Antimicrobial resistance among Shigella in New Zealand

Abstract

Aim: We undertook a national survey to provide current information on antimicrobial resistance among Shigella isolated in New Zealand.

Methods: Diagnostic laboratories are requested to refer all Shigella isolates to the Institute of Environmental Science and Research (ESR) for epidemiological typing as part of the national surveillance of shigellosis. The antimicrobial susceptibility of 263 non-duplicate Shigella isolates referred to ESR in 2015 and 2016 was tested.

Results: The 263 Shigella comprised 141 (53.6%) S. sonnei, 113 (43.0%) S. flexneri, 7 (2.7%) S. boydii and 2 (0.8%) S. dysenteriae. Among the 141 S. sonnei, the majority were either biotype g (90) or biotype a (50). Rates of resistance to the two currently recommended first-line antibiotics, co-trimoxazole and fluoroquinolones, were relatively high at 56.7% and 22.8%, respectively. Azithromycin is considered a second-line treatment option, but 11.0% of Shigella were categorised as having a non-wildtype (NWT) azithromycin phenotype (ie, having some mechanism of azithromycin resistance although not necessarily clinically resistant). There were several significant differences in resistance between the two most prevalent S. sonnei biotypes, with resistance being significantly more prevalent among biotype g isolates. Shigella from patients who had not travelled overseas were significantly more likely to be azithromycin NWT than isolates from patients who had recently travelled (20.7 vs 5.6%). Azithromycin NWT was more prevalent among Shigella from males than females (13.9 vs 7.7%).

Conclusions: These results suggest there is an immediate need to revise the currently recommended first-line treatment for shigellosis, especially when treatment is given on an empirical basis. Equally concerning is the fact that resistance to the second-line antibiotic for shigellosis, azithromycin, appears to be emerging in New Zealand. As diagnostic laboratories increase their use of culture-independent testing, it is recommended that they should continue to culture specimens from all shigellosis cases so that isolates are available for susceptibility testing and epidemiological typing.