Development of a simplified model for assessing bovine leukemia virus transmission risk using enzyme-linked immunosorbent assay sample/positive ratios.

The bovine leukemia virus (BLV) is the primary cause of enzootic bovine leukosis in cattle, resulting in considerable economic impacts, including decreased milk production, increased disease susceptibility, and limitations on trade. While quantifying proviral load (PVL) through quantitative PCR (qPCR) effectively assesses BLV transmissibility, its high cost restricts widespread implementation. This study developed a simplified enzyme-linked immunosorbent (ELISA)-based method utilizing the sample-to-positive (S/P) ratio to estimate BLV transmission risk. Blood samples were obtained from 496 cattle (Japanese Black and Holstein-Friesian) in Japan. BLV infection was identified using ELISA and qPCR, with PVL measured as the number of BLV genome copies per 100,000 cells. The findings showed a positive correlation between the S/P ratio and PVL. Through receiver operating characteristic analysis, an S/P ratio cutoff of 1.87 was determined to differentiate ultra-low-risk cows (PVL < 600 copies/105 cells) from those at elevated transmission risk, achieving 93.0 % sensitivity and 77.8 % specificity. This S/P ratio threshold accurately classified 94.3 % of ultra-low-risk animals and effectively identified cattle with a higher likelihood of horizontal transmission. The approach offers a cost-effective and scalable alternative to qPCR, particularly suitable for large-scale farms or areas with limited resources. Although less accurate than PVL-based techniques, S/P ratio-driven risk evaluation could significantly improve BLV management by facilitating the prompt detection and control of high-risk cattle.