Testes de elisa e vírus-neutralização na detecção de anticorpos contra o vírus da diarréia viral bovina no soro e leite

Abstract

The success of control/eradication programs of bovine viral diarrhea virus (BVDV) infection necessarily includes the identification and elimination of persistently infected (PI) animals. Since these animals continuously shed virus, the prevalence and titers of antibodies in herds with PI animals are often high. Hence, bulk milk and serum samples were subjected to two serological tests in order to establish the most appropriate for herd screening. For this, 767 samples of bulk milk and 701 serum samples were analyzed by a commercial ELISA kit and by virus-neutralization test (VNT). The toxic effects of milk for cell cultures on VNT was abolished by increasing the final volume of medium during the incubation. Among the tested milk samples, 177 and 139 were positive in VNT and ELISA, respectively. Thus, the VNT showed a sensitivity (Se) of 76.8% and a specificity (Sp) of 99.5%. The Kappa index (k) was 0.82, demonstrating an excellent agreement between the two techniques. The analysis of the correlation coefficient between the absorbance values in ELISA (OD) and antibody titers in positive samples in VNT showed a moderate positivity (r = 0.57) with p<0.05. In serum samples, 442 and 602 were positive in ELISA and VNT, respectively. This resulted in a VNT sensitivity (Se) of 92.7% and specificity (Sp) of 27.1%, with (k) of 0.23, reflecting a weak agreement between the two techniques. The correlation coefficient between OD and antibody titers showed a moderate positivity (r = 0.54) with p<0.05. Testing of both milk and serum revealed a considerable number of samples with high VN titers and low ODs. On the other side, many samples with low VN titers showed high ODs in the ELISA. Based on the above mentioned results, and assuming that the presence of PI animals usually results in VN titers ≥ 80, we conclude that the VNT is more appropriate to conduct herd screening in bulk milk when the objective is to detect herds with high titers of neutralizing antibodies, potentially harboring PI animals.