Nekouei Omid, VanLeeuwen John, Stryhn Henrik, Kelton David, Keefe Greg
Department of Ecosystem and Public Health, Faculty of Veterinary Medicine, University of Calgary, Calgary, AB T2N 4Z6, Canada.
Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PE C1A 4P3, Canada.
Prev Vet Med. 2016 Oct 1;133:1-9. doi: 10.1016/j.prevetmed.2016.09.011. Epub 2016 Sep 12.
Enzootic bovine leukosis (EBL) is an economically important disease of dairy cattle caused by bovine leukemia virus (BLV). The economic impacts of the infection have been debated in the literature. The present study was conducted to determine the lifetime effects of BLV infection on longevity and milk production of dairy cows in Canada. The data were aggregated from a combination of two data sets: 1) BLV serum-ELISA test results from Canada-wide surveys of production limiting diseases, which took place between 1998 and 2003 in 8 provinces, and 2) longitudinal production data for all cows in the former study, extracted from the Canadian dairy herd improvement database. All participant cows had been culled or died by the onset of this study. A historical cohort study was designed, including cows which tested positive to BLV-antibodies in their first lactation (positive cohort, n=1858) and cows which tested negative in their second or later lactations (negative cohort, n=2194). To assess the impacts of infection with BLV on longevity (the number of lifetime lactations), a discrete-time survival analysis was carried out. The effect of BLV on the lifetime milk production (the sum of all life 305-day milk production) was evaluated using a multilevel linear regression model. Overall, 4052 cows from 348 herds met the eligibility criteria and were enrolled in the study. In the longevity model, the interaction term between time (lactation number) and BLV-status was highly significant. Cows which were positive to BLV had consistently greater probabilities of being culled (or dying) than the test-negative cows. In the milk production model, the interaction term between BLV-status and longevity of the cows was highly significant; indicating that lifetime BLV effects on the total milk production was dependent on the lactation in which the study cows were culled/died. Infected cows with 2 and 3 lactations showed significantly lower life milk productions [-2554kg (-3609 to -1500) and -1171kg (-2051 to -292), respectively] compared with their negative counterparts with 2 and 3 lactations. As the cows lived longer (>3 lactations), the differences in life milk production between the two cohorts were no longer significant. Overall, it was predicted that the test-positive cows produced substantially lower milk compared to the test-negative cows throughout their study lifespans. With the high prevalence of BLV in Canadian dairy cows and its detrimental economic impacts, pursuing broad-based control programs in Canada should be evaluated.
地方流行性牛白血病（EBL）是由牛白血病病毒（BLV）引起的一种对奶牛具有重要经济影响的疾病。该感染的经济影响在文献中一直存在争议。本研究旨在确定BLV感染对加拿大奶牛寿命和产奶量的终生影响。数据来自两个数据集的组合：1）1998年至2003年在8个省份进行的全国性生产限制疾病调查中的BLV血清酶联免疫吸附测定（ELISA）检测结果，以及2）前一项研究中所有奶牛的纵向生产数据，这些数据从加拿大奶牛群改良数据库中提取。在本研究开始时，所有参与的奶牛均已被淘汰或死亡。设计了一项历史性队列研究，包括首次产奶时BLV抗体检测呈阳性的奶牛（阳性队列，n = 1858）和第二次或以后产奶时检测呈阴性的奶牛（阴性队列，n = 2194）。为了评估BLV感染对寿命（终生产奶次数）的影响，进行了离散时间生存分析。使用多级线性回归模型评估BLV对终生产奶量（所有305天终生产奶量之和）的影响。总体而言，来自348个牛群的4052头奶牛符合纳入标准并被纳入研究。在寿命模型中，时间（产奶次数）和BLV状态之间的交互项非常显著。BLV检测呈阳性的奶牛被淘汰（或死亡）的概率始终高于检测呈阴性的奶牛。在产奶量模型中，BLV状态和奶牛寿命之间的交互项非常显著；这表明BLV对终生总产奶量的影响取决于研究奶牛被淘汰/死亡时的产奶次数。与具有2次和3次产奶的阴性奶牛相比，具有2次和3次产奶的感染奶牛的终生产奶量显著更低[分别为-2554千克（-3609至-1500）和-1171千克（-2051至-292）]。随着奶牛寿命延长（>3次产奶），两个队列之间的终生产奶量差异不再显著。总体而言，预计在整个研究寿命期间，检测呈阳性的奶牛产奶量大幅低于检测呈阴性的奶牛。鉴于BLV在加拿大奶牛中的高流行率及其有害的经济影响，应评估在加拿大推行广泛控制计划的可行性。
