Evaluation of three serological tests for diagnosis of Maedi-Visna virus infection using latent class analysis

Maedi-Visna virus (MVV) infection in sheep is present in several European countries, including Norway. The current Norwegian surveillance and control programme for MVV infection uses three serological tests: an agar gel immunodiffusion test (AGID) and two commercially available indirect ELISAs (Institut Pourquier, P-ELISA and HYPHEN BioMed, H-ELISA). From 18 flocks with suspected or confirmed MVV infection, sera from naturally infected sheep were obtained, and sensitivity (Se) and specificity (Sp) of the three tests were estimated in absence of a perfect reference test using latent class models in a Bayesian analysis. The AGID had higher Sp (95% posterior credibility interval (PCI) 98.4; 99.9]) than either ELISA (95% PCI: P-ELISA, 95.1; 99.0]; H-ELISA, 91.4; 96.6]), but much lower Se (95% PCI: AGID, 41.4; 59.8]; P-ELISA, 92.7; 100.0]; H-ELISA, 90.9; 99.4]). Currently the P-ELISA is used for screening and positive samples are subsequently confirmed by a setup using all three tests in a serial reading. The Se and Sp of the serial interpretations with and without the H-ELISA were estimated. The results suggested that the H-ELISA could be dropped as a confirmatory test as the Se of the three test serial reading was reduced significantly without adding a significant improvement of the Sp compared to the serial reading of the P-ELISA and AGID alone. However, the perceived cost of false positives versus false negatives will influence this decision. Estimates of the predictive values for the tests and combinations suggested that the P-ELISA is a good choice of screening, but confirmatory tests are needed to achieve acceptable levels of positive predictive values.