Therapeutic caveats: Difficult urinary tract infections

Aug 01, 2005

As discussed in last month's Diagnote, the underlying causes of bacterial urinary tract infections (UTIs) encompass transient or persistent abnormalities in host defenses.

It follows that protocols designed to safely and effectively eliminate UTIs should include:

  • Selection of appropriate antimicrobial agents to eradicate microbial pathogens, and
  • Detection and treatment of host defense abnormalities that allow bacteria to colonize and invade the urinary tract.

Table 1
The objective of the second part of this series is to summarize caveats derived from our experience with initial treatment of bacterial UTIs. Caveats associated with treatment of recurrent bacterial UTIs will follow in next month's Diagnote.

Empirical use Should empirical choice of antimicrobial drugs be considered to treat bacterial UTIs?

Table 2
In context of bacterial UTI, empirical treatment consists of selection of an antibacterial drug without knowledge of the susceptibility of bacteria to that drug. It is based on: 1) previous experience with the types of bacterial pathogens most likely to cause UTI, 2) epidemiologic data about the susceptibility of these bacteria to various antimicrobial agents (Table 1), and 3) knowledge that the drug will attain an appropriate concentration in urine (Table 2, p. 6S).

The most commonly reported bacterial isolates from dogs with UTIs have been Escherichia coli (~45 percent), Staphylococcus spp. (~10 percent), Proteus spp. (~10 percent), Klebsiella spp. (~10 percent), Enterococcus spp. (~5 percent) and Streptococcus (~5 percent).

Caveat: Patients with acute onsets of uncomplicated bacterial UTIs often "respond" to empirical antimicrobial treatment. Therefore, making an empirical, but educated, "guess" of the proper antibiotic to treat patients with acute onsets of uncomplicated bacterial UTI has been an accepted standard of practice.

However, the standard of practice also includes appropriate follow-up evaluation of the patient to determine the efficacy of therapy. Empirical selection of antimicrobial agents is not recommended for patients that have 1) a history of frequently recurrent clinical signs, and 2) been given antibacterial drugs to treat signs of urinary tract disease in the past three to six weeks.

Susceptibility testing When should antimicrobial agents be selected on the basis of antimicrobial susceptibility tests?

Table 3
Evaluation of the susceptibility of infecting bacteria to antimicrobial drugs is advisable as the most reliable guide for choice of therapeutic agents because different bacterial pathogens isolated from dogs and cats with UTI may vary widely in their susceptibility to specific antimicrobial agents. Especially Pseudomonas aeruginosa and Klebsiella spp., but also Enterobacter spp., Escherichia coli and Proteus spp. are examples of urinary pathogens that may be associated with polyresistant strains. There are some circumstances when antimicrobial susceptibility tests are essential (Table 3).

Caveat: Once bacterial urinary tract pathogens that have been exposed to one or more antimicrobial drugs, they often acquire resistance to many commonly used antimicrobial agents. In this setting, the susceptibility of bacterial pathogens is unpredictable, and therefore cannot be accurately predicted from tables derived from untreated patients (Table 1). If a patient with recurrent signs of UTI has recently been given antimicrobics, it is essential that the therapy be re-evaluated on the basis of quantitative culture and antimicrobic susceptibility tests to determine whether changes in susceptibility have occurred.