Summary
Technically speaking, a urinary tract infection (UTI) is an infection by microbial organisms (usually bacteria) of any part of the urinary tract.
While the kidneys, ureters, bladder, and urethra all comprise the urinary tract, the bladder and urethra are the most common sites of infection. This is largely because bacteria tend to enter the urethra and bladder via the opening of the urethra, where contamination from local skin irritation, fecal material, and/or the genitals can occur. This is referred to commonly as an "ascending" UTI.
For the reasons outlined in the paragraph above, this article will deal exclusively with UTIs that affect the bladder and/or urethra alone.
UTIs are common in both dogs and cats, though their causes differ significantly. The signs of disease, usually more frequent and painful urination, are similar in both species. Bloody urine may or may not be evident.
The causes of UTIs are many and various between and within each species.
Causes For Dogs and Cats
Diabetes mellitus (sugar in the urine feeds nearby bacteria and leads to UTIs)
Skin diseases, diarrhea, vaginitis, prepucial infections (or anything else that leads bacteria to proliferate abnormally near the opening of the urethra)
Bladder stones (these can lead to irritation and subsequent infection, but stones can also be caused by chronic UTIs, too)
Cancer affecting the bladder or ureter
Neurologic diseases (that lead to urine retention)
Medications or diseases that affect the body's normal immune function adversely
Cat-specific Causes
Complications arising from feline idiopathic cystitis (an inflammatory condition for which bacterial infection is currently considered an unlikely component)
Dog-specific Causes
Cushing's disease (this disease decreases the body's immune response and leads to skin disease as well)
Hypothyroidism (this can predispose dogs to skin diseases)
Causes For Female Dogs
Allergic skin disease (in females, skin infections surrounding the vulva are common in patients with allergic skin disease)
Recessed ("hypoplastic") vulva (urine tends to collect on the skin in dogs with this conformational problem that can be inherited or exacerbated by excess weight)
Hormone-related incontinence (this common condition affects spayed female dogs and is characterized by an incompetent bladder sphincter and "leaking" urine, thereby allowing for ascending infections)
Ectopic ureter (this congenital, inherited condition allows female dogs to urinate or "leak" abnormally, too)
As should be evident from the list above, female dogs are more likely to get UTIs than males. Yet no breed predisposition has been noted for UTIs in either dogs or cats. However, some of the underlying causes of urinary tract infections may be influenced by genetic factors that predispose pets to these infections.
Symptoms and Identification
UTI symptoms in dogs and cats can vary. Dogs and cats with UTIs will typically urinate small volumes frequently. Smelly, cloudy, or bloody urine may be appreciated. Straining to urinate (squatting or heading to the litterbox frequently) is considered good evidence of the pain or irritation typical of infected bladders and urethra.
Definitive diagnosis of UTIs are achieved via urinalysis and a "culture and sensitivity," a test that determines the kind of bacteria growing in the urine as well as the antibiotics it's susceptible to.
Additional testing is considered very important to determine the cause of the UTI, if it's not already evident, and to investigate its the underlying causes, too:
Complete blood count
Serum biochemistry analysis
Abdominal X-rays
Abdominal ultrasound
Cystoscopy
CT scans
Testing for immunosuppressive conditions
Allergy testing (in dogs)
Thyroid hormone testing (in dogs)
Affected Breeds
Any breed of dog or cat can be affected. A genetic predisposition for diabetes, Cushing's disease, hypothyroidism, ectopic ureters, hypoplastic vulvas, and allergic skin disease (among other conditions) is presumed.
Treatment
Treatment of UTIs invariably requires targeted antimicrobial therapy (antibiotics), along with treatment of any underlying condition.
Prevention
Preventing UTIs is accomplished by identifying predisposing conditions early and treating them before microbes can colonize the urinary tract.
References
Hess RS, Kass PH, Ward CR. Association between hyperadrenocorticism and development of calcium-containing uroliths in dogs with urolithiasis. J Am Vet Med Assoc 1998;212(12):1889-1891.
Hostutler RA, Chew DJ, DiBartola SP. Recent concepts in feline lower urinary tract disease. Vet Clin North Am Small Anim Pract 2005;35(1):147-170.
Martin GJ, Rand JS. Current understanding of feline diabetes: Part 2, treatment. J Feline Med Surg 2000;2:3-17.
Norris AM, Laing EJ, Valli VEO, et al. Canine bladder and urethral tumors: a retrospective study of 115 cases (1980-1985). J Vet Intern Med 1992;6(3):145-153.
Osborne CA, Unger LK, Lulich JP. Canine and feline nephroliths. In: Bonagura JD, Kirk RW, eds. Kirk's current veterinary therapy XII: small animal practice. Philadelphia, Pa: WB Saunders Co, 1995;981-985.
Smith AL, Radlinsky MG, Rawlings CA. Cystoscopic diagnosis and treatment of ectopic ureters in female dogs: 16 cases (2005-2008). J Am Vet Med Assoc 2010;237(2):191-195.