Antimicrobial Resistance in Swiss Laying Hens: Prevalence and Risk Factors

For E. coli, 30 per cent of the isolates from hens on 99 egg farms were susceptible to at least two of the antimicrobials tested, which compared with around 40 per cent for two Enterococcus spp., according to researchers based in Bern. There were no consistent patterns between housing or management and antimicrobial resistance.
calendar icon 17 September 2011
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Antimicrobial resistance is an emerging concern to public health, and food-producing animals are known to be a potential source for transmission of resistant bacteria to humans. In a paper published this month in the journal, Zoonosis and Public Health, M. Harisberger of the University of Bern in Switzerland and co-authors there and at the University of Zurich and University of Ghent in Belgium write that, as legislation of the European Union requires to ban conventional cages for the housing of laying hens on the one hand, and a high food safety standard for eggs on the other hand, further investigations about the occurrence of antimicrobial resistance in alternative housing types are required.

In their study, they determined antimicrobial resistance in indicator bacteria from 396 cloacal swabs from 99 Swiss laying hen farms using four alternative housing types during a cross-sectional study. On each farm, four hens were sampled and exposure to potential risk factors was identified with a questionnaire.

The minimal inhibitory concentration was determined using broth microdilution in Escherichia coli (n=371) for 18 antimicrobials and in Enterococcus faecalis (n=138) and Enterococcus faecium (n=153) for 16 antimicrobials. All antimicrobial classes recommended by the European Food Safety Authority for E. coli and enterococci were included in the resistance profile.

Sixty per cent of the E. coli isolates were susceptible to all of the considered antimicrobials and 30 per cent were resistant to at least two antimicrobials. In E. faecalis, 33 per cent of the strains were susceptible to all tested antimicrobials and 40 per cent were resistant to two or more antimicrobials, whereas in E. faecium, these figures were 14 per cent and 39 per cent, respectively.

Risk factor analyses were carried out for bacteria species and antimicrobials with a prevalence of resistance between 15 per cent and 85 per cent. In these analyses, none of the considered housing and management factors showed a consistent association with the prevalence of resistance for more than two combinations of bacteria and antimicrobial.

The researchers concluded that the housing and management practices that they considered on the egg farms had a low impact on antimicrobial resistance in the laying hens.


Harisberger M., S. Gobeli, R. Hoop, J. Dewulf, V. Perreten and G. Regula. 2011. Antimicrobial resistance in Swiss laying hens, prevalence and risk factors. Zoonoses and Public Health, 58 (): 377–387. DOI: 10.1111/j.1863-2378.2010.01376.x

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September 2011
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