Introduction
Urinary tract infections (commonly called “UTIs”) are very common in dogs, especially in females, although they certainly occur in males as well. They are not common in cats. Many dogs with UTIs do not show any recognizable clinical signs. Because so many of these infections are asymptomatic, it is especially important to rely on veterinary laboratory tests for accurate diagnoses. Urinary tract infections are usually caused by intestinal or environmental bacteria that enter and ascend the urethra into bladder; these are called uncomplicated or simple UTIs. Dogs can also develop UTIs if they have stones in the bladder or elsewhere in the urinary tract (uroliths), or if they have other predisposing medical conditions or diseases; these are called complicated, persistent or recurrent UTIs and are much more difficult to resolve.
Causes of Uncomplicated (Simple) Urinary Tract Infections in Dogs
Bacterial
Uncomplicated bacterial UTIs (called “cycstitis”) are those with no underlying structural or functional abnormalities causing the infection (immunosuppression, congenital deformities of the urinary tract, etc.). These ascending infections normally involve only one type of bacteria, although some dogs develop mixed infections. Bacterial urinary tract infections are more common in females because they have a much shorter urethra, and therefore bacteria have a shorter journey from the external genital environment to the bladder. Dogs born with narrow urethras may also be predisposed to simple bacterial urinary tract infections. Almost all bacterial UTIs in dogs involve the lower urinary tract (urethra and bladder). Occasionally, bacteria ascend into the upper urinary tract as well, through the ureters into the kidneys, causing more serious infection.
Other
Virtually all uncomplicated UTIs in dogs are caused by bacterial ascension. However, in rare cases, other organisms are the culprit; these can include Chlamydia, mycoplasma, viruses and fungi.
Causes of Complicated (Persistent or Recurrent) Urinary Tract Infections in Dogs
Complicated UTIs are defined as those having some underlying reason for recurring, usually a defect in the dog’s host defense mechanisms. They are much more difficult to diagnose and treat, because the underlying condition must be identified and resolved, if at all possible. Because UTIs are far less prevalent in males than in females, any UTI in a male dog should be considered to be a complicated infection.
Underlying etiologies that can predispose dogs to UTIs include anatomical defects or disorders such as bladder tumors or polyps, uroliths (“stones”), pyelonephritis, prostatitis, strictures and congenital urachal remnants. Other potential causes of recurrent UTIs in dogs include neoplasia (cancer), kidney insufficiency or failure, neurological disorders associated with the ability to urinate properly (such as masses or intervertebral disc abnormalities affecting the dogs ability to empty its bladder) and elevated urine pH from any cause, which facilitates bacterial overgrowth. One of the more common underlying etiologies is immunosuppression, steroid-induced or otherwise, which can be seen with hyperadrenocorticism and with long-term corticosteroid treatment. Diabetes mellitus is also associated with persistent or recurrent UTI, because elevated blood and urine glucose levels create a supportive environment for bacterial growth. Finally, an emerging and unfortunate cause of recurrent canine urinary tract infections is bacterial resistance to antimicrobial drugs.