Antimicrobial resistance in swine respiratory bacterial pathogens in the USA

Antimicrobial resistance in swine respiratory bacterial pathogens in the USA

On this Monday morning, we would like to share with you this article published in Research in Veterinary Science by PhD-candidate Shivdeep Hayer advised by Dr. Julio Alvarez. The peer-reviewed publication is a summary of antimicrobial resistances (AMR) in swine respiratory isolates between 2006 and 2016.


  • AMR data for bacterial swine pathogens associated with Swine Respiratory Disease Complex are lacking
  • AMR data on swine bacterial pathogens collected over 11 years in the U.S.A was analyzed for changes in AMR prevalence
  • AMR in S. suis and P. multocida isolates mostly remained low
  • There were statistically significant changes in AMR in A. suis and H. parasuis
  • Use of surrogate breakpoints can lead to different AMR estimates for certain bacteria



Almost 20,000 isolates were included in the study. 45% came from Minnesota farms and another 45% from the top 10 swine producing states in the country. They divided as follows:

  • 42.3 % Pasteurella multocida
  • 34.6% Streptococcus suis
  • 14.7% Actinobacillus suis
  • 8.3% Haemophilus (now Glaesserella) parasuis

They were tested for antimicrobial resistance against 13 antimicrobials: ampicillin, ceftiofur, chlortetracycline, enrofloxacin, florfenicol, gentamicin, oxytetracycline, penicillin, spectinomycin, sulphadimethoxine, tiamulin, trimethoprim-sulphamethoxazole and tulathromycin.

For more information regarding the cut-offs used to qualify isolates as sensitive, intermediate, or resistant, please refer back to the full manuscript on the journal’s website.


Actinobacillus suis

No breakpoints were available for any of the antimicrobials for A. suis. However, no statistically significant change in the odds of resistance to enrofloxacin, trimethoprim-sulphamethoxazole and gentamicin was noted. There was a significant annual decrease of 15–29% in the odds of being resistant to ceftiofur, spectinomycin, sulphadimethoxine and tiamulin.

Glaesserella parasuis

No breakpoints available for any of the antimicrobials for H. parasuis. There was no change in the odds of resistance to chlortetracycline, tiamulin, trimethoprim-sulphamethoxazole and spectinomycin. However, there was a consistent annual increase of 9–20% in the odds of ampicillin, florfenicol, gentamicin and penicillin resistance. This annual increase in odds of resistance was even higher for ceftiofur, enrofloxacin and tulathromycin, with the change in odds ranging between 23 and 34%.

Pasteurella multocida

Between 0 and 8% of isolates were resistant against eight of the studied antimicrobials (ampicillin, ceftiofur, enrofloxacin, florfenicol, gentamicin, penicillin, spectinomycin and tulathromycin). There were no change in prevalence of resistance to enrofloxacin, florfenicol, gentamicin and tulathromycin and a 4–19% decrease in annual odds of resistance to ampicillin, ceftiofur, penicillin and spectinomycin. Resistance against tetracyclines varied widely and ranged between 28 and 56% (chlortetracycline) and 52–72% (oxytetracycline) during the study period.

Streptococcus suis

The overall prevalence of resistance to ampicillin, ceftiofur, enrofloxacin, florfenicol and trimethoprim-sulphamethoxazole was low (<3%). The overall prevalence of penicillin and ceftiofur resistance showed a 9% and 19% annual increase in odds, respectively. Prevalence of florfenicol resistance decreased from 1.42% in 2006 to 0.51% in 2016. Levels of oxytetracycline and chlortetracycline resistance were always high and ranged between 93 and 97% over the years without any statistically significant change.

To read the results in more details, refer to the full manuscript here.


Swine respiratory disease complex (SRDC) causes massive economic losses to the swine industry and is a major animal welfare concern. Antimicrobials are mainstay in treatment and control of SRDC. However, there is a lack of data on the prevalence and trends in resistance to antimicrobials in bacterial pathogens associated with SRDC. The objective of this study was to estimate the prevalence and changes in resistance to 13 antimicrobials in swine bacterial pathogens (Streptococcus suisPasteurella multocidaActinobacillus suis and Haemophilus parasuis) in the U.S.A using data collected at University of Minnesota Veterinary Diagnostic Laboratory between 2006 and 2016. For antimicrobials for which breakpoints were available, prevalence of resistance remained below 10% except for tetracycline in S. suis and P. multocida isolates, and these prevalence estimates remained consistently low over the years despite statistical significance (p < .05) in trend analysis. For antimicrobial-bacterial combinations without available breakpoints, the odds of isolates being resistant increased by >10% annually for 7 and 1 antimicrobials in H. parasuis and S. suis isolates respectively, and decreased >10% annually for 4 and 1 antimicrobials in A. suis and H. parasuis isolates, respectively, according to the ordinal regression models. Clinical implications of changes in AMR for A. suis and H. parasuis should be interpreted cautiously due to the lack of interpretive criteria and challenges in antimicrobial susceptibility tests in the case of H. parasuis. Future studies should focus on surveillance of antimicrobial resistance and establishment of standardized susceptibility testing methodologies and interpretive criteria for these animal pathogens of critical importance.