Most infectious diseases of the urinary system in small animals are aerobic bacterial infections. Common organisms include Escherichia coli, Staphylococcus, Enterococcus, and Streptococcus. Less common organisms causing infection include Klebsiella, Proteus, and Pseudomonas. Pasteurella is more common in cats than dogs. Mycoplasma is an uncommon cause of urinary tract infection and is usually found as a coinfection with bacteria. Leptospirosis is a worldwide zoonotic disease caused by filamentous bacteria that infect the kidney and many other organs. Fungi, yeast, and parasites uncommonly infect the urinary system.
Bacterial infections of the urinary tract typically ascend from the urethra into the bladder and in some cases into the kidneys. Predisposing factors include abnormalities of urine flow (eg, urine retention), decreased urothelial defense mechanisms, decreased systemic immune defense mechanisms, or inadequate urine concentration. Female dogs are more susceptible to urinary tract infections than male dogs, except for bacterial prostatitis in older, uncastrated male dogs. Dogs with concurrent diseases (eg, chronic kidney disease or hyperadrenocorticism) are at greater risk. Cats are relatively resistant to bacterial cystitis compared with dogs. Less than 2% of cats 1–10 yr old with lower urinary tract signs have a positive culture for bacterial infection. Adults cats with clinical signs of lower urinary tract pain and inflammation typically have a noninfectious cause, such as idiopathic cystitis, interstitial cystitis, or urinary bladder stones. Geriatric cats, immunosuppressed cats, or those with systemic diseases (eg, chronic kidney disease, hyperthyroidism) are more prone to bacterial urinary tract infections. Urinary tract pathogens, except for Leptospira interrogans, are not considered zoonotic. However, the potential for multidrug-resistant bacteria to localize in the urinary system is a concern for both animal and human health. Subtherapeutic treatment regimens and inappropriate antibiotic selection are contributing factors. Animals that receive chronic antibiotic treatment or are immunocompromised may (rarely) become infected with Candida spp. Systemic fungal infections (eg, blastomycosis, aspergillosis, protothecosis) can involve the urinary system; the kidneys and prostate are the most likely sites.