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.
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.
Giguère
Steeve
S
Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, Athens, GA 30602, USA. gigueres@uga.edu
Lee
Elise
E
Williams
Elliott
E
Cohen
Noah D
ND
Chaffin
M Keith
MK
Halbert
Natalie
N
Martens
Ronald J
RJ
Franklin
Robert P
RP
Clark
Carol C
CC
Slovis
Nathan M
NM
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
D000900:Anti-Bacterial Agents; D018942:Macrolides; D012293:Rifampin