Urolithiasis occurs in all species but the problem assumes greatest economic importance in fattening steers being fed heavy concentrated rations (Radostits et al. 2000). The incidence of bovine urolithiasis has decreased worldwide in recent years because of the improvement in the management of cattle (Larson 1996). Although the occurrence of obstructive urolithiasis is usually sporadic, outbreaks affecting a large number of animals have been recorded (Manning & Blaney 1986, Radostits et al. 2000). In the beef feedlot studied here, after the introduction of adequate amounts of fiber feedstuff in the diet of the feedlot, no additional cases of urolithiasis or other metabolic disorders were observed. It is suggested that important contributing factors for the development of obstructive urolithiasis in that feedlot included rations with high levels of phosphorus, diminished water intake and dietary deficiency or lack of adequate amounts of fiber.
Rupture of the urinary bladder secondary to complete urethral obstruction by uroliths with leakage of urine into the abdominal cavity causes gradual accumulation of fluid from uroperitoneum resulting in a severely distended abdomen as suggested in the present report (Oehme & Tillmann 1965, Radostits et al. 2000). Osmotic pressure from hypertonic urine together with acute fibrinous peritonitis promotes rapid movement of large amounts of extracellular water into the peritoneal cavity resulting in abdominal distension and dehydration. Chemical peritonitis associated with uroperitoneum (Oehme & Tillmann 1965) and cardiovascular disturbances caused by the abnormally high thoracic pressure induced by the abrupt release of large quantities of urine to the abdomen (Gasthuys et al. 1993) could also have contributed to some extent to the death of the animals.
In the present report, ammonium and phosphate were the principal mineral components of the uroliths. Phosphate calculi have usually been described as smooth, soft, numerous, small, sand-like particles (Larson 1996). Both chemical and morphologic findings from our cases match with those from previous reports of phosphate urolithiasis in feedlot cattle (Emerick 1988, Larson 1996). Chemical composition of uroliths together with environmental and dietary factors should be considered when establishing adequate preventive measures for urolithiasis.
The treatment of obstructive urolithiasis is primarily surgical (Larson 1996, Van Metre et al. 1996). Perineal urethrostomy resulted in complete recovery of one animal from that herd. It should be emphasized that immediate slaughter for salvage is advisable when urethral obstruction is diagnosed since therapy for this condition is unrewarding. Critical preventive measures such as providing a calcium to phosphorus ratio of 2:1 in the complete ration, increasing the salt level to 4% of the diet in order to stimulate water consumption and to increase urine volume and the maintenance of adequate and abundant water supplies should be highly considered in feedlot management.
Acknowledgements.– The authors would like to thank Mr. Joel Machado, undergraduate student of veterinary medicine, Universidade Federal de Pelotas, RS, Dr. Fábio Alexandre Rigos Alves, Laboratórios CEPAV Tecnologia em Saúde Animal, São Paulo, SP, Dr. Alexandre de Mello Kessler, Laboratório de Nutrição Animal, and Dr. Clesio Gianello, Laboratório de Análise de Solo, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, for their technical assistance. Financial support was entirely provided by the Conselho Nacional para o Desenvolvimento Científico e Tecnológico (CNPq-PRONEX), Brazil.