Resistance in zoonotic bacteria and animal pathogens

In 2021 macrolide (erythromycin) resistance in Campylobacter jejuni was less than one percent in human isolates and resistance was not detected in cattle and broiler isolates.

Fluoroquinolone (ciprofloxacin) resistance remained common in C. jejuni isolates from human cases (52%), broilers (32%) and cattle (25%). The decrease observed in isolates from broilers in 2020 continued in 2021.

The level of azithromycin resistance in Salmonella Typhimurium was less than 1% in human isolates, while one of 39 isolates from Danish pork showed resistance. Unlike previous years, where azithromycin resistance was not detected in isolates from pigs, in 2021, three of 34 isolates of S. Typhimurium from pigs were azithromycin resistant.

Fluoroquinolones may be used for treatment of human Salmonella infections and resistance to fluoroquinolones is monitored using ciprofloxacin. Ciprofloxacin resistance was observed in 7% of S. Typhimurium isolates from domestically acquired infections and in 3 of 13 travel-associated cases. The resistance level in domestic isolates in 2021 was higher than the level reported in 2019 and 2020, where 4% and 1% resistance, were reported, respectively. Historically, ciprofloxacin resistance has predominantly been observed in isolates from travel-associated cases. Fluoroquinolone resistance has not been recorded in S. Typhimurium from Danish pigs and pork since 2010 and 2007, respectively.

Resistance to the critically important 3rd generation cephalosporins and carbapenems is rare in S. Typhimurium. In 2021 cephalosporin resistance was observed in one human isolate and no carbapenem resistance was observed. In line with the previous years, these resistances were not observed in S. Typhimurium isolates from Danish pigs and pork.

Pathogenic bacteria from pigs, including Actinobacillus pleuropneumoniae, haemolytic Escherichia coli and Streptococcus suis, displayed similar levels of resistance as in previous years.

There was a high concordance between antimicrobial susceptibility testing results and presence/absence of corresponding resistance genes/point mutations in pathogenic bacteria from pigs (100% for A. pleuropneumoniae, 96% for haemolytic E. coli and 90% for S. suis).

Resistance to the critically important 3rd generation cephalosporin cefotaxime was identified in 7% of the haemolytic E. coli isolates from pigs and was in all cases associated with presence of either the bla_CTX-M-1 gene or point mutations in the ampC promoter.

The optrA gene, which has been associated with resistance to oxazolidinones (e.g. linezolid used in human medicine) and phenicols (e.g. florfenicol used in veterinary medicine), was identified in 2% of the S. suis isolates from pigs.

A point mutation in the pmrB gene associated with resistance to colistin was identified in 14% of the haemolytic E. coli isolates from pigs, but only one of the 15 isolates with this point mutation was phenotypically resistant to colistin.