10.6.5 Urinary tract infections
Urinary tract infections are less common in horses
than other animals. Cystitis is the most common
bacterial infection of the urinary tract, and is often
associated with bladder dysfunction or other predisposing
factors (Table 10.7). As such, recurrent
infections can be encountered. Pyelonephritis is
uncommon, however proper therapy is essential
because of the potential consequences of disease. The
advantage of treating urinary tract disease is the ability
of many antimicrobials to reach high concentrations
in urine, including penicillin, cephalosporins
and trimethoprim–sulfonamides. As a result, pathogens
that are reported as resistant in vitro may be
susceptible in vivo. However, the underlying bladder
wall (i.e. biophase for infection) is protected against
xenobiotics, including drugs, by the uppermost cells
of the urothelium at the inner surface of the bladder,
known as umbrella cells. One important point to note
is that while many sulfonamides concentrate in urine,
this is not true for all. Sulfamethoxazole is largely
metabolized before urinary excretion, and is therefore
less likely to be effective in urinary tract disease. In
addition, urine pH may influence local antibacterial
activity.