TABLE 1: Antimicrobials and Dosages Used to Treat Feline AMP
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After the susceptibility of the mycobacteria causing AMP is known, clinicians are often left with multiple drug choices, as
most strains are susceptible to several classes of antimicrobials. The ideal antimicrobial agent should 1) be effective against
the infecting strain, 2) be approved for use in cats, 3) have favorable pharmacologic and pharmacokinetic parameters in the
intended species and disease for which it will be prescribed, and 4) have minimal or clinically acceptable side effects. Of
the most commonly used agents for the treatment of AMP, fluoroquinolones have several advantages. Fluoroquinolones, such as
enrofloxacin, are bactericidal, have good penetration into dermal and subcutaneous structures, and achieve adequate concentrations
within phagocytic immune cells.1,9 Their use is well-tolerated in cats within the recommended oral dose range (Table 1). Retinal degeneration has recently been described in cats receiving long-term, extralabel dosages of enrofloxacin10 and has been documented when used to treat a cat with AMP,11 so be sure to inform owners of this possible complication when long-term therapy with enrofloxacin is anticipated. Another
concern commonly voiced in human medicine about fluoroquinolone usage is the possibility for the rapid development of antimicrobial
resistance,9 but currently there is insufficient clinical evidence suggesting that this is problematic in cats with AMP.
Doxycycline, a bacteriostatic tetracycline, has also been shown to be efficacious and safe for treating AMP (Table 1) and is a more economical choice than enrofloxacin. Routine doxycycline use may also reduce the risk of the development of
resistant, mutant strains of mycobacteria.1 The potentially reduced risk is based on experience with people that indicates that doxycycline has low mutational resistance.1 It is also based on doxycycline's high lipid solubility as well as on other pharmacokinetic data on area-under-the-curve
concentration. Vomiting and inappetence are potential dose-limiting adverse effects of doxycycline. Increasing a cat's doxycycline
dosage is best achieved by a gradual escalation over two or three weeks.1 Doxycycline has been reported to cause esophageal strictures in cats, and oral therapy should be immediately stopped in
any animal that develops signs of regurgitation or dysphagia.12 Doxycycline should be given with food or followed immediately by oral fluid.
Although studies in cats are limited, based on previous analyses of the general susceptibility profiles of multiple mycobacterial
isolates1,3 and the previous discussion, administration of doxycycline, ciprofloxacin, or enrofloxacin alone or in combination has been
recommended for AMP cases in which individual susceptibility data cannot be obtained. At this time, if susceptibility data
are available, routine use of multiple concurrent antimicrobials to initially treat AMP offers no important advantages over
monotherapy.1
Some agents that should not be considered for empirical use, as they can be associated with variable susceptibility profiles
or potentially serious toxicities, do deserve mention as potential agents for treating AMP if bacterial culture and antimicrobial
sensitivity results indicate that the mycobacterial isolate in a specific case is sensitive to these drugs. Most strains of
M. smegmatis have reportedly been susceptible to potentiated sulfonamides, such as trimethoprim-sulfamethoxazole (Table 1), although one study reported that multiple isolates of M. fortuitum were uniformly resistant to trimethoprim.1 Some of the newer macrolide derivatives, including clarithromycin and azithromycin, have been useful for treating atypical
mycobacterial infections in people and cats (Table 1).1,7 Although these macrolides have broad antimicrobial spectrums and prolonged pharmacologic effects, their susceptibility profiles
are often unpredictable.1 Many mycobacterial species from cats with AMP will also be susceptible to aminoglycosides such as gentamicin (Table 1). However, routine aminoglycoside use can be hindered by poor penetration into adipose tissues, potential nephrotoxicity,
and a requirement for parenteral administration.1,7
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