An evaluation of serum gentamicin concentrations and bacterial susceptibility to gentamicin in equine practice.

BACKGROUND: Therapeutic drug monitoring and minimum inhibitory concentration (MIC) data allow more informed use of gentamicin. HYPOTHESIS/OBJECTIVES: To measure peak and trough serum gentamicin concentrations in horses after a 6.6 mg/kg dose of gentamicin given IV and the MIC of gentamicin of bacteria for which gentamicin might be selected. METHODS: Retrospective analysis of hospital records. Peak and trough plasma gentamicin concentrations were measured after 6.6 mg/kg gentamicin IV in 339 hospitalized horses. The MIC of gentamicin was measured for 503 isolates from ambulatory practice and 33 from hospital practice. The distribution of gentamicin concentrations and MIC results were compared to current recommendations for MIC breakpoints. RESULTS: The median serum gentamicin concentration at 60 minutes after administration (C60min ) was 21.4 µg/mL with a distribution indicating that bacteria with MIC ≥2 µg/mL were unlikely to be exposed to sufficient gentamicin for effective killing. Approximately 90% of isolates from ambulatory practice and 36% of hospital isolates had MICs at or below breakpoints for susceptibility with most of the remainder unlikely to be responsive, even to higher IV doses. CONCLUSIONS AND CLINICAL IMPORTANCE: Gentamicin at a dosage of 6.6 mg/kg IV is likely to be effective against the majority of infections encountered in ambulatory practice, but less effective in an equine hospital. Because there was a dichotomy of most bacteria as being clearly susceptible or clearly resistant to gentamicin, it appears unlikely that higher doses would have been more efficacious, especially in the hospitalized population in our study.

The role of nutrition in colic.

Nutritional intolerances manifesting as colic in the horse may be largely explained by divergence from the diet and ingestive behaviors to which the feral ancestors of modern domesticated equids had become accustomed and adapted. High-starch diets and abrupt dietary changes are probably foremost in the risk factors for diet-associated colic in the horse and have their basis in disruption of the stability of microbial populations resident within the equine hindgut. Although some general associations between colic and diet may be inferred from several epidemiologic studies, data derived from studies of single and specific disease processes associated with colic allow more effective practical application of corrective dietary management strategies in situations where colic risk is judged to be increased.