ORIGINAL ARTICLE Genetic diversity and description of transmission routes for Campylobacter on broiler farms by amplified-fragment length polymorphism G. Johnsen, H. Kruse and M. Hofshagen National Veterinary Institute, Oslo, Norway Introduction Campylobacteriosis is currently the most frequently reported bacterial enteric infection in humans in Norway as well as in many other developed countries (Blaser et al. 1983; Altekruse et al. 1999; Friedman et al. 2000). During the 1990s, there was a significant increase in campylobac- teriosis in Norway, with a doubling in the incidence rate from 1997 to 2001 (Nyga ˚rd and Kapperud 2005). Similar tendencies on increasing incidences have been reported from other developed countries (Friedman et al. 2000). Two case–control studies conducted in Norway, during 1989–1990 and 1999–2000, identified consumption of poultry meat purchased raw as a significant risk factor in regard to campylobacteriosis (Kapperud et al. 1992, 2003). In order to reduce the prevalence of contaminated broiler meat at retail sale, minimizing the prevalence of positive flocks at farm level is considered as the most effective means (Phillips 1995; Altekruse et al. 1999). In Norway, an action plan against Campylobacter in broilers was implemented in 2001 to reduce the consum- ers’ exposure to Campylobacter through domestically produced broiler meat. The action plan consists of three parts: (1) a surveillance programme including on-farm and at-slaughter sampling of all Norwegian broiler flocks slaughtered before 50 days of age, (2) a follow-up advi- sory service on farms with Campylobacter-positive flocks and (3) surveys of broiler meat products at retail level (Hofshagen and Kruse 2005). The surveillance pro- gramme has revealed that several farms repeatedly Keywords amplified-fragment length polymorphism, broiler, Campylobacter, environment, farm, poultry, transmission. Correspondence Gro Johnsen, National Veterinary Institute, P.O. Box 8156 Dep, N-0033 Oslo, Norway. E-mail: sarogde@online.no 2005/1170: received 4 October 2005, revised 24 February 2006 and accepted 28 February 2006 doi:10.1111/j.1365-2672.2006.02995.x Abstract Aims: To investigate the genetic diversity of Campylobacter in broilers and in the environment of broiler farms, to compare the genetic profiles and describe critical factors for transmission to broilers. Methods and Results: Flocks at three of four investigated farms became colon- ized with Campylobacter. The total proportion of Campylobacter-positive sam- ples at different farms varied from 20% to 42%. The farm with the poorest biosecurity routines had broilers that became infected earliest, the highest pro- portion of positive samples and the highest genetic diversity among the broiler Campylobacter isolates. Campylobacter isolates within common amplified-frag- ment length polymorphism (AFLP) clusters (95–100%) were found to be pre- sent in outdoor environment and in broilers at adjacent farms before they were found in the broilers. A large presence of Campylobacter in the farm environ- ment was demonstrated after the broilers were infected. A high genetic diversity was found among Campylobacter present in the outdoor environment, where certain Campylobacter clusters were found for periods of up to 6 weeks. Conclusion: Confirmation by AFLP indicates adjacent poultry farms and out- door environment as major sources of Campylobacter infection of broilers, this being the novel achievements. Significance and Impact of the Study: The results provide more exact know- ledge on transmission of Campylobacter at farm level, helpful for developing optimal preventive strategies. Journal of Applied Microbiology ISSN 1364-5072 1130 Journal compilation ª 2006 The Society for Applied Microbiology, Journal of Applied Microbiology 101 (2006) 1130–1139 ª 2006 National Veterinary Institute