Short communication Time-patterns of antibiotic exposure in poultry production—A Markov chains exploratory study of nature and consequences C. Chauvin a, * , C. Clement b , M. Bruneau c , D. Pommeret b a AFSSA, French Food Safety Agency, Pig and Poultry Veterinary Research Laboratory, Zoopo ˆle, BP 53, 22440 Ploufragan, France b ENSAI, National School in Statistical Analysis, Campus de Ker Lann, BP 37203, 35172 Bruz Cedex, France c AFSSA, French Food Safety Agency, Veterinary Medicine and Disinfectant Research Laboratory, La Haute Marche, Javene ´, 35302 Fouge `res, France Received 22 September 2006; received in revised form 21 February 2007; accepted 23 February 2007 Abstract This article describes the use of Markov chains to explore the time-patterns of antimicrobial exposure in broiler poultry. The transition in antimicrobial exposure status (exposed/not exposed to an antimicrobial, with a distinction between exposures to the different antimicrobial classes) in extensive data collected in broiler chicken flocks from November 2003 onwards, was investigated. All Markov chains were first-order chains. Mortality rate, geographical location and slaughter semester were sources of heterogeneity between transition matrices. Transitions towards a ‘no antimicrobial’ exposure state were highly predominant, whatever the initial state. From a ‘no antimicrobial’ exposure state, the transition to beta-lactams was predominant among transitions to an antimicrobial exposure state. Transitions between antimicrobial classes were rare and variable. Switches between antimicrobial classes and repeats of a particular class were both observed. Application of Markov chains analysis to the database of the nation-wide antimicrobial resistance monitoring programme pointed out that transition probabilities between antimicrobial exposure states increased with the number of resistances in Escherichia coli strains. # 2007 Elsevier B.V. All rights reserved. Keywords: Antibiotic; Markov models; Poultry; Treatment patterns; Resistance www.elsevier.com/locate/prevetmed Preventive Veterinary Medicine 80 (2007) 230–240 * Corresponding author. Tel.: +33 296016266; fax: +33 296016253. E-mail address: c.chauvin@afssa.fr (C. Chauvin). 0167-5877/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2007.02.010