Serological monitoring on milk and serum samples in a BVD eradication program: A field study in Belgium showing antibody ELISA performances and epidemiological aspects.

In a cross-sectional field study involving 51 cattle herds in Belgium, 3159 serum samples and 557 individual milk samples were collected and tested by four different commercial antibody (Ab) ELISAs on serum and two Ab ELISAs on milk. A virus neutralization test (VNT) was performed on serum samples with discording ELISA results and on all samples from non-vaccinating herds. An epidemiological survey was carried out in the same herds to collect information about herd characteristics, management practices, BVD vaccination and BVD infection status. The objective of the study was to evaluate the performances of the Ab ELISAs relatively to the VNT, to assess the possibility of using pooled samples and to give recommendations regarding serological monitoring of BVD-free herds in the context of the Belgian national BVD eradication program which started early 2015. Depending on the assays, for ELISAs on serum, the diagnostic sensitivity (DSe) was estimated to be between 93.0 and 98.7% and the diagnostic specificity (DSp) between 94.3% and 99.1%. For the two ELISAs on milk, the DSe were 91.3% and 96.7% and the DSp 94.0% and 100% respectively and the Cohen's agreement coefficients between serum and milk samples were 0.75 and 0.85. Positive serum and milk samples diluted in negative samples to mimic different pool sizes were not detected by all ELISAs at dilutions above 1:5 or 1:10, leading to the conclusion that the testing of pooled samples should be used cautiously for serological monitoring and only with ELISAs with high sensitivity. The epidemiological analysis and the seroprevalence study, based on a general estimating equation model, showed that several factors had a significant influence on overall animal seroprevalence and within-herd seroprevalence such as age class, herd size, BVD herd infection status, BVD vaccination of young and/or adult cattle and the number of stables in the farm. This study showed that the best performances obtained with commercial Ab ELISAs are observed on individual serum samples, which should therefore be the preferred matrix to monitor BVD-free herds in the context of the Belgian eradication program. By regularly testing a limited number of samples from young (6-18 months) unvaccinated cattle it is possible to confirm the BVD-free herd status or to detect a recent infection.