title: 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.
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.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  38 isolates classified as resistant to macrolide antimicrobials or rifampin received from 9 veterinary diagnostic laboratories between January 1997 and December 2008.
  
  
  METHODS
  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
  Interpretation of the results emphasized the importance of microbiological culture and antimicrobial susceptibility testing in foals with pneumonia caused by R equi.
authors.affiliation: Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, USA. gigueres@uga.edu
authors.forename: Steeve
authors.initials: S
authors.lastname: Giguère
authors.forename: Elise
authors.initials: E
authors.lastname: Lee
authors.forename: Elliott
authors.initials: E
authors.lastname: Williams
authors.forename: Noah D
authors.initials: ND
authors.lastname: Cohen
authors.forename: M Keith
authors.initials: MK
authors.lastname: Chaffin
authors.forename: Natalie
authors.initials: N
authors.lastname: Halbert
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Martens
authors.forename: Robert P
authors.initials: RP
authors.lastname: Franklin
authors.forename: Carol C
authors.initials: CC
authors.lastname: Clark
authors.forename: Nathan M
authors.initials: NM
authors.lastname: Slovis
mesh_terms: D000193:Actinomycetales Infections / Q000188:drug therapy / Q000382:microbiology / Q000662:veterinary*; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology* / Q000627:therapeutic use*; D024881:Drug Resistance, Bacterial; D006734:Horse Diseases / Q000188:drug therapy / Q000382:microbiology*; D006736:Horses; D018942:Macrolides / Q000494:pharmacology; D008826:Microbial Sensitivity Tests; D016929:Rhodococcus equi / Q000187:drug effects* / Q000472:pathogenicity; D012293:Rifampin / Q000494:pharmacology; D016896:Treatment Outcome; D014774:Virulence
chemical_list: D000900:Anti-Bacterial Agents; D018942:Macrolides; D012293:Rifampin