Hushållningssällskapet Västernorrland, Trädgårdsgatan 7, Härnösand, Sweden.
Acta Vet Scand. 2012 Nov 20;54(1):65. doi: 10.1186/1751-0147-54-65.
Staphylococcus aureus is one of the most common causes of intramammary infections in dairy cows at dry off. Reliable identification is important for disease management on herd level and for antimicrobial treatment of infected animals. Our objective was to evaluate the test characteristics of PathoProof ™ Mastitis PCR Assay and bacteriological culture (BC) in diagnosing bovine intramammary infections caused by S. aureus at dry off at different PCR cycle threshold (Ct)-value cut-offs.
Sterile quarter samples and non-sterile composite samples from 140 animals in seven herds were collected in connection with the dairy herd improvement (DHI) milk recording. All quarter samples were analyzed using BC whereas all composite samples were analyzed with PathoProof ™ Mastitis PCR Assay. Latent class analysis was used to estimate test properties for PCR and BC in the absence of a perfect reference test. The population was divided into two geographically divided subpopulations and the Hui-Walter 2-test 2-populations model applied to estimate Se, Sp for the two tests, and prevalence for the two subpopulations.
The Se for PCR increased with increasing Ct-value cut-off, accompanied by a small decrease in Sp. For BC the Se decreased and Sp increased with increasing Ct-value cut-off. Most optimal test estimates for the real-time PCR assay were at a Ct-value cut-off of 37; 0.93 [95% posterior probability interval (PPI) 0.60-0.99] for Se and 0.95 [95% PPI 0.95-0.99] for Sp. At the same Ct-value cut-off, Se and Sp for BC were 0.83 [95% PPI 0.66-0.99] and 0.97 [95% PPI 0.91-0.99] respectively. Depending on the chosen PCR Ct-value cut-off, the prevalence in the subpopulations varied; the prevalence increased with increasing PCR Ct-value cut-offs.
Neither BC nor real-time PCR is a perfect test in detecting IMI in dairy cows at dry off. The changes in sensitivity and prevalence at different Ct-value cut-offs for both PCR and BC may indicate a change in the underlying disease definition. At low PCR Ct-value cut-offs the underlying disease definition may be a truly/heavily infected cow, whereas at higher PCR Ct-value cut-offs the disease definition may be a S. aureus positive cow.
金黄色葡萄球菌是奶牛干奶期乳腺炎最常见的病原体之一。可靠的鉴定对于群体疾病管理和感染动物的抗菌治疗非常重要。我们的目的是评估 PathoProof ™乳腺炎 PCR 检测和细菌培养（BC）在不同 PCR 循环阈值（Ct）值截断值下诊断奶牛干奶期金黄色葡萄球菌引起的乳腺炎的检测特性。
在与奶牛群体改良（DHI）牛奶记录相关的研究中，从 7 个牛群中的 140 头动物中采集无菌乳头样本和非无菌复合样本。所有乳头样本均采用 BC 进行分析，而所有复合样本均采用 PathoProof ™乳腺炎 PCR 检测进行分析。使用潜在类别分析来估计无完美参考测试时 PCR 和 BC 的测试特性。该人群分为两个地理上分开的亚群，并应用 Hui-Walter 2-测试 2-群体模型来估计两个测试的 Se、Sp，以及两个亚群的患病率。
随着 Ct 值截断值的增加，PCR 的 Se 增加，Sp 略有下降。对于 BC，随着 Ct 值截断值的增加，Se 降低，Sp 增加。实时 PCR 检测的最佳测试估计值为 Ct 值截断值为 37；0.93[95%后验概率区间（PPI）0.60-0.99]用于 Se，0.95[95% PPI 0.95-0.99]用于 Sp。在相同的 Ct 值截断值下，BC 的 Se 和 Sp 分别为 0.83[95% PPI 0.66-0.99]和 0.97[95% PPI 0.91-0.99]。根据选择的 PCR Ct 值截断值，亚群中的患病率不同；随着 PCR Ct 值截断值的增加，患病率增加。
BC 和实时 PCR 都不是检测奶牛干奶期乳腺炎感染的完美检测方法。PCR 和 BC 的不同 Ct 值截断值下的灵敏度和患病率变化可能表明潜在疾病定义的变化。在较低的 PCR Ct 值截断值下，潜在疾病定义可能是真正/严重感染的奶牛，而在较高的 PCR Ct 值截断值下，疾病定义可能是金黄色葡萄球菌阳性的奶牛。
