Determination of the prevalence of antimicrobial resistance to macrolide antimicrobials or rifampin in Rhodococcus equi isolates and treatment outcome in foals infected with antimicrobial-resistant isolates of R equi

J Am Vet Med Assoc. 2010 Jul 1;237(1):74-81. doi: 10.2460/javma.237.1.74.

Abstract

Objective: To determine the prevalence of antimicrobial resistance to macrolide antimicrobials or rifampin in Rhodococcus equi isolates and to describe treatment outcome in foals infected with antimicrobial-resistant isolates of R equi.

Design: Cross-sectional study.

Sample population: 38 isolates classified as resistant to macrolide antimicrobials or rifampin received from 9 veterinary diagnostic laboratories between January 1997 and December 2008.

Procedures: For each isolate, the minimum inhibitory concentration of macrolide antimicrobials (ie, azithromycin, erythromycin, and clarithromycin) and rifampin was determined by use of a concentration-gradient test. Prevalence of R equi isolates from Florida and Texas resistant to macrolide antimicrobials or rifampin was determined. Outcome of antimicrobial treatment in foals infected with antimicrobial-resistant isolates of R equi was determined.

Results: Only 24 of 38 (63.2%) isolates were resistant to >or= 1 antimicrobial. Two isolates were resistant only to rifampin, whereas 22 isolates were resistant to azithromycin, erythromycin, clarithromycin, and rifampin. The overall prevalence of antimicrobial-resistant isolates in submissions received from Florida and Texas was 3.7% (12/328). The survival proportion of foals infected with resistant R equi isolates (2/8 [25.0%]) was significantly less, compared with the survival proportion in foals that received the same antimicrobial treatment from which antimicrobial-susceptible isolates were cultured (55/79 [69.6%]). Odds of nonsurvival for foals infected with resistant R equi isolates were 6.9 (95% confidence interval, 1.3 to 37) times the odds for foals infected with susceptible isolates.

Conclusions and clinical relevance: Interpretation of the results emphasized the importance of microbiological culture and antimicrobial susceptibility testing in foals with pneumonia caused by R equi.

MeSH terms

  • Actinomycetales Infections / drug therapy
  • Actinomycetales Infections / microbiology
  • Actinomycetales Infections / veterinary*
  • Animals
  • Anti-Bacterial Agents / pharmacology*
  • Anti-Bacterial Agents / therapeutic use*
  • Drug Resistance, Bacterial
  • Horse Diseases / drug therapy
  • Horse Diseases / microbiology*
  • Horses
  • Macrolides / pharmacology
  • Microbial Sensitivity Tests
  • Rhodococcus equi / drug effects*
  • Rhodococcus equi / pathogenicity
  • Rifampin / pharmacology
  • Treatment Outcome
  • Virulence

Substances

  • Anti-Bacterial Agents
  • Macrolides
  • Rifampin