Ovine pulmonary adenocarcinoma (OPA)
Ovine pulmonary adenocarcinoma (OPA) is a contagious lung tumour of sheep resulting from infection with a betaretrovirus called Jaagsiekte sheep retrovirus (JSRV). The disease is also commonly known as Jaagsiekte. Tumour growth replaces the normal lung structure and consequently impairs lung function. In addition the tumour cells may secrete large volumes of fluid, also impairing lung function. Transmission of JSRV occurs mostly through the aerosol route but also via colostrum and milk.
OPA is common in the UK but the disease is grossly under-reported because few sheep deaths are investigated. It is generally considered a chronic wasting disease with progressive respiratory distress and is invariably fatal. In the field, the incubation period from infection with JSRV to the appearance of clinical signs is many months or years with clinical disease most commonly observed in three to four year-old sheep, though it is occasionally seen at less than one year old.Ultrasonography has been used to detect OPA lesions as small as 2 cm in diameter. Trans-thoracic ultrasound examination should be considered for a second opinion on an initial diagnosis of OPA, for screening purchased adult flock replacements for OPA, or for screening sheep in a known OPA-affected flock but a negative scan cannot provide a guarantee that the animal is free of OPA as lesions can develop in just 8 weeks.
Callum has in the past been involved with scanning a hill flock of blackies for OPA. He scanned a total of 831 sheep with a detection rate of 5% (national average). Animals with lesions have been culled. In the absence of any better diagnostic options he will repeat the scan every year for the foreseeable future with a view to improving the health status of the flock as much as possible. Callum hopes to demonstrate how a scan and cull policy can help reduce incidence of OPA in a hill flock.
