Evidence for vertical transmission of avian reovirus in chickens.

Avian reovirus (ARV) has emerged as a highly prevalent pathogen in Chinese poultry, characterized by its high mutation rate, environmental resilience, and dual vertical-horizontal transmission routes, leading to rising chick morbidity and substantial economic losses. To evaluate vertical transmission, 945 dead embryos and 58 weak chicks from a Guangdong hatchery between January 2023 and December 2024 were tested, yielding a 9.6 % (96/1003) ARV positivity rate. Three strains (GD/SB/202301, WMC06, LN02) were successfully isolated. Phylogenetic analysis classified GD/SB/202301 and WMC06 as genotype II, while LN02 clustered within genotype III. Experimental challenge of broilers with GD/SB/202301 elicited severe arthritis and tenosynovitis. Vertical transmission was confirmed in SPF breeder chickens. Offspring from challenged hens and roosters exhibited clinical signs including lethargy, leg splaying, and joint redness. Hatchability declined in challenged hens (33.6 %) and roosters (40.5 %) versus controls (44.9 %), with 31.8 % and 24.4 % weak chicks, respectively. ARV detection rates in challenged groups significantly exceeded natural infection rates, confirming vertical transmission independent of parental sex. Horizontal transmission studies showed 100 % ARV positivity in challenged chicks by 14 days and 100 % transmission to co-housed SPF chicks by 28 days. Necropsy revealed joint swelling, subcutaneous hemorrhage, diffuse articular bleeding. Comparable viral loads in cecal tonsils/tendons and histopathological evidence of tendon lesions confirmed vertically infected chicks as persistent horizontal spreaders. These findings demonstrate ARV's bidirectional transmission capacity and emphasize the critical role of vertical transmission in sustaining outbreaks, providing vital insights for ARV control strategies.