Bovine leukemia virus (BLV) belongs to the genus Deltaretrovirus in the family Retroviridae, and is etiologically associated with Enzootic Bovine Leukosis (EBL). The majority of BLV-infected cattle remain asymptomatic throughout their productive lives, whereas approximately 5%-10% of infected cattle develop EBL. Data accumulated recently indicate that whole blood proviral load (PVL) levels of BLV-infected cattle could be an indicator of disease progression in the field. However, a few cross-sectional studies have been reported. Here, we prospectively evaluated 866 cattle enrolled between August 2015 and December 2015, and followed until November 2018, identifying 407 asymptomatic BLV-infected cattle. There were no significant differences in the median PVL level among the category of herd seroprevalence (p = 0.57), herd size (p = 0.19), nor among the category of past EBL history in the herd (p = 0.31). During the study period, 12 cattle developed EBL. The PVL levels of EBL cattle at the time of enrollment were significantly higher than that of cattle that did not progress to EBL (median, 90,695 vs 39,139 copies/105 cells, p = 0.0005). Moreover, the adjusted hazard ratio for the increase in PVL was 2.61 (95% CI, 1.51-4.53) as estimated by the Cox proportional hazards frailty model. These results indicate that a high PVL level is a significant risk factor for progression to EBL, and could potentially be used as an indicator for the identification of cattle to be culled from the herd long before the progression of EBL. This knowledge might be useful to design a strategy for decreasing economic loss from EBL or even eradicating it from herds.

中文翻译：

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牛白血病病毒 (BLV) 前病毒载量的增加是地方性牛白血病进展的危险因素：日本的一项前瞻性研究。


牛白血病病毒 (BLV) 属于逆转录病毒科 Delta 逆转录病毒属，在病因学上与牛地方性白血病 (EBL) 相关。大多数感染 BLV 的牛在其生产生活中始终保持无症状，而大约 5%-10% 的受感染牛会出现 EBL。最近积累的数据表明，感染 BLV 的牛的全血前病毒载量 (PVL) 水平可能是现场疾病进展的指标。然而，已经报道了一些横断面研究。在此，我们前瞻性评估了 2015 年 8 月至 2015 年 12 月期间登记的 866 头牛，并跟踪至 2018 年 11 月，确定了 407 头无症状 BLV 感染牛。在猪群血清流行率类别（p = 0.57）、猪群规模（p = 0.19）以及猪群过去 EBL 历史类别（p = 0.31）之间，PVL 中位水平没有显着差异。在研究期间，12 头牛出现了 EBL。 EBL 牛在入组时的 PVL 水平显着高于未进展为 EBL 的牛（中位数，90,695 vs 39,139 拷贝/105 个细胞，p = 0.0005）。此外，根据 Cox 比例风险脆弱模型估计，PVL 增加的调整后风险比为 2.61（95% CI，1.51-4.53）。这些结果表明，高 PVL 水平是进展为 EBL 的一个重要风险因素，并且有可能用作在 EBL 进展之前很久识别从牛群中宰杀的牛的指标。这些知识可能有助于设计减少 EBL 经济损失甚至从牛群中根除 EBL 的策略。