Antimicrobial susceptibility of bacterial isolates from ambulatory practice and from a referral hospital

An updated description of bacterial pneumonia in adult horses and factors associated with death

BACKGROUND

Available descriptive studies on equine pneumonia are outdated or focus on specific horse or bacterial populations.

OBJECTIVES

To describe the clinical presentation and bacterial isolates of adult horses with bacterial pneumonia and identify factors associated with death.

ANIMALS

One hundred sixteen horses >2 years old with bacterial pneumonia.

METHODS

Retrospective case series. Data regarding history, physical examination, clinicopathologic features, treatment, bacterial culture and sensitivity, and outcome were collected and analyzed retrospectively.

RESULTS

Historical risk factors were present for 60% of cases, whereas abnormal vital signs on intake were present for <50%. Most horses (58%) underwent at least 1 change of antimicrobial treatment, and 67% received the highest-priority critically important antimicrobials. Streptococcus zooepidemicus was the most isolated bacteria (44%), followed by Escherichia coli (19%), Klebsiella spp. (18%), other Streptococcus species (17%), and Bacillus spp. (13%). Fusobacterium spp. were the most common anaerobic isolates (11%). Antimicrobial susceptibility varied widely. Survival to discharge was 73%. Heart rate at presentation (odds ratio [OR] 1.08, 95% confidence interval [CI] 1.008-1.17, P = .03) and higher creatinine (OR 14.1, 95% CI 1.56-127.6, P = .02) increased the risk of death. Higher lymphocyte count (OR 0.27, 95% CI 0.08-0.94, P = .04) reduced risk.

CONCLUSIONS AND CLINICAL IMPORTANCE

Contrasting older literature, Fusobacterium spp. were the most common anaerobes. Streptococcus zooepidemicus remained the most common isolate and was predictably susceptible to penicillin. Antimicrobial susceptibility was otherwise variable and broad applicability is limited as this was a single-center study. Increased risk of death associated with tachycardia and abnormally high serum creatinine concentration is consistent with previous studies.

Antimicrobial susceptibility of bacterial isolates from ambulatory practice and from a referral hospital

BACKGROUND

Responsible use of antimicrobials in equine practice relies on knowledge of common bacterial isolates and their antimicrobial sensitivities.

OBJECTIVES

To assess the frequency of bacterial resistance to a combination of parenteral penicillin and gentamicin and to trimethoprim and sulfamethoxazole for PO use in a selection of clinical isolates, and subsequently to determine the prevalence of resistance to antimicrobials that might then be used as alternatives to first-line antimicrobials for the same isolates.

METHODS

Retrospective analysis of minimal inhibitory concentrations (MICs) of antimicrobials for 6354 bacterial isolates from 365 ambulatory practices and 519 isolates from a referral hospital. The MICs were used to indicate sensitivity or resistance to commonly used antimicrobials and the prevalences of resistance were compared between origin of the isolates, and among antimicrobial drugs.

RESULTS

Isolates from the referral hospital were significantly (P < .05) more likely to be resistant to the antimicrobials tested than those derived from ambulatory practice. Overall, 91% of the ambulatory isolates and 64% of the hospital isolates were sensitive to penicillin-gentamicin. For trimethoprim-sulfamethoxazole combination, 82% of the ambulatory practice isolates and 56% of the referral hospital isolates were sensitive.

CONCLUSIONS AND CLINICAL IMPORTANCE

Most isolates were sensitive to penicillin and gentamicin as well as trimethoprim-sulfamethoxazole. No predictable efficacious second choice antimicrobial was identified for those isolates resistant to the first-line antimicrobials. The likelihood of isolates being sensitive to second choice antimicrobials was variable but generally higher for ambulatory isolates compared to referral isolates. Bacterial identification and measurement of MIC are essential to make the appropriate antimicrobial choice.