6 Resistance in zoonotic bacteria

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Report Highlights 2024:

Resistance in zoonotic bacteria

In 2024, resistance in Campylobacter jejuni isolates from humans remained lower in domestically acquired cases (ciprofloxacin 44%, tetracycline 22%) than in travel-associated cases (ciprofloxacin 85%, tetracycline 62%), but higher than in isolates recovered from broilers (ciprofloxacin 29%, tetracycline 17%) and cattle (ciprofloxacin 26%, tetracycline 1%), respectively. This is similar to previous years, albeit with some fluctuations. The proportion of fully susceptible C. jejuni has remained stable over time (69% in broilers, 73% in cattle and 46% in human cases for 2024). Erythromycin resistance remained rare in C. jejuni from cattle and was not observed in isolates from broilers and humans.

Campylobacter coli showed higher levels of resistance in human and broiler isolates compared to C. jejuni, with particularly high levels of resistance to ciprofloxacin (83% in travel-related cases, 66% in domestic cases and 54% in broilers) and tetracycline (75% in travel-related human cases, 72% in domestic human cases and 58% in broilers). Resistance to erythromycin was observed in 25% of the travel-related and 7% of the domestic human isolates and in 17% of isolates from broilers. This is an important finding as macrolides are used for the treatment of human patients in severe cases.

Phenotypic resistance in Salmonella Typhimurium and monophasic S. Typhimurium isolates remained overall higher in domestic pork than in humans. In addition to results from phenotypic resistance testing, DANMAP 2024 presents results from whole-genome sequencing of all Salmonella isolated between 2020 and 2024.

In 2024, two large human outbreaks caused by S. Typhimurium and monophasic S. Typhimurium affected the overall levels of phenotypic resistance in human isolates, the monophasic outbreak being caused by an atypical fully susceptible clone. In all 823 human isolates tested, resistance to third-generation cephalosporins remained low, and no resistance to meropenem was detected. Six human isolates (1%) were simultaneously resistant to azithromycin, third-generation cephalosporins and ciprofloxacin. Genetic determinants conferring resistance to quinolones, azithromycin, gentamicin, and third-generation cephalo-sporins were detected sporadically from 2020 to 2024. The yearly occurrence of commonly found resistance determinants, blaCARB-2, blaTEM-1, floR, sul1, sul2, tet(A), tet(B), tet(G), was influenced by the outbreak-associated clones.

Among Salmonella isolates from pork, the level of full susceptibility in S. Typhimurium and monophasic S. Typhimurium increased compared to 2023. Phenotypic ampicillin-, sulfamethoxazole-, and tetracycline resistance (ASuT) remained common in monophasic S. Typhimurium isolates and the most common genetic profile conferring ASuT resistance was reported. Phenotypic fluoroquinolone (ciprofloxacin) resistance remained undetected. Unlike in 2023, phenotypic azithromycin resistance was detected in two monophasic S. Typhimurium isolates from pork. The occurrence of phenotypic resistance to gentamicin remained similar to that in 2022 and 2023. Genes conferring resistance to amikacin and gentamicin were detected in monophasic S. Typhimurium, and the aac(3)-IV gene, conferring resistance to gentamicin and apramycin, was found in three S. Typhimurium isolates.