Development and evaluation of an antibody ELISA for sarcoptic mange in sheep and a comparison with the skin-scraping method

Abstract

In this work an indirect ELISA for detecting serum-specific IgG antibodies in sheep was developed using a crude saline extract from Sarcoptes scabiei var. ovis mites and then the repeatability of the ELISA outcomes was estimated. Subsequently, its diagnostic accuracy was evaluated by Receiver Operating Characteristics (ROC) analysis using a sample collected from the entire sheep population of western Castile and Leon region in Spain, and then compared with that of the skin-scraping method. The reference method used was a combination of clinical examination, skin-scraping analysis and epidemiological surveys, but it introduced selection and probably information biases. Furthermore, we attempted to identify biological factors useful to predict the sensitivity or specificity of the ELISA as determined by comparison with the reference method. Additionally, conventional latent-class analysis [Hui, S.L., Walter, S.D., 1980. Estimating the error rates of diagnostic tests. Biometrics 36, 167–171] was also used to estimate accuracy parameters. The between-run coefficient of variation (CV) for a standard serum was 8.8% and the within-run CV 4.3%. No significant deviation between the OD% means and strength positive correlation between the OD% values ( r = 0.98) were found for the results from two different batches of antigen. When compared to the reference method, the Area Under the ROC curve (AUC) for the reference population was 0.967 (95% CI: 0.949–0.985) for the ELISA and 0.915 (95% CI: 0.863–0.968) for the skin-scraping method. By logistic regression analysis, one explanatory biological factor—result to the skin-scraping method—and four explanatory biological factors–Tyroglyphidae individual status, Trichophyton verrucosum individual status, Oestrus ovis status of the flock and presence of adjacent animals with a clinical disease neighbour to S. scabiei infection—were found for diagnostic sensitivity and specificity of the ELISA, respectively, although this depended on the OD% cut-off value used. Latent-class analysis, carried out for the ELISA at 17.8 OD% cut-off value (mean plus 3 SDs of sheep considered negative to anti- S. scabiei antibodies), showed a marked difference between the estimated diagnostic sensitivity of the ELISA (87.6%) and the skin-scraping method (62.8%), but closer diagnostic specificities (95.9% vs. 100%, respectively). These results demonstrate that the developed ELISA is valid for different applications in clinical as well as in epidemiological contexts.