a semipurified, basal diet containing varying levels of moisture. 67 The cats consuming a diet containing 10% moisture had an average daily urine volume of 63 milliliters (ml). This volume increased to 112 ml/day when the moisture content of the diet was increased to 75%. Urine specific gravity was also slightly higher in cats that were fed the low-moisture food. In both of these studies, the differences in urine volume were attributed to lower total water intake in the cats that were consuming low-moisture foods.
However, in contrast to these studies, two other groups of investigators found no difference in water consumption between cats fed dry diets and those fed canned diets. It appears that diet composition, especially fat content and caloric density, influences water turnover in cats fed different types of commercial diets. In a study examining the effects of diet type, composition, and digestibility on water-excretory patterns in cats, a comparison of three canned diets showed that when cats were fed diets containing high levels of fat (34% and 28% of DM), significantly less DM was consumed than when cats were fed a canned diet containing a relatively low level of fat (14%). 68 Fecal DM and fecal water content were lower in cats fed the high-fat diets. Because total water intake was the same for all cats, the cats consuming the high-fat diets excreted significantly higher volumes of water in their urine to achieve water balance. Further evidence supporting the importance of caloric density and fat content is demonstrated by a comparison of a low-fat canned food to three dry cat foods in the same study. Water volume in urine and feces was similar between cats fed the low-fat, canned ration and cats fed the three dry diets. Other than the large difference in water content, the nutrient content of the low-fat, canned food was very similar to that of the dry diets. Energy digestibility of the canned diet was also equivalent to that of the dry diets (79.3% and 78.7%, respectively) and was significantly lower than the mean digestibility of the high-fat, canned diets (90.3%). Statistical analysis of these data revealed that the percentage of water excreted in the urine of cats is directly related to the fat and energy content of the diet, with correlation coefficients of 0.96 and 0.94, respectively.
Some investigators have advocated feeding only canned cat food to cats with a history of urolithiasis. 69. and 70. The intent is to increase water intake and cause a resultant increase in urine volume and decrease in urine specific gravity. However, the water content of the diet is probably not as important as are caloric density, fat content, and digestibility of the food. As was evident in the previously mentioned study, a poorly digestible canned cat food may not contribute to increased urine volume if large amounts of water are excreted in the feces. Conversely, the consumption of a cat food (canned or dry) that is energy dense and highly digestible will result in lower total DM intake. This decrease will be accompanied by decreased fecal volume and fecal water and increased urine volume. These effects may be beneficial in preventing urolithiasis in cats because urine will contain a lower concentration of the mineral components that lead to urolith formation. In addition, an increase in urine volume stimulates an increased frequency of urination, thus decreasing the time available for struvite formation.
In addition to urine pH, decreased urine volume and the production of highly concentrated urine are important risk factors for the development of urolithiasis in cats. Diets that cause a decrease in total fluid turnover can result in decreased urine volume and increased urine concentration, both of which may contribute to struvite formation. Dietary factors that contribute to water turnover include the amount of water included in the food, and the food’s caloric density, fat content, and digestibility.
Feeding Method
The postprandial alkaline tide occurs as a result of meal ingestion and the subsequent excretion and loss of gastric acids. 71 Many factors affect its duration and magnitude. Domestic cats are nibblers by nature. When fed free-choice, most cats eat small meals every few hours throughout the day. 72. and 73. In general, this feeding regimen has the effect of reducing the magnitude of the alkaline tide but prolonging its duration. In contrast, depending on the alkalinizing effects of the diet, meal-feeding may cause greater fluctuations of shorter duration. The effects of a feeding regimen are further complicated by the type of diet fed, the eating patterns of the cat, and various dietary components.
In one study, cats were fed a dry, commercial food, either on a free-choice basis or once daily. The urine pH of cats fed ad libitum was maintained between 6.5 and 6.9 throughout the day. In cats fed the same diet once daily, urine pH increased 2 hours after the meal to 7.7 and then gradually decreased for the remainder of the day. 51 Another group of researchers fed cats two dry foods and three canned foods on an ad libitum basis and recorded urine pH throughout a 24-hour period. 37 One of the dry foods and two of the canned foods resulted in constant urine pH values of less than 6.3. However, the other dry and canned foods produced pH values that ranged from 6.5 to higher than 7.0. When the same foods were meal-fed once daily, all of the foods except one dry and one canned product resulted in peak urine pH values of greater than 7.0 within 4 hours after the start of the meal. These values all declined to less than 6.5 by approximately 16 hours after the meal. One dry and one canned food maintained pH values of 6.6 or less, even when meal-fed. These differences were attributed to different urine-acidifying components/ingredients present in the foods. More recently, a study that examined the long-term effects of acidifying diets found that ad libitum feeding was essential to maintain a mean urine pH of less than 6.5, even when an acidifying diet was fed. 27 The lower urine pH in cats fed ad libitum was attributed to the consumption of numerous small meals throughout the day, which minimized the amount of gastric acid secreted for each meal and subsequently decreased the postprandial alkaline tide.
In addition to urine pH, the effects of feeding regimen on urine volume and composition are important considerations. In a study that examined the relationship between method of feeding, food and water intake, urine volume, and urine composition, the period of highest urinary excretion of magnesium and phosphorus occurred preprandially and therefore did not coincide with the daily alkaline tide. 74 This study also found that ad libitum–fed cats had increased frequency of urination and greater total urine volume when compared with meal-fed cats. These effects may be beneficial in minimizing urolith formation. Although these results indicate that the highest concentration of composite minerals does not occur during the time they would be most likely to precipitate, this may not be a necessary condition for struvite formation. Research has shown that urine pH is directly related to the size of the meal, and this relationship can be described by a simple linear model. 75 In other words, as the size of the meal increases, so does postprandial urine pH. These data also showed that as postprandial urine pH increased, the presence of struvite crystals increased accordingly. Struvite did not form when urine pH was maintained at less than 6.6.
Dietary Management
In clinical cases in which obstruction has occurred, immediate care involves stabilization of the cat’s condition, fluid replacement therapy, and relief of bladder distention and urethral obstruction. Removal of the obstruction can usually be accomplished by either flushing the urolith or urethral plug out of the urethra or by cystocentesis. However, while cystocentesis immediately relieves the distended bladder, it usually does not remove the obstructing uroliths. When a bacterial urinary tract infection is present, appropriate antimicrobial therapy should be initiated. Long-term dietary management involves the removal or dissolution of any remaining struvite uroliths and the feeding of an appropriate food to minimize the likelihood of recurrence.
Remaining struvite uroliths that are present in the urinary tract can be removed either through surgical means or via diet-induced dissolution. Surgical intervention provides immediate relief to the animal, followed by recovery within 3 to 7 days. On the other hand, dietary dissolution is a noninvasive procedure but can take several weeks to months to be effective. When dietary intervention is used to dissolve existing struvite uroliths, a food that produces an acidic urine with a pH of 6.3 or lower and contains reduced magnesium should be selected. 76 Adding sodium chloride to the diet has been suggested as an agent to induce polydipsia and resultant polyuria with the intent of producing a more dilute urine and increasing the frequency of urination. 77 However, feeding increased amounts of sodium to cats increases renal excretion of calcium and so may contribute to the formation of calcium oxalate uroliths. 76 For this reason, supplementing a cat’s normal food with sodium chloride is not recommended.
Depending on the size and number of the uroliths present, complete dietary dissolution usually takes between 5 and 7 weeks. 78 There is also evidence suggesting that infection-induced struvite uroliths may take longer for dissolution than sterile struvite uroliths. 32 Regardless of the difference in time for dissolution, the eradication of infection caused by urease-producing bacteria is the most important factor in cats with infection-induced struvite urolithiasis.
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