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Bacterial Cystitis in Small Animals

By Scott A. Brown, VMD, PhD, DACVIM, Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, University of Georgia

Bacterial cystitis is infection and inflammation of the urinary bladder. Clinical signs are pollakiuria, hematuria, dysuria, and urinating in inappropriate places. Hematuria may be more noticeable at the end of the urine stream. An animal may exhibit pain on palpation of the caudal abdomen, and the bladder may feel thickened or irregular. Bacterial cystitis is occasionally diagnosed in an asymptomatic animal when a routine urinalysis is performed. Chronic glucocorticoid administration, hyperadrenocorticism, chronic kidney disease, and diabetes mellitus may be associated with asymptomatic urinary tract infections. Occasionally, animals with no concurrent disease will have bacteria in the urine without evidence of a true infection (ie, bladder mucosal invasion and inflammation); this condition is termed asymptomatic bacteriuria.

Urinalysis often shows increased protein and hemoglobin on the urine dipstick analysis. The WBC part of the dipstick (ie, nitrate) is inaccurate in dogs and cats and should not be used. The urine pH may be alkaline (7.5–9) if the bacteria are urease positive (eg, Staphylococcus or Proteus). However, an alkaline urine pH by itself is not abnormal, because diet and other factors can affect urine pH. Urine sediment should be examined microscopically. Increased numbers of WBCs, RBCs, and/or bacteria are consistent with cystitis. Bacteria can be confused with stain precipitate; filtering the stain or evaluating the sediment without staining is advised. Lack of visible bacteria in the sediment does not exclude urinary tract infection.

If clinical signs and/or urinalysis are suggestive of infection, a urine culture and antimicrobial susceptibility test should be performed. Cystocentesis is the preferred method for sample collection, followed by sterile urethral catheterization or a midstream free catch into a sterile collection cup. A quantitative culture is necessary to interpret the result, unless the sample was collected by cystocentesis. Ideally, the culture should be set up within 2 hr of collection. If the laboratory is off-site, the sample should be refrigerated and processed by the laboratory within 24 hr. If the specimen cannot be refrigerated, commercial collection kits that contain preservatives can be used to maintain a stable bacterial population at room temperature for 24 hr. Laboratories that can provide both quantitative culturing and a minimum inhibitory concentration–based method for antimicrobial susceptibility testing are preferred.

Antibiotic regimens for infections involving soft tissues, such as bacterial cystitis with prostatic involvement, should be similar to pyelonephritis (see Pyelonephritis in Small Animals). Simple bacterial cystitis is treated for 2 wk with a broad-spectrum antibiotic that achieves a high concentration in the urine. Appropriate initial choices include amoxicillin (10–20 mg/kg, PO, bid-tid), cefadroxil (22–30 mg/kg, PO, bid), cefpodoxime (5–10 mg/kg/day, FDA approved for dogs only), ormetoprim-sulfadimethoxine (27 mg/kg, PO, day one, then 13.5 mg/kg/day, PO), or cefovecin (8 mg/kg, SC, day one, which may be repeated once 7 days later). A repeat urine culture 3–7 days after oral therapy or on day 21 if cefovecin is used is recommended. If positive, another antibiotic based on the new susceptibility test results is given for a longer treatment period (eg, 3–4 wk). Very resistant or recurrent infections should be treated for 4–6 wk. Every course of treatment should be followed by a urine culture, even if the signs have resolved. In animals that have a history of chronic or recurrent infections, a urine culture should be done 3–7 days and 3 mo after successful therapy. If both of these cultures are negative, then a urine culture should be repeated at 6 and 12 mo after therapy. Because resistance to antibiotics can develop during therapy, antimicrobial susceptibility testing should be performed on every positive urine culture.

Animals with recurrent bacterial cystitis should be evaluated for an underlying cause. A recurrent infection caused by the same bacterial organism is termed a relapse and is essentially a treatment failure. This is typically caused by inappropriate antibiotic therapy (ie, wrong drug, dosage, or treatment duration) and often occurs because of an unrecognized complicating factor (eg, deep-seated bladder wall infection, bladder polyps, renal or prostatic involvement, concurrent disease). A recurrent infection in which different organisms are causative is termed a reinfection and is usually caused by host defense problems such as disorders of micturition (eg, urethral incompetence), anatomic abnormalities (eg, hooded vulva, patent urachus, ectopic ureters, uroliths), and/or concurrent disease (eg, chronic kidney disease, hyperadrenocorticism, chronic glucocorticoid administration). Abdominal radiographs are frequently diagnostic for uroliths but negative survey films should be followed by ultrasonography, cystoscopy, and/or double-contrast cystourethrography to exclude radiolucent urocystoliths, anatomic defects, polyps, and neoplasia. The history may reveal chronic glucocorticoid use. A serum biochemical profile, CBC, and complete urinalysis are important to exclude predisposing systemic diseases, such as chronic kidney disease, hyperadrenocorticism, and diabetes mellitus. Other diagnostic considerations include feline immunodeficiency virus, feline leukemia virus, and hyperthyroidism in cats or hyperadrenocorticism in dogs.

In cases that respond to therapy but continue to have frequent bouts of cystitis without an identifiable cause, low-dose prophylactic antibiotics can be used to prevent ascending bacteria from establishing an infection according to the following protocol: 1) a therapeutic course of an antibiotic for the current infection is completed, 2) no antibiotics are given for 3 days, to allow collection of urine for a culture after treatment, and 3) the prophylactic protocol is immediately started. Prophylaxis consists of using a broad-spectrum antibiotic (eg, amoxicillin, cefadroxil) at one-third of the total daily dose, given at bedtime, indefinitely. Every 6–8 wk, the antibiotic should be stopped for 3–7 days to obtain a sample for repeat urinalysis and culture. Every new infection should be treated with a therapeutic course of an antibiotic based on culture and susceptibility results. The treatment antibiotic will likely be different than the prophylactic antibiotic. Oral cranberry extract and d-mannose may be useful adjuncts. The most valuable therapeutic antibiotics (eg, fluoroquinolones, second- or third-generation cephalosporins) should be reserved for resistant infections. If the recurrent infection is resistant to the prophylactic antibiotic, this antibiotic can still be used for future prophylaxis after the infection is eradicated. Encouraging frequent voiding during the daytime is helpful to prevent recurrent infections. The major disadvantage of using a prophylactic antibiotic protocol is development of multidrug-resistant bacteria.