Minimum Inhibitory and Minimum Bactericidal Concentrations of Gamithromycin against European swine respiratory bacterial field isolates

Respiratory infections remain a significant problem for the swine industry and result in substantial economic loss. Swine Respiratory Disease (SRD) is a complex condition involving viral agents and bacterial agents such as Actinobacillus pleuropneumoniae (App), Haemophilus parasuis (Hp), Bordetella bronchiseptica (Bb) and Pasteurella multocida (Pm).

The in vitro activity of gamithromycin, macrolide sub-class, 7a-azalide antimicrobial, was evaluated against common field strains isolated from outbreaks of clinical disease across Europe.

A total of 358 isolates (100 Pm, 100 App, 91 Bb and 67 Hp) were sourced from the VETPATH III European collection coordinated by the European Animal Health Study Centre (Centre Européen d’Etudes pour la Santé Animale; CEESA). The strains were isolated between 2010 and 2012 in eight European Countries (Belgium, Denmark, France, Germany, The Netherlands, Poland, Spain and the UK) and exclusively originated from clinically sick pigs aged from 3 weeks to 6 months and showing depression, hyperthermia (> 39.8°C), with one or more of these respiratory signs: polypnea, dyspnea, cough and/or sneezing. The animals had not been exposed to any form of antimicrobial treatment for at least 15 days prior to sample collection. The isolates were epidemiologically unrelated (one isolate per outbreak, per farm without resampling within a six month period).

Minimum Inhibitory concentration (MIC) and Minimum Bactericidal Concentrations (MBC) were generated using harmonized and internationally recognized broth microdilution methods published by the Clinical & Laboratory Standards Institute (CLSI) in guidelines M100-S23 (2013), VET01-A4 (2013) and M26A (1999) . MIC values were determined for the whole collection and MBC values were assessed for half of the isolates (50 Pm, 50 App, 46 Bb, 67 Hp) selected at random within each MIC category.μ

The MIC ranges were 0.25-2 μg/mL for Pm, 2-16 μg/mL for App, 1-4 μg/mL for Bb and 0.06-4 μg/mL for Hp. MIC50 and MIC90 of the isolates occurred at 0.5/1 μg/mL, 4/4 μg/mL, 1/2 μg/mL and 0.25/0.5 μg/mL respectively. MBC killing of 50% and 90% of the isolates occurred at 1/2 μg/mL for Pm, 2/4 μg/mL for App, 2/4 μg/mL for Bb and 0.5/0.5 μg/mL for Hp. For the majority of the isolates (167/181), MBC ≤ 2 x MIC. Only 14 isolates out of 181 showed MBC > 2 x MIC

MIC and MBC for gamithromycin were determined for recent swine bacterial respiratory pathogens representative of distinct areas of the EU. This study revealed a very high susceptibility of all the tested strains (App, Hp, Bb and Pm) to gamithromycin and evidenced a clear bacteriostatic and predominantly bactericidal effect on these SRD pathogens.

A. Richard-Mazet Conflict with: MERIAL S.A.S., J. Hayward: None Declared, E. Siegwart: None Declared, A. Pfefferkorn Conflict with: MERIAL S.A.S., P. Dumont Conflict with: MERIAL S.A.S.