Booth Richard E, Brownlie Joe
Royal Veterinary College, Hawkshead Lane, North Mymms, Hertfordshire, AL9 7TA, UK.
BMC Vet Res. 2016 Aug 26;12(1):177. doi: 10.1186/s12917-016-0797-2.
This paper examines the use of Bulk Milk antibody (BM Ab), Youngstock (YS) serology (Check Tests) and Bulk Milk PCR (BM PCR) for determining the presence or absence of animals persistently infected (PI) with Bovine Viral Diarrhoea Virus (BVDV) within a herd. Data is presented from 26 herds where average herd sizes were 343 and 98 animals for dairy and beef units respectively. Seventeen herds had sufficient data to analyse using Receiver Operating Characteristic (ROC) and probability curves enabling calculation of the sensitivity and specificity of BM Ab and YS Check tests for determining the presence of PI animals within herds in this dataset.
Using BM Ab to screen a herd for the presence of PI animals, achieved a herd level sensitivity and specificity of 80.00 % (44.39-97.48 %) and 85.71 % (42.13-99.64 %) respectively (95 % confidence intervals quoted). Sensitivity and specificity of YS Check Tests at a cut off of 3/10 Ab positive YS were 81.82 % (48.22-97.72 %) and 66.67 % (22.28-95.67 %) respectively (95 % confidence interval). These results were achieved by comparing the screening tests to whole herd PI searches that took place 1-19 months after the initial screen with a mean interval of 8 months. Removal of this delay by taking BM samples on the day of a whole herd test and simulating a YS Check Test from the herd test data produced improvements in the reliability of the Check Tests. BM Ab sensitivity and specificity remained unchanged. However, the Check Test sensitivity and specificity improved to 90.9 % (58.72-99.77 %) and 100 % (54.07-100 %) respectively (95 % confidence interval) at a cut of off 2.5/10 Ab positive animals. Our limited BM PCR results identified 5/23 dairy farms with a positive BM PCR result; two contained milking PIs, two had non-milking PIs and another had no PIs identified.
Delaying a PI search following an initial herd screen decreased the diagnostic accuracy and relevance of our results. With careful interpretation, longitudinal surveillance using a combination of the techniques discussed can successfully determine farm status and therefore allow changes in BVDV status to be detected early, thus enabling prompt action in the event of a BVDV incursion.
本文探讨了使用散装牛奶抗体（BM Ab）、幼畜（YS）血清学检测（检查试验）和散装牛奶聚合酶链反应（BM PCR）来确定牛群中是否存在持续感染牛病毒性腹泻病毒（BVDV）的动物。文中给出了来自26个牛群的数据，其中奶牛场和肉牛场的平均牛群规模分别为343头和98头。17个牛群有足够的数据可用于使用受试者工作特征（ROC）和概率曲线进行分析，从而能够计算出BM Ab和YS检查试验在该数据集中确定牛群中是否存在持续性感染（PI）动物的敏感性和特异性。
使用BM Ab对牛群进行PI动物筛查时，畜群水平的敏感性和特异性分别达到80.00%（44.39 - 97.48%）和85.71%（42.13 - 99.64%）（引用95%置信区间）。当YS检查试验的临界值为3/10 Ab阳性YS时，其敏感性和特异性分别为81.82%（48.22 - 97.72%）和66.67%（22.28 - 95.67%）（95%置信区间）。这些结果是通过将筛查试验与初始筛查后1 - 19个月进行的全群PI搜索进行比较得出的，平均间隔为8个月。通过在全群检测当天采集BM样本并从群检数据模拟YS检查试验来消除这种延迟，提高了检查试验的可靠性。BM Ab的敏感性和特异性保持不变。然而，当临界值为2.5/10 Ab阳性动物时，检查试验的敏感性和特异性分别提高到90.9%（58.72 - 99.77%）和100%（54.07 - 100%）（95%置信区间）。我们有限的BM PCR结果显示，23个奶牛场中有5个BM PCR结果为阳性；其中两个含有正在产奶的PI动物，两个有非产奶的PI动物，另一个未检测到PI动物。
在初始牛群筛查后延迟进行PI搜索会降低我们结果的诊断准确性和相关性。通过仔细解读，结合所讨论的技术进行纵向监测能够成功确定农场状况，从而能够早期发现BVDV状况的变化，以便在发生BVDV入侵时能够迅速采取行动。
