Changing Patterns of Antimicrobial Sensitivity Patterns in Equine Isolates Submitted to a UK Laboratory: 1999–2012





Antimicrobial resistance (AMR) is considered one of the key developing health concerns globally. Knowledge of AMR and changes in resistance patterns over time may allow for more appropriate empirical antimicrobial use and may help prevent the development of multidrug resistance (MDR).


Records from The Royal Veterinary College Diagnostic Laboratory between 1999 and 2012 were searched for equine samples from which a either Escherichia coli or Streptococcus spp. was identified. Sensitivity to enrofloxacin, ceftiofur, gentamicin, penicillin G, trimethoprim sulfamethoxazole (TMPS) and tetracyclines was noted. Isolates were divided into those identified between 1999–2004 (‘Early’) and 2007–2012 (‘Late’). The proportion of isolates resistant to each antimicrobial, and MDR isolates (resistant to 3 or more antimicrobial classes) was compared between time periods.


A total of 530 isolates from 483 horses were identified. 297 isolates (262 horses) were identified in the ‘Early’ period, and 233 isolates from 221 horses in the ‘Late’ period. There was a significant increase between time periods in the percentage of E. coli isolates resistant to ceftiofur (6.7% [9/135] to 22.7% [20/88] P<0.001); to gentamicin (29.6% [40/135] to 51.7% [46/89] P<0.001); and to tetracyclines (46.6% [34/73] to 73% [65/89] P<0.001). There was a significant increase between time periods in the percentage of Streptococcus spp. resistant to enrofloxacin (5.6% [7/125] to 48.1% [65/135]) and tetracyclines (4.1% [4/98] to 35.7% [41/115]). Multiple drug resistance was not identified in Streptococcus spp. There was no significant increase in the percentage of E. coli showing MDR (33% [24/73] early; to 47% [42/89] P = 1.0).

Conclusion and practical significance

An increase in resistance over time of common equine pathogens to a number of widely used antimicrobials supports the responsible use of antimicrobials in an attempt to minimise AMR.

Ethical animal research

Not required by this Congress: retrospective analysis of laboratory records. Sources of funding: None. Competing interests: None.