A new era of coccidiosis control: Eimeria and Clostridium perfringens dynamics in vaccinated broiler flocks after the ionophore phase-out in Norway.

Norway discontinued the prophylactic use of coccidiostats in broiler production in 2016, and all broilers are now raised without the routine use of coccidiostats. As an alternative strategy to control coccidiosis, all broiler flocks in Norway are now routinely vaccinated at hatch with vaccines based on live attenuated Eimeria oocysts. In this study, we investigated the population dynamics and interactions of Eimeria spp. and Clostridium perfringens in Norwegian broiler flocks vaccinated against coccidiosis in field settings. We also explored how this may be influenced by farm management practices. Fresh pooled fecal samples were collected as part of a longitudinal time-series and through a case-control design from commercial broiler flocks diagnosed with coccidiosis, necrotic enteritis (NE) or both. Samples were analyzed using conventional and molecular methods targeting the relevant pathogens. Management data were analyzed using multivariable logistic regression models to identify factors associated with coccidiosis and NE. The occurrence of Eimeria oocysts showed varying dynamics over time across individual flocks and companies, and Eimeria species composition largely reflected the content of the vaccine used. Higher numbers of E. maxima and lower overall management scores were identified as risk factors associated with NE. Coccidiosis appeared to be well controlled under the current production practices, whereas NE seems to increase after the abolishment of coccidiostats. This may be related to the indirect effects of coccidiostats on Clostridium spp. and is a consequence that should be carefully monitored in any circumstance where there is a move towards the use of anticoccidial vaccines in broiler production.