Evaluation of a recombinant major envelope protein (F1L) based indirect- ELISA for sero-diagnosis of orf in sheep and goats.

Orf or contagious ecthyma, is a highly contagious transboundary disease of sheep and goats. For sero-diagnosis of orf, recombinant antigen based assays are considered as alternatives to conventional approaches such as serum neutralization test (SNT) and counter-immuno-electrophoresis (CIE). A major envelope protein of orf virus (ORFV), F1L, is highly immunogenic and is a candidate for use in these assays. In this study, the F1L gene of the ORFV-59/05 strain encoding a recombinant mature F1L protein (1M-D302 aa) with a C- terminal truncation, was produced as a fusion protein (∼50 kDa) in Escherichia coli. The immunogenic potential of purified rF1L was confirmed by detecting specific anti-F1L antibody responses in sera collected from immunized rabbits and guinea pigs using ELISA and SNT. An indirect-ELISA based on rF1L was developed and optimized. In comparison to SNT by ROC analysis in the detection of ORFV specific antibodies, this new assay exhibited a diagnostic specificity of 94.04% and 92.53% with sheep and goat sera, respectively, while the sensitivity was 89.22% and 94.25%, for sheep and goat sera. No cross reactivity was noted with sera collected from small ruminants infected with other transboundary diseases (goatpox, sheeppox, peste des petits ruminants, foot-and-mouth disease and bluetongue). Furthermore, the rF1L-ELISA applied to screen the vaccinated/challenged goat sera resulted in better detection (30%) than by SNT (28%) in spite of lower levels of antibodies which could be due to predominant cell mediated immune response in vaccinated animals. This study highlighted the potential utility of rF1L protein as a safe and novel diagnostic reagent in comparison to live virus antigen, in the development of sero-diagnostic assay for surveillance of ORFV infection in sheep and goats.