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.
Chickens and turkeys from 3-30 weeks old, mostly broilers and broiler breeders, are susceptible to this chronic to debilitating disease. Broilers develop problems from 5-8 weeks and breeder pullets at 10-18 weeks of age. The disease is caused by a reovirus (respiratory, enteric, orphan).
Mode of transmission
Spread laterally from bird to bird or vertically from hen to offspring. Faecal to oral or via respiratory tract transmission can also occur.
It commonly causes downgrading and trimming in broilers and culling in breeders.
Incubation period of 9-13 days.
Birds are down on the hocks, have stunted growth, reluctant to move and have an uneven gait. Swollen tendons above and below the hocks and ruptured a gastrocnemius tendon is also seen
Bilateral swelling of tendons (digital flexor and metatarsal extensor) is diagnostic. There is an increase in clear, thick fluid in joints and swelling of the footpad, straw coloured or blood-tinged exudate can be seen in lesions.
Ruptured tendons, haemorrhaging of synovial membrane, erosion of cartilage of distal tibiotarsus can also occur. The condyles (joint capsule) may also be involved.
Isolation of virus from lesions on CAM of inoculated embryos and serologic test for VA antibodies (VN or ELISA) are of limited use because birds are routinely infected with nonpathogenic reoviruses. Histopathology, reveals oedema, coagulation necrosis, heterophil accumulation and perivascular infiltration. Hypertrophy and hyperplasia of synovial cells, infiltration of lymphyocytes, macrophages and reticular cells are also evident.
VA causes excessive or clear straw coloured fluid; Mycoplasma synoviae causes honey coloured fluid and Staphylococcus produces thick white fluid in swollen joints. Gross and microscopic lesions are diagnostic.
Vaccination of pullets with live and killed S1133 strain of reovirus to prevent them from getting the disease and to pass maternal immunity to the progeny. New antigenic viruses have arisen.