Urolithiasis is a common clinical problem in dogs. Several risk factors for urolith formation, such as breed, sex, age, diet composition, water intake, infection of the urinary tract, environment and drug administration, have been recognised(Reference Stevenson, Rutgers, Pibot, Biourge and Elliott1). Struvite and calcium oxalate are the predominant mineral types in urolithiasis in dogs, representing, overall, more than 80 % of total reported urolithiasis cases(Reference Osborne, Lulich and Polzin2, Reference Picavet, Detilleux and Verschuren3).
The aim of the present study was to compare the effect of two different dry foods for the management of struvite urolithiasis with different anion–cation balance on urinary pH.
Materials and methods
A total of twelve privately owned adult dogs (4·3 (sem 1·2) years old; live weight 20·2 (sem 10) kg) of different breeds showing struvite urolithiasis, confirmed by urolith composition assay, were divided into two groups (A and B), and fed for 3 months one of two dry diets (Hill's prescription diet c/d named A v. an experimental diet named B) formulated in order to dissolve struvite uroliths. After the first evaluation (time 0), all the dogs were returned to their owner, and the following recommendations were specifically indicated: (1) increase the water administration in order to dilute the urine; (2) increase the outdoor time in order to promote the voluntary urination. The diets were analysed for chemical and mineral composition(4, Reference de Ruig5), and the anion–cation balance (mEq/kg DM) was calculated according to the equation proposed by Langendorf(Reference Langendorf and Lang6):
Urine analyses were repeated six times (every 15 d). Urine was collected in the morning immediately after the food administration.
All procedures were approved by the Federico II Ethics Committee.
Data were processed using PROC GLM of Statistical Analysis Systems (SAS Institute, Cary, NC, USA)(7).
Results
The chemical and mineral composition of the diets is reported in Table 1. The chemical and mineral composition of both diets was appropriate for dissolving struvite uroliths(Reference Lulich, Osborne, Bartges, Ettinger and Feldman8, 9). In particular, the diets were characterised by moderate protein amounts in order to reduce urinary urea availability for urease-producing bacteria. The fibre contents were lower than in maintenance diets in order to improve the digestibility and to reduce water losses via faeces. Regarding mineral composition, both diets were characterised by low Mg and P supplementation. Although in both diets the anion–cation balance was negative, diet A showed an anion–cation balance lower than diet B. This was due to the differences in the amount of P, S and Mg.
Table 1 Chemical and mineral composition of the diets
CP, crude protein; EE, diethyl ether extract; CF, crude fibre.
* Hill's prescription diet c/d named A v. an experimental diet named B.
† Anion–cation balance calculated on the basis of food content in g/kg DM: mEq/kg DM = 49·9 × Ca+82·3 × Mg+43·5 × Na+25·6 × K − 64·6 × P − 62·4 × S − 28·2 × Cl.
The mean values of urinary pH and density registered during the trial are reported in Table 2. For urine analysed at the first time point, the mean pH values were 8·0 (sem 0·5) and 8·0 (sem 0·8) for groups A and B, respectively, and bacteria were found in about 70 % of the samples. For this reason, an antimicrobial (fluoroquinolones in tablet form) was given for 1 week. All urine samples analysed after 15 d did not show bacteria.
Table 2 Mean values of urinary pH and density (kg/m3) registered for groups A (n 6) and B (n 6)
(Mean values with their standard errors)
a,b Mean values within a row with unlike superscript letters were significantly different for diets A and B (P < 0·05).
Both groups showed a progressive decrease in pH values. This could be affected to the chosen measuring time (each 15 d in the morning immediately after the food administration), instead individual urine pH changes during the day(Reference Kienzle and Wilms-Eilers10). From the 30 d sampling, dogs fed diet A showed pH values significantly (P < 0·05) lower than dogs fed diet B. This could be due to the lower anion–cation balance of diet A. After 45 d of therapy, in both cases, the pH values recommended for stone dissolution were on average 5·9–6·1(Reference Stevenson, Rutgers, Pibot, Biourge and Elliott1). From 45 d sampling, group A showed pH values lower than 5·9. The pH values, registered after 75 d of dietary treatment, indicated the necessity to change the diet in both cases in order to avoid the risk of other diseases (e.g. urate urolithiasis(Reference Stevenson, Rutgers, Pibot, Biourge and Elliott1)). At the end of the trial, none of the urine samples showed crystals as has been reported in other studies(Reference Osborne, Lulich and Polzin11, Reference Rinkardt and Houston12).
Conclusion
The combination of antimicrobial and dietary therapy allowed the dissolution of struvite uroliths in both groups, even if the utilisation of the diet characterised by the lower anion–cation balance seems to reduce the urinary pH more rapidly.
Acknowledgements
The authors' contributions were as follows: S. C. and R. T. performed the chemical and mineral analysis of the diets. S. B. and M. I. C. chose the diets and calculated the individual ratios. S. C. were involved in the statistical analysis. A. D. B. and M. G. performed the diagnoses and urine analysis. M. I. C. prepared the experimental design. In any case, all the authors were equally involved in the interpretation of the results and manuscript preparation. The present study was supported by the Italian Ministry of Education, University and Research (individual research funding M. I. C.).
