Antimicrobial susceptibility of 150 isolates of Brachyspira hyodysenteriae recovered from Spanish swine dysentery outbreaks during 2011-2015.

The antibiotic resistance of field strains of Brachyspira hyodysenteriae has increased in the last years in Spain as well as in other countries due to the high use of antibiotics for the control of swine dysentery (SD). For this reason, antimicrobial susceptibility tests are becoming essential for an effective control of this disease.

Our objective was to study the antibiotic susceptibility of Spanish field strains of Brachyspira hyodysenteriae in order to provide swine production companies updated data on the most appropriate antibiotic treatment.

Bacterial strains: 150 field strains of B. hyodysenteriae isolated in Spain during 2011 to 2015 were investigated.

Antimicrobial susceptibility test: Each strain was suspended in a commercial plate (VetMIC™ Brachy v2) following the manufacturer’s instructions. The plates have six rows, each containing different concentrations of one of the following antibiotics: tiamulin, valnemulin, doxycycline, tylvalosin, tylosin and lincomycin.

The minimum inhibitory concentration (MIC) was determined as the lowest concentration of the antibiotic that prevented visible growth.

The lowest concentrations that completely inhibited the growth of 50% and 90% of the isolates, MIC₅₀ and MIC₉₀, respectively, were calculated for each antibiotic.

During the studied period, the value of the MIC₅₀ has increased from 1 to 2 μg/ml for tiamulin, from 1 to 4 μg/ml for valnemulin and from 4 to 16 μg/ml for tylvalosin. Regarding to doxycycline and lincomycin, this value has ranged between 0,5-1 μg/ml in the case of doxycycline and between 16-32 μg/ml for the lincomycin. Finally, the value has been > 128 μg/ml for the tylosin during the five years.

The values of the MIC₉₀ have remained stable in all the antibiotics except tylvalosin, which has increased from 8 to 32 µg/ml.

Along the past five years, the MIC₅₀ values for three of the six most relevant antimicrobials used to control SD have increased in the Spanish isolates of Brachyspira hyodysenteriae. These results imply that this bacteria is becoming resistant to most commonly available antibiotics used to deal with SD. That fact produces a serious health problem in pigs, increasing the risk of infections among animals, and negative influences on swine production.

Careful use of antibiotics and the use of other alternatives such as autovaccines or natural feed additives will minimise the development of more antibiotics resistant strains and provide a possible solution to the global problem of antibiotic resistance.

Disclosure of Interest: None Declared.