Occurrence: Worldwide. Species affected: All. Age affected: 3-30 weeks. Broilers develop problems from 5-8 weeks and breeder pullets at 10-18 weeks of age.

Causes: Double stranded RNA reovirus.

Effects: Incubation period is 9-13 days. Birds are down on the hocks and have stunted growth, are reluctant to move and have an uneven gait. Swollen tendons above and below the hocks and a ruptured gastrocnemius muscle are also seen.