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Internist and Fetch dvm360 conference speaker Dr. Michael Wood offers practical insights into this frustrating veterinary condition.
When it comes to urinary tract infections (UTIs) in pets, the good news is that most dogs with simple uncomplicated UTIs will recover from the episode and go on to live a happy healthy life, says Michael Wood, DVM, PhD, DACVIM (SAIM), of the University of Wisconsin-Madison School of Veterinary Medicine. The not-so-good news? About 25% of pets that develop a UTI in their lifetime will experience recurrence.
Recurrent infections are a problem for a couple of major reasons, Dr. Wood says: One, how do you manage your patient in a way that minimizes antibiotic resistance, both in the individual and more broadly in the population? Two, how do you decide whether bacteria in the urine should be treated or not? During a recent Fetch dvm360 conference, Dr. Wood gave attendees some guidance in these areas.
Step one when faced with a UTI, Dr. Wood says, is to know your local resistance patterns. “Veterinarians have generally not done a very good job in this area,” Dr. Wood says.
Yes, you can look up published data on antibiotic resistance in veterinary patients, but this data may or may not match what's happening in your immediate patient population. The best approach is to keep track in your own hospital by recording culture and sensitivity results for every patient, building a database over time. This will help you know which antibiotics are likely to be more effective and less effective in your own patients when an empiric choice is needed. Antibiotics with resistance rates greater than 20% should be used cautiously if at all empirically.
Step two is to understand a specific patient's individual risk factors for resistance. For cats, risk increases with the number of antibiotics used in the last three months and the number of days the cat has been hospitalized-more days means more likelihood of resistance. In dogs, prior use of antibiotics is a risk factor, especially fluoroquinolones. Dogs are also at higher risk of developing resistance if they've been hospitalized for three days or more or if they consume a raw meat diet.
Step three is to perform a urine culture. The challenge, of course, is obtaining the urine sample. While Dr. Wood says cystocentesis will provide the most reliable results, free catch is also acceptable-you just have to interpret the results accordingly. With a free catch urine sample, you need to see more than 100,000 colony-forming units/ml to be confident you have a true infection rather than a contaminated sample.
While you're waiting for culture and sensitivity results, Dr. Wood says empiric therapy is justified if the patient has had limited previous antibiotic use, if you know the likely pathogen, if you know local susceptibility, and if the patient is showing clinical signs. According to the International Society for Companion Animal Infectious Diseases (ISCAID), appropriate first-tier empiric antibiotics are amoxicillin, amoxicillin-clavulanate (Clavamox-Zoetis), and trimethoprim sulfa. But this is where it's important to know your local resistance rates, Dr. Wood says. Your own first, second and third choices may differ accordingly.
Second-tier antibiotic choices, which should be prescribed based on culture and sensitivity results, are fluoroquinolones, third-generation cephalosporins and nitrofurantoin; the latter is best for maintaining urine sterility but does have side effects that should be discussed with the owner, Dr. Wood says. He notes that an important exception outlined by ISCAID is pyelonephritis. Here a fluoroquinolone is an appropriate first-line treatment because of its ability to penetrate into kidney tissue where other water-soluble antibiotics are less effective.
ISCAID proposes that nonsteroidal anti-inflammatory drugs (NSAIDs) may be effective as an antibiotic alternative to treat UTI in veterinary patients based on human studies. The thinking is that they provide the patient relief from clinical signs while the body clears the infection on its own. However, Dr. Wood says, in the human studies, people who use NSAIDS in this manner are shown to experience clinical signs one day longer than those treated with antibiotics, and increased rates of pyelonephritis were detected. Given the challenges of diagnosing pyelonephritis in our veterinary patients, Dr. Wood doesn't use NSAIDs to treat UTI with one exception: If a patient with known recurrent UTIs presents showing clinical signs, NSAIDs can be used as an effective symptom reliever while you're waiting for culture and sensitivity testing results.
Dr. Wood explains that recurrent bacteriuria-defined as the detection of bacteria more than three times in a year or twice in six months-falls into one of four categories.
1. Persistent UTI: When a urine culture is positive seven to 10 days after the beginning of treatment, this means bacteria has become resistant, the patient's immune system is compromised, or the antibiotic cannot achieve a high enough concentration to wipe out the infection either for endogenous or exogenous reasons. All require further investigation to manage the infection appropriately.
2. Relapse: This means the urine is initially cleared of bacteria, but bacterial reservoirs remain, allowing recolonization of the bladder. This condition can be seen in patients with urolithiasis, prostatitis and pyelonephritis and may call for a longer course of treatment or an antibiotic that better penetrates the area of the infection, Dr. Wood says. Another example, though unproven in cats in dogs, is intracellular bacterial communities where bacteria remain quiescent within the urothelium only to proliferate again when the urothelium is turned over.
3. Reinfection: This occurs when the UTI is cleared by an antimicrobial, but abnormal host defenses (for example, low urine-specific gravity) prime the host for another infection. “You can treat each of these infection episodes as an uncomplicated infection with five to seven days of an antibiotic,” Dr. Wood says.
4. Subclinical bacteriuria: More discussion on this below.
Persistence, relapse and reinfection are true UTIs that require treatment if the patient is experiencing clinical signs. But what do you do with subclinical bacteriuria, where you have a positive urine culture but clinical signs are absent?
In people, asymptomatic bacteriuria is rarely treated, Dr. Wood says. In fact, doctors restrict screening of diabetics, people with spinal cord injuries and immunocopromised patients for bacteriuria because they assume it will be present-and it may actually be protecting the patient from a more virulent organism, Dr. Wood says.
ISCAID agrees with this approach, stating that if a veterinary patient is not showing clinical signs, it should not be treated. But Dr. Wood says this approach is not always practical in clinical practice.
“It depends on how much you trust the owner to recognize subtle clinical signs,” he says. “Are they likely to notice slight changes in frequency, urgency or volume?” Dr. Wood says if he's confident an owner would pick up on these things in their pet, he doesn't treat subclinical bacteriuria.
However, he warns, “This is by no means a foolproof method and hence must only be considered with a strong understanding of the patient's overall health and after discussing the pros and cons of not treating the bacteriuria with owners.”