Avian Reovirus in Israel, Variants and Vaccines-A Review.

Avian reovirus (ARV) has been determined to be the etiologic agent of viral arthritis/tenosynovitis. In Israel, meat-type chickens, including broilers and breeders, are the most affected. Severe disease symptoms can appear in broiler flocks at a very young age because of early exposure and vertical transmission, causing significant welfare problems. Jewish laws define birds with inflamed, damaged, or torn gastrocnemius and digital flexor tendons as religious condemnations (non-kosher), resulting in severe economic losses for the poultry industry. Vaccination of breeders is a strategy to control the disease by reducing vertical transmission and providing maternal-derived antibodies to the progeny. This review describes Israel's ARV variants and the various vaccines developed over the years. Identification of co-circulating variants triggered the development of multivalent autogenous inactivated vaccines. However, the genotype-matched vaccines failed to provide protection, resulting in an increased prevalence of Cluster II ARV (classified as genotyping cluster 5 in the ARV common world classification). Since 2014, ARV Cluster II has been dominant in Israel. In 2015, the dominant variant s7585 tropism changed the virus pathogenesis and affected broilers with severe clinical signs between 12 and 15 days of age. A new vaccine approach developed in Israel used controlled exposure of the breeding flock to virulent ARV at the age when they are resistant to infection. This approach significantly reduced clinical field cases and reovirus isolations of breeding and broiler flocks between 2020 and 2022.