An approach to diagnosis of Jaagsiekte sheep retrovirus infection in sheep based on assessment of agreement between macroscopic examination, histopathologic examination and reverse-transcriptase polymerase chain reaction

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA). JSRV infection is usually detected post-mortem by macroscopic and histological examination of lungs for lesions of OPA. Subsequently, the presence of JSRV may be confirmed using polymerase chain reaction (PCR) on tumour tissue. Our goal was to determine the most effective way of combining macroscopic and histological examination with reverse transcriptase PCR (RT-PCR) to detect JSRV infection post-mortem. Lungs of slaughtered sheep (n = 369) with macroscopic lesions were examined macroscopically and histologically to identify lesions consistent with OPA, and subsequently subjected to RT-PCR for JSRV. Positive (Ppos) and negative (Pneg) agreement and Cohen’s Kappa were calculated between RT-PCR and: 1) macroscopic examination; 2) histological examination; 3) macroscopic and histological examinations combined in series, and 4) in parallel. The highest Ppos was between macroscopic and histological examination in parallel and RT-PCR (0.38). Conversely, Pneg for all combinations of RT-PCR and macroscopic and histological examinations was high (0.95-0.96). All Kappa values were low (0.1-0.33). This indicates that macroscopic and histological examination combined in parallel is the most effective way to identify animals that should be tested using RT-PCR for JSRV. If a positive result is obtained on macroscopic examination and/or histological examination, RT-PCR for JSRV should be carried out. The high Pneg indicates that if a negative result is obtained on macroscopic and histological examination, RT-PCR testing is not merited, as the result is likely to be negative. This provides an evidence-base for the diagnosis of JSRV infection.