Simple SummaryAvian orthoreovirusescause significant losses to the poultry industry worldwide. Reovirus-associated diseases in chickens are frequently reported even in vaccinated flocks due to the emergence and spread of novel strains. Herein we studied the genetic features of reovirus strains originating from parts of East-Central Europe. The isolated strains from the study area displayed large genetic variability and shared low genetic similarity with vaccine strains. These findings highlight the importance of immune pressure from vaccination in the epizootiology of reovirus infection in chickens.AbstractThe concern that the vaccines currently used againstAvian orthoreovirus(ARV) infections are less efficient in the field justifies the need for the close monitoring of circulating ARV strains. In this study, we collected necropsy samples from various chicken breeds and tested for ARV by virus isolation, RT-PCR assay and sequence analysis. ARVs were isolated from birds showing runting-stunting syndrome, uneven growth, lameness or increased mortality, with relative detection rates of 38%, 35%, 6% and 25%, respectively. Partial σC gene sequences were determined for nearly 90% of ARV isolates. The isolates could be classified into one of the major genetic clusters. Interestingly, cluster 2 and cluster 5 were isolated from vaccinated broiler breeders, while clusters 1 to 4 were isolated from unvaccinated broilers. The isolates shared less than 75% amino acid identities with the vaccine strains (range, 44.3–74.6%). This study reaffirms the global distribution of the major genetic clusters of ARVs in chicken. The diversity of ARV strains isolated from unvaccinated broilers was greater than those detected from vaccinated animals, however, the relative importance of passive and active immunity on the selection of novel strains in different chicken breeds needs to be better understood.