A caprine arthritis-encephalitis virus (CAEV)/maedi-visna virus (MVV) indirect enzyme-linked immunosorbent assay (iELISA) was validated with samples from U.S. sheep and by the use of radioimmunoprecipitation as the standard for comparison. The sensitivity and the specificity were 86.0% (؎5.8%) and 95.9% (؎2.9%), respectively. The iELISA format and phylogenetic differences based on the MVV gag sequence contribute to the reduced sensitivity.The diagnosis of small ruminant lentivirus (SRLV) infections in sheep and goats is most commonly determined by the detection of anti-SRLV antibodies in serum by an enzymelinked immunosorbent assay (ELISA) that is typically created by the use of maedi-visna virus (MVV) or caprine arthritisencephalitis virus (CAEV) isolates from sheep or goats of a given region or country (1). ELISA formats are typically validated against reference standard tests, including the agar gel immunodiffusion (AGID) assay, the radioimmunoprecipitation (IP) assay, or Western blot analysis. Although most seropositive sheep and goats do not show clinical signs of SRLV disease, they are persistent and potential reservoirs for transmission. Therefore, highly specific and sensitive serological diagnostic assays are essential for the early detection of SRLV.Three hundred ten of 332 serum samples from U.S. sheep from a previous CAEV competitive ELISA (cELISA) validation study (4) were tested in duplicate by using a Chekit CAEV/MVV antibody test kit (IDEXX Laboratories, The Netherlands), according to the manufacturer's instructions. The CAEV/MVV indirect ELISA (iELISA) results were compared with those of the ovine progressive pneumonia virus (OPPV) WLC1 radio-IP assay, which has been described previously (4). The CAEV/MVV iELISA utilizes whole virus from Swiss MVV strain OLV as the antigen (15, 16). With a value of Ն60% being defined as a CAEV/MVV iELISA-positive serum sample, the sensitivity and the specificity of the CAEV/MVV iELISA were 74.0% (Ϯ7.6%) (95% confidence interval) and 98.3% (Ϯ2.0%), respectively, compared to the results of the radio-IP assay. Since the sensitivity was less than adequate, we reassessed the cutoff by calculating the mean value (in percent) Ϯ 2 standard deviations for the radio-IP assay-negative serum samples. The results of that analysis placed the cutoff mean value at 33.1%. By using the new cutoff value, the sensitivity of the iELISA improved to 86.0% (Ϯ5.8%) and the specificity decreased slightly to 95.9% (Ϯ2.9%) compared to the results of the radio-IP assay. However, compared to the CAEV cELISA, which has a sensitivity of 98.6% and a specificity of 96.9% when the results of the radio-IP assay are used as the reference standard, the iELISA had a reduced sensitivity.Since the sera were taken from a number of different U.S. sheep kept under different husbandry and management conditions, we also wanted to test the performance of the CAEV/ MVV iELISA with sera from one flock in which the sheep are exposed to the same husbandry and management conditions. Sera from an Idaho sheep flock...
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