Association between the level of antibodies in bulk tank milk and bovine respiratory syncytial virus exposure in the herd

Klem, T. B.; Tollersrud, T.; Østerås, O.; Stokstad, M.

BTM samples used in the first and the third studies were collected at the farm by the driver of the milk truck and transported the same day at 4°C to the dairy and were stored at -20°C until they were dispatched by overnight delivery to the laboratory, where they were kept frozen at the same temperature until analysis. The individual samples used in the second and third studies and the BTM samples in study 2 were collected from the herds by the same veterinary surgeon and dispatched by overnight delivery to the laboratory on the same day. Blood and milk samples were centrifuged and the serum or skimmed milk was extracted before being stored at -20°C until they were analysed. An indirect ELISA (SVANOVIR BRSV-Ab, Svanova Biotech AB, Uppsala, Sweden) was used to analyse for antibodies against BRSV, following the manufacturer's instructions. In brief, the optical density (OD) reading of 450 nm was corrected by the subtraction of OD for the negative control antigen, and PP was calculated as (corrected OD/positive control corrected OD)×100. The serum was diluted 1:25 with PBS-Tween buffer and the milk was analysed undiluted as recommended by the manufacturer. A sample was considered positive if PP≥10 and negative if PP<10. Positive and negative controls were used and reproducibility was monitored by use of in-house controls. The ELISA kit is indicated for use in serum and milk samples. The sensitivity and specificity of the tests reported by the manufacturer were 94.6 per cent and 100 per cent, respectively.