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Institute of Food, Nutrition and Human Health, Massey University;
* Landcare Research, Palmerston North, New Zealand and
Department of Physiology, Institute of Medical Biology, University of Southern Denmark, Odense, Denmark
2To whom correspondence should be addressed. E-mail: W.Hendriks{at}massey.ac.nz.
KEY WORDS: • feral cat • urine pH • urine composition • reference values
EXPANDED ABSTRACT
Cats have evolved over many years, thriving on a natural diet consisting mainly of animal tissues, and as a result have developed a specialized metabolism like other true carnivores such as mink and ferrets. In the wild, cats are predominantly predators of small mammals. In one New Zealand study, small mammals (rats, rabbits, possums, mice, stoats) made up 93% (by weight) of the diet of feral cats over 3 y (1). However, the feral diet varies with seasonal availability of prey species (2) and is therefore difficult to characterize.
Domestic cats can suffer from a number of urinary tract diseases in which the diet is implicated as a major causal factor. An example of this is urolithiasis, a common condition in which uroliths (crystals or stones) of various types form in the urinary tract. It has been shown that the potential for struvite (MgNH4PO4·6H2O) crystal formation is reduced if urine pH is <6.6 (3), whereas calcium oxalate crystal formation is less likely to occur at a higher urinary pH (4). Cook (5) stated that a carnivorous diet, is known to produce acidic urine, which is assumed to be entirely safe for cats. When cats are fed commercial diets, it is possible to lower the urine pH by addition of acidifying agents (such as ammonium chloride, calcium chloride and methionine) to the diet to prevent struvite formation (6). Currently, it is recommended to maintain urine pH of adult cats between 6.0 and 6.4 to minimize the risk of struvite urolithiasis. However, urine acidification together with a low magnesium intake increases the risk of calcium oxalate formation in domestic cats (7).
Urine composition and characteristics are directly related to the diet of cats (8) and because the natural diet is difficult to characterize, it is difficult to establish "normal" reference values. Vondruska (9) found a mean urine pH of 6.98 in cats fed a "seminatural diet" of rat carcasses. The latter study can be criticized, however, given that the rat carcasses were canned and heat sterilized. The approach used in this study to obtain "normal" biological reference values for cat urine was to determine the composition of feral cat urine because these animals are ingesting a "natural diet." Thus by measuring the various constituents and characteristics of feral cats’ urine it is possible to provide a set of baseline data for the evaluation of urinary compositional characteristics of domestic cats fed commercial pet foods.
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MATERIALS AND METHODS |
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 TOP MATERIALS AND METHODS RESULTSDISCUSSIONLITERATURE CITED |
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Standard predator trapping techniques were used to catch a total of 85 adult feral cats in four areas, two in the lower North Island and two in the upper South Island of New Zealand, over a 3-mo period (March–May). Only cats weighing >2.0 kg were classed as adults and included in the analyses. Victor Soft-Catch leg-hold traps (size 1
) (Oneida Victor, Eastlake, OH) baited with fresh rabbit meat were set at 100- to 200-m intervals, usually over 5–10 nights, and checked daily. Bait was replaced as required. Traps were set in all suitable habitats likely to be frequented by cats, such as gullies and areas with signs of high rabbit numbers. Animals were humanely killed at the trap site where they were captured, and urine was manually expressed immediately after death. The animal capture and handling procedures were approved by the Landcare Animal Ethics Committee (Landcare Research, Palmerston North, New Zealand).
Analyses
Urinary pH was determined on the warm urine (immediately postmortem) using a mobile pH-meter (Orion 250A, Orion Research, Boston, MA), calibrated before each measurement, using two buffers (pH 4.0 and 7.0) and using automatic temperature compensation. The urines were frozen, transported to the laboratory and the rewarmed urine was analyzed in duplicate for osmolality and specific gravity, using an Advanced DigiMatic Osmometer 3D2 (Advanced Instruments, Needham Heights, MA) and a urine refractometer (UG-1; Atago, Tokyo, Japan), respectively. Samples of urine were also analyzed for protein, ammonia, urea, uric acid, magnesium, calcium, phosphate and creatinine using commercially available kits (protein, kit no. 610A and ammonia, kit no. 171A, Sigma, St. Louis, MO; urea, kit no. 2052369, uric acid, kit no. 2052326, magnesium kit no. 1551353, calcium kit no. 1489216, creatinine kit no. 2052237, phosphate kit no. 2052288; Hoffmann La-Roche, Basel, Switzerland). Total nitrogen was analyzed by the Kjeldahl technique. Statistical analyses were carried out using ANOVA (10), with gender as the dependent variable.
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RESULTS |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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DISCUSSION |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The second method, determining the urine composition of cats assumed to be eating a completely "natural" diet under natural conditions, was used in the present study. We found that the urine pH of feral cats was significantly different between the sexes, with females exhibiting, on average, a lower pH (5.97) compared to that of males (6.37). The pH values are presented as a box plot in Figure 1. Our urine pH data support the recommendation to maintain urine pH of adult cats between 6.0 and 6.4, although the urine pH of one cat was as high as 7.39. Urinary composition and pH are directly and markedly related to the mineral composition (15) and the origin (natural or synthetic) of the diet (12). This was also the case in the present study, where increased urinary acidity (low pH) was associated with increased excretion of phosphate and, hence, augmented excretion of titratable acid in the urine of the feral cats. In an earlier study (W. H. Hendriks, unpublished data), we caught 198 feral cats from the same areas as reported in this study, and the bladder including urine was investigated for the presence of urinary stones. The urine present in the bladder was quantitatively transferred into a container and filtered (Wattman No. 1; Maidston, UK). The opened bladder and filter paper were placed under a stereo dissection microscope and examined for the presence of stones. No bladder stones were found in any of the 198 feral cats examined. In a large number of cats examined, urinary crystals were found to be present in the urine and bladder. This crystal formation is likely to have occurred during the freezing/thawing of the bladder and not in the bladder of the live cats.
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One factor that may have influenced the results in the present study is stress associated with the capture. The cats are likely to have been excited and stressed while being caught in the trap. In the stressed animal, the increasing activity of the sympathetic nerves and the adrenal glands will most likely lead to an increased metabolism, including catabolic conversion of proteins, which, in turn, increases sulfuric acid production and lowers urinary pH. However, the possible effects of stress are not easily distinguishable from those of long-term inanition and a thorough study of the effects of stress and excitement on urine pH in cats is clearly warranted. In contrast, it has been reported (7) that the urine pH of a cat increased by 1.4 U when transported from its normal home to a veterinary clinic. Buffington and Chew (7) concluded that the most likely cause was anxiety-induced hyperventilation (excessive panting) (alkaline urine is known to be caused by hyperventilation). Based on these results (7), if the urine pH of the cats in the present study was altered because of stress, this would indicate that our values are overestimates, which would seem unlikely.
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FOOTNOTES |
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LITERATURE CITED |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Fitzgerald, B. M. & Karl, B. J. (1979) Foods of feral house cats (Felis catus L.) in forest of the Orongorongo Valley, Wellington. N.Z. J. Zool. 6:107-126.
2. Cowan, B. J. & Brunner, H. (1972) Food habits of the feral house cat in Victoria. J. Wildl. Manage. 36:848-853.
3. Finke, M. D. & Litzenberger, B. A. (1992) Effect of intake on urine pH in cats. J. Small Anim. Pract. 33:261-265.
4. Hesse, A., Steffes, H.-J. & Graf, C. (1998) Pathogenic factors of urinary stone formation in animals. J. Anim. Phys. Anim. Nutr. 80:108-119.
5. Cook, N. E. (1985) The importance of urinary pH in the prevention of Feline Urologic Syndrome. Pet Food Ind. 27:24-31.
6. Skoch, E. R., Chandler, E. A., Douglas, G. M. & Richardson, D. P. (1991) Influence of diet on urine pH and the feline urological syndrome. J. Small Anim. Pract. 32:413-419.
7. Buffington, C. & Chew, D. (1996) Intermittent alkaline urine in a cat fed an acidifying diet. J. Am. Vet. Med. Assoc. 209:103-104.[Medline]
8. Burger, I. H. & Smith, P. M. (1987) Effects of diet on the urine characteristics of the cat. Edney, A.T.B. eds. Nutrition, Malnutrition and Dietetics in the Dog and Cat 1987:71-73 British Veterinary Association U.K. .
9. Vondruska, J. F. (1987) The effect of a rat carcass diet on the urinary pH of the cat. Comp. Anim. Pract. Fel. Nutr. 1:5-9.
10. SAS (1999) The SAS system for windows, release 6.12 1999 SAS Institute Cary, NC.
11. Hendriks, W. H., Emmens, M.M.A., Trass, B. & Pluske, J. R. (1999) Heat processing changes the protein quality of canned cat foods as measured with a rat bioassay. J. Anim. Sci. 77:669-676.
12. Finco, D. R., Barsanti, J. A. & Brown, S. A. (1989) Influence of dietary source of phosphorus on fecal and urinary excretion of phosphorus and other minerals by male cats. Am. J. Vet. Res. 50:263-266.[Medline]
13. Pastoor, F.J.H., Opitz, R., Van’t Klooster, A. T. & Beynen, A. C. (1995) Dietary phosphorous restriction to half the minimum required amount slightly reduces weight gain and length of tibia, but sustains femur mineralization and prevents nephrocalcinosis in female kittens. Br. J. Nutr. 74:85-100.[Medline]
14. Adams, L. G., Polzin, D. J., Osborne, C. A. & O’Brien, T. A. (1992) Correlation of urine protein/creatinine ratio and twenty-four hour urinary protein excretion in normal cats and cats with surgically induced chronic renal failure. J. Vet. Int. Med. 6:36-40.
15. Kienzle, E., Schuknecht, A. & Meyer, H. (1991) Influence of food consumption on the urine pH in cats. J. Nutr. 121:S87-S88.
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