Bacterial cystitis is inflammation of the urinary bladder caused by bacteria. Bacterial cystitis should be distinguished from subclinical bacteriuria.
Etiology and Pathogenesis of Bacterial Cystitis in Small Animals
Bacterial cystitis typically arises from organisms ascending from the perineum into the lower urinary tract, reaching the bladder and, potentially, the upper urinary tract (see Pyelonephritis).
Most cases of bacterial cystitis involve a single organism. Gram-negative bacteria in the family Enterobacteriaceae are the most common organisms causing bacterial cystitis, with Escherichia coli as the most common species.
Mycoplasma spp are a less common cause of infection but are a consideration for cystitis with no growth on routine urine culture. Infection with Corynebacterium spp would be another consideration for this clinical scenario because urine must be incubated for a longer time frame for these slow-growing bacteria.
The urinary tract has a number of defense mechanisms to impede development of infections:
In the urine, antimicrobial peptides help prevent bacteria from proliferating and adhering to the urothelium.
The high osmolality of concentrated urine creates a hostile environment for invading organisms.
The urothelium provides a physical barrier and performs processes that aid in expulsion of bacteria through normal voiding.
Additionally, the immune system response plays a role in both elimination of bacteria and regeneration of normal tissue.
As with other forms of UTI, bacterial cystitis occurs when a failure in host defenses allows a pathogen to infect the bladder and cause clinical disease. Decreased urinary defense mechanisms, including the following, are risk factors for UTIs:
immunosuppression
dilute urine
abnormalities in urinary flow (urinary retention or urinary incontinence)
anatomical abnormalities (eg, recessed vulva, urinary bladder masses)
Clinical Characteristics of Bacterial Cystitis in Small Animals
Although bacterial colonization in the urinary tract can occur in any small animal, female dogs and cats, as well as cats ≥ 10 years old, are at increased risk. Comparatively, females have shorter urethras that are in closer proximity to the anus. The increased frequency of UTIs seen in older cats may be due to the increased frequency of metabolic diseases and neoplasia that compromise host defenses. As such, clinical signs of lower urinary tract disease in a young cat are usually from a noninfectious disease.
Clinical signs of bacterial cystitis include pollakiuria, dysuria, stranguria, hematuria, periuria, and urinary incontinence.
Some animals with bacteriuria, with or without concurrent pyuria, may exhibit no clinical signs (see discussion of subclinical bacteriuria).
A bacterial infection resulting in clinical cystitis can be classified as either sporadic or recurrent:
A patient with < 3 confirmed or suspected episodes of bacterial cystitis in the preceding 12 months is classified as having sporadic bacterial cystitis.
A patient with ≥ 3 episodes of bacterial cystitis within the preceding 12 months, ≥ 2 episodes of bacterial cystitis within the preceding 6 months, or 1 episode of bacterial cystitis within the preceding 3 months of the most recent one is classified as having recurrent bacterial cystitis. Recurrent bacterial cystitis may be from a relapsing infection or reinfection.
Diagnosis of Bacterial Cystitis in Small Animals
Diagnosis of bacterial cystitis is based on the presence of lower urinary tract signs with confirmation of bacteriuria on urinalysis or bacterial culture.
Documentation of clinical signs associated with bacterial cystitis, as well as their frequency, through a detailed history, is important for classifying bacterial cystitis. Presence of clinical signs is one of the most important indications for treatment.
At minimum, urinalysis including a urine sediment examination should be performed to diagnose bacterial cystitis. Urine culture and susceptibility testing is recommended but does not need to be performed in all cases of sporadic bacterial cystitis.
Urine samples should ideally be collected via cystocentesis. Because some organisms, including Leptospira, are zoonotic, infection control barriers should be utilized as necessary.
The presence of bacteriuria on urinary sediment examination is predictive of positive results of urine culture. The prevalence of a positive urine culture when the urine sediment is concurrently inactive is 3.4% (1). Debris must not be misinterpreted as bacteria, particularly cocci. The leukocyte portion of the urine dipstick is inaccurate in small animals and should not be used. (See photomicrographs of bacteriuria and pyuria in a cat.)
Pearls & Pitfalls
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Courtesy of the Purdue University Veterinary Hospital Clinical Pathology Laboratory.
Urease-producing bacteria (primarily Staphylococcus spp and Proteus spp, but also some Klebsiella spp, Pseudomonasspp, and E coli) can cause an alkaline urinary pH and subsequent development of struvite crystalluria with the risk of urolithiasis; however, other factors can also affect urine pH.
Quantitative urine culture remains the gold standard for diagnosis of bacterial cystitis. Bacterial growth of ≥ 1,000 CFUs/mL from urine collected by cystocentesis is usually considered indicative of clinically relevant bacteriuria. Veterinary point-of-care urine culture systems are available and some show good correlation with cultures performed by microbiology laboratories. Samples should be processed immediately or refrigerated and processed within 24 hours of collection.
Most cases of bacterial cystitis involve a single organism. With recurrent infections involving multiple organisms, presence of a rectourethral fistula should be considered as a differential diagnosis.
Follow-up evaluation of urine is not indicated if clinical signs resolve with empirical antimicrobial therapy.
If clinical signs do not resolve with empirical antimicrobial therapy in cases of bacterial cystitis, a urine culture and diagnostic evaluation are indicated to rule out other causes of lower urinary tract disease.
Documentation of urine culture and susceptibility results is especially helpful to distinguish between relapse infection (same organism cultured) versus reinfection (different organism cultured). Additionally, the time frame of recurrence can help distinguish relapse infection from reinfection, given that clinical signs will recur shortly after antimicrobial discontinuation with relapse versus a longer time frame with reinfection.
A patient with recurrent bacterial cystitis should undergo further diagnostic evaluation to assess for a cause of relapse and risk factors for reinfection. A nidus for infection (eg, a urolith, polyp, tumor, infection of the prostate or kidneys, or deep-seated infection) is a potential cause of relapse infections. Some risk factors for reinfection include urinary incontinence, urine retention, recessed vulva, and suppression of immune function.
Treatment of Bacterial Cystitis in Small Animals
Advancements in veterinary antimicrobial stewardship are being made through growing evidence that shorter treatment durations are sufficient for sporadic bacterial cystitis. Currently, treatment durations of 3–5 days are recommended for sporadic bacterial cystitis, with amoxicillin or trimethoprim-sulfonamide being reasonable empirical choices in most areas.
A study found a short duration course of trimethoprim-sulfamethoxazole (15 mg/kg, PO, every 12 hours for 3 days) to be noninferior to a conventional course of cephalexin (20 mg/kg, PO, every 12 hours for 10 days) in dogs with sporadic bacterial cystitis (2). Another study found a high-dose, short duration course of enrofloxacin (18–20 mg/kg, PO, every 24 hours for 3 days) to be noninferior to a conventional course of amoxicillin-clavulanic acid (13.75–25 mg/kg, PO, every 12 hours for 14 days) in dogs with sporadic bacterial cystitis (3). The efficacy of shorter courses has not been studied for all antimicrobial classes.
Certain antimicrobials, such as fluoroquinolones or third-generation cephalosporins, should be reserved and only used if indicated based on urine culture and susceptibility testing.
The 2019 International Society for Companion Animal Infectious Diseases urinary tract infection guidelines are a good resource for current antimicrobial dosing recommendations. See also Pharmacotherapeutics in Bacterial Urinary Tract Infections in Animals.
Prolonged courses of antimicrobials are no longer recommended for all cases of recurrent infections. If a nidus of infection is identified as a cause of relapse infections, it should be eliminated, if possible. Antimicrobial treatment durations of > 3–5 days are likely needed for relapse infections and should be based on urine culture and susceptibility results; however, a lack of evidence exists in veterinary medicine to help guide appropriate treatment durations.
Alterations in antimicrobial type and longer durations of treatment are recommended in cases of pyelonephritis, bacterial prostatitis, or deep-seated infections. Most reinfection cases should be treated with short courses similar to treatments for sporadic bacterial cystitis.
Identifying and addressing risk factors for reinfection (eg, performing an episioplasty in a dog with a hooded vulva) can help decrease the frequency of reinfection. These cases can be difficult to manage long-term.
Poor evidence exists for the use of cranberry extract, D-mannose, probiotics, or oral glycosaminoglycans.
Administration of nitrofurantoin at night can be beneficial for some dogs; however, bacterial resistance can develop.
Intravesicular administration of a live E coli biotherapeutic product (asymptomatic bacteriuria E coli 212) is a promising nonantimicrobial therapy but is not commercially available.
If a longer course of an antimicrobial is prescribed, rechecking urine cultures 5–7 days into treatment as well as 5–7 days after the last dose can be considered. If bacterial growth occurs during antimicrobial therapy, the patient may have a refractory or persistent UTI. These cases may be due to poor client compliance, poor oral bioavailability, inadequate dosing regimen, or decreases in renal elimination of the antimicrobial.
Post-treatment urine culture helps to document that treatment resulted in a microbiological cure and can aid in determining the classification of a recurrent UTI.
Subclinical Bacteriuria in Bacterial Cystitis
Subclinical bacteriuria (ie, the presence of bacteria in urine as determined by positive results of bacterial culture from a properly collected urine specimen in the absence of clinical evidence of infectious urinary tract disease such as cystitis or pyelonephritis) may be self-limiting or persistent. It can occur in healthy patients as well as those with various diseases, such as myelopathies, endocrinopathies, and chronic kidney disease.
Subclinical bacteriuria is unlikely to lead to complications in small animals and usually does not require treatment.
Presence of pyuria or antimicrobial resistance is not an indication for treatment. Treatment of bacteriuria in a patient without clinical signs should be considered in certain clinical situations (eg, growth of a urease-producing organism in a dog with a history of struvite urolithiasis or growth of Corynebacterium spp, due to concern for encrusting cystitis).
Key Points
Quantitative aerobic culture of urine is the gold standard for diagnosis of bacterial cystitis; however, bacteriuria on urine sediment examination may be predictive of a positive culture.
Short treatment durations of 3–5 days are recommended for sporadic bacterial cystitis.
Recurrent bacterial cystitis can be categorized as refractory, relapse, or reinfection, and a thorough clinical evaluation is needed to determine the cause of recurrence.
Subclinical bacteriuria does not require treatment in most cases.
For More Information
Weese JS, Blondeau J, Boothe D, et al. International Society for Companion Animal Infectious Diseases (ISCAID) guidelines for the diagnosis and management of bacterial urinary tract infections in dogs and cats. Vet J. 2019;247:8-25.
Weese JS, Giguère S, Guardabassi L, et al. American College of Veterinary Internal Medicine (ACVIM) consensus statement on therapeutic antimicrobial use in animals and antimicrobial resistance. J Vet Int Med. 2015;29(2):487-498.
References
Strachan NA, Hales EN, Fischer JR. Prevalence of positive urine culture in the presence of inactive urine sediment in 1049 urine samples from dogs. J Vet Intern Med. 2022;36(2):629-633. doi:10.1111/jvim.16378
Clare S, Hartmann FA, Jooss M, et al. Short- and long-term cure rates of short-duration trimethoprim-sulfamethoxazole treatment in female dogs with uncomplicated bacterial cystitis. J Vet Intern Med. 2014;28(3):818-826. doi:10.1111/jvim.12324
Westropp JL, Sykes JE, Irom S, et al. Evaluation of the efficacy and safety of high dose short duration enrofloxacin treatment regimen for uncomplicated urinary tract infections in dogs. J Vet Intern Med. 2012;26(3):506-512. doi:10.1111/j.1939-1676.2012.00914.x
