Susceptibility of Actinobacillus pleuropneumoniae isolated from swine in Italy to trimethoprim plus sulphadimethoxin

Antimicrobials continue to be an effective measure for the control of pleuropneumonia outbreaks in swine. The Italian ministry of health recommends not to systematically use new molecules as first line treatment (Manuale Biosicurezza e uso corretto e razionale degli antibiotici in zootecnia, Sezione suini, 2012). Thus, susceptibility to alternative treatment must be documented. Trimethoprim-sulphonamide (TMP+S) combination is recommended as first choice antimicrobial for pleuropneumonia therapy but a recent bacterial susceptibility study to this antimicrobial family reported an increasing antibioresistance level to TMP+S combination, particularly in historical collections (1994-2009) of Actinobacillus pleuropneumoniae (App) strains collected in Italy. This study aimed at updating the knowledge on App resistance to Trimethoprim-sulphadimethoxine (TMP+SDM) (Prazil® N Orale, Merial Italia Spa, Milano, Italy).

The study was performed using a collection of 42 App strains belonging to IZSLER, recently isolated in clinical outbreaks of pleuropneumonia in Italy during 2012-2014. The Minimal Inhibitory Concentrations (MIC) for TMP+SDM was determined using a broth dilution technique. In addition, the antimicrobial susceptibility testing was carried out by the agar disk diffusion test trimethoprim-sulphamethoxazole (SXT) (1.25/23.75 µg). The methods described above were performed following the CLSI guidelines.

Susceptibility rate to TMP+SDM was 76% (32/42) according to the MIC results. Using the disc diffusion test the strains were classified as sensitive (40.5%), intermediate (30.9%) and resistant (28.6%) to TMP+SXT. These results show high sensitivity of recent App isolates to TMP+S combination and suggested a high efficacy of TMP+S combination against App in the field.

MIC technique can be considered a gold standard for antibiotic-resistance monitoring, in particular for some antimicrobials (such as TMP+SDM) for which disc diffusion testing could underestimate the bacterial susceptibility in vitro. Our results prompt to consider TMP+SDM as valuable in the treatment of pig pleuropneumoniae.

Susceptibility to Trimethoprim-sulphadimethoxine of a collection of Actinobacillus pleuropneumoniae strains isolated in clinical outbreaks between 2012 and 2014 in Italy was 76%.

Disclosure of Interest: G. Leotti Conflict with: MERIAL Italia SpA, A. Luppi: None Declared, Y. Gherpelli: None Declared, M. Dottori: None Declared.