Gentamicin

Gentamicin In Dogs and Cats: Uses, Dosage and Side Effects

Overview

• Aminoglycosides irreversibly bind to the 30S ribosome subunit inhibiting bacterial protein synthesis.
• They require an oxygen-rich environment to be effective, thus they are ineffective in
  low-oxygen sites (abscesses, exudates), making all obligate anaerobic bacteria resistant.
• Their mechanism of killing is concentration-dependent, leading to a marked post-antibiotic effect, allowing pulse-dosing regimens which may limit toxicity.

Uses of Gentamicin

• Active against Gram-negative bacteria, but some staphylococcal and streptococcal (Streptococcus faecalis) species are also sensitive.
• All obligate anaerobic bacteria and many haemolytic streptococci are resistant.
• Use in domestic animals is limited by nephrotoxicity and, more rarely, ototoxicity and neuromuscular blockade.
• Cats are more sensitive to toxic effects.
• When used for empiric therapy of serious infections, gentamicin is usually given in conjunction with penicillin and/or metronidazole to provide broad-spectrum cover.
• Aminoglycosides are more active in an alkaline environment.
• Geriatric animals or those with reduced renal function should only be given this drug systemically when absolutely necessary, although dosing q24h should reduce the likelihood of nephrotoxicity.
• Sepsis, dehydration, hypokalaemia, prolonged treatment and fever all increase the risk of nephrotoxicity.
• Therapeutic drug monitoring should be considered if possible and is highly
  recommended if nephrotoxicity risk factors are present.
• A peak level (30–60 minutes post i.v. dose) should be >20 μg (micrograms)/ml and
  a trough level should be <1 μg/ml.

Dose of Gentamicin in Dogs and Cats

Dogs:

• Otic: 4–8 drops (depending on weight of the animal) in affected ear or apply ointment to the affected area q12h.
• Easotic preparation: apply 1 ml to each ear q24h using a metered or single dose delivery system.
• Ophthalmic: 1 drop per eye q6–8h. 1 cm gel per eye q8–12h.
• Systemic: 5–10 mg/kg slowly i.v. (over 30 min), i.m., s.c. q24h.

Cats:

Ophthalmic: Dose as for dogs.
Systemic: 5–8 mg/kg slowly i.v. (over 30 min), i.m., s.c. q24h.

Side Effects of Gentamicin in Dogs and Cats

• Gentamicin delays the epithelial healing of corneal ulcers and may cause local irritation.
• Nephrotoxicity and cytotoxicity (auditory and vestibular) are potential side effects.
• Cellular casts in urine sediment are an early sign of impending nephrotoxicity;
  however, urine must be examined immediately to detect their presence, and their absence is not a guarantee of safety.
• Serum creatinine levels rise later and fatal acute renal failure may be inevitable
  when they do.
• Gentamicin should not be used during pregnancy.

Contraindications of Gentamicin in Dogs and Cats

• Do not use the aural preparation if the tympanum is perforated.
• Do not use it in conjunction with other drugs considered to be nephrotoxic.
• For systemic use do not exceed 7 days treatment duration.

Some Notes:

• Avoid concurrent use of other nephrotoxic, ototoxic or neurotoxic agents (e.g. amphotericin B, furosemide).
• Increase monitoring and adjust dosages when these drugs must be used together.
• Aminoglycosides may be chemically inactivated by beta-lactam antibiotics (e.g. penicillins, cephalosporins) or heparin in vitro (avoid mixing in the same syringe).
• The effect of nondepolarizing muscle relaxants (e.g. atracurium, pancuronium,
  vecuronium) may be enhanced by aminoglycosides.
• Synergism may occur when aminoglycosides are used with beta-lactam antimicrobials.
• Microbial resistance is a concern, although many bacteria resistant to gentamicin may be susceptible to amikacin.