In vitro and in vivo characterization of avian reoviruses. I. Pathogenicity and antigenic relatedness of several avian reovirus isolates.

Pathogenicity, pathogenesis, and antigenic relatedness of four avian reovirus isolates obtained from commercially reared broilers were investigated. Chickens of various ages were inoculated both orally and intratracheally with reovirus. Based on disease signs, mortality, weight depression, tissue lesions, invasiveness, and viral persistence in chickens inoculated at 1 day of age, the isolates were classified as being of low, intermediate, or high pathogenicity. The low-pathogenicity isolate (2177) did not cause mortality, weight depression, or clinical disease. The isolate of intermediate pathogenicity (2035) produced low mortality rates (8%), some weight reduction by 7 weeks postinoculation, and microscopic lesions in the intestine and gastrocnemius tendons. The pathogenic isolates, 2408 and 1733, caused severe clinical disease characterized by stunting, feathering abnormalities, mortality as high as 84%, and microscopic lesions in the liver, intestine, pancreas, and/or gastrocnemius tendon. Highly pathogenic isolates also persisted longer in tissues of infected birds and elicited a more prompt and prolonged antibody response. Birds inoculated at 1 day or 1 week of age were more susceptible to reovirus-induced disease than birds inoculated at 2 weeks, suggesting an age-associated resistance. All isolates produced mortality with equal frequency in embryos. The isolates characterized were found to be antigenically similar based on cross-neutralization and cross-protection studies.