Independent researcher, Amqui, Québec, Canada G5J 2N5.
Faculté de Médecine Vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec, Canada J2S 2M2.
J Dairy Sci. 2024 Sep;107(9):7221-7229. doi: 10.3168/jds.2024-24693. Epub 2024 May 23.
The objective of this cross-sectional study was to estimate the validity of laboratory culture, Petrifilm and Tri-Plate on-farm culture systems, as well as luminometry to correctly identify IMI at dry-off in dairy cows, considering all tests to be imperfect. From September 2020 until December 2021, we collected composite milk samples from cows before dry-off and divided them into 4 aliquots for luminometry, Petrifilm (aerobic count), Tri-Plate, and laboratory culture tests. We assessed multiple thresholds of relative light units (RLU) for luminometry, and we used thresholds of ≥100 cfu/mL for the laboratory culture, ≥50 cfu/mL for Petrifilm, and ≥1 cfu for Tri-Plate tests. We fitted Bayesian latent class analysis models to estimate the sensitivity (Se) and specificity (Sp) for each test to identify IMI, with 95% credibility interval (BCI). Using different prevalence measures (0.30, 0.50, and 0.70), we calculated the predictive values (PV) and misclassification cost terms (MCT) at different false negative-to-false-positive ratios (FN:FP). A total of 333 cows were enrolled in the study from one commercial Holstein herd. The validity of the luminometry was poor for all thresholds, with an Se of 0.51 (95% BCI = 0.43-0.59) and Sp of 0.38 (95% BCI = 0.26-0.50) when using a threshold of ≥150 RLU. The laboratory culture had an Se of 0.93 (95% BCI = 0.85-0.98) and Sp of 0.69 (95% BCI = 0.49-0.89); the Petrifilm had an Se of 0.91 (95% BCI = 0.80-0.98) and Sp of 0.71 (95% BCI = 0.51-0.90); and the Tri-Plate had an Se of 0.65 (95% BCI = 0.53-0.82) and Sp of 0.85 (95% BCI = 0.66-0.97). Bacteriological tests had good PV, with comparable positive PV for all 3 tests, but lower negative PV for the Tri-Plate compared with the laboratory culture and the Petrifilm. For a prevalence of IMI of 0.30, all 3 tests had similar MCT, but for prevalence of 0.50 and 0.70, the Tri-Plate had higher MCT in scenarios where leaving a cow with IMI untreated is considered to have greater detrimental effects than treating a healthy cow (i.e., FN:FP of 3:1). Our results showed that the bacteriological tests have adequate validity to diagnose IMI at dry-off, but luminometry does not. We concluded that although luminometry is not useful to identify IMI at dry-off, the Petrifilm and Tri-Plate tests performed similarly to laboratory culture, depending on the prevalence and importance of the FP and FN results.
本横断面研究的目的是评估实验室培养、Petrifilm 和 Tri-Plate 农场培养系统以及发光计在奶牛干奶期正确识别 IMI 的有效性，考虑到所有测试都不完美。从 2020 年 9 月至 2021 年 12 月，我们在奶牛干奶前收集了复合牛奶样本，并将其分为 4 份进行发光计、Petrifilm（需氧计数）、Tri-Plate 和实验室培养测试。我们评估了发光计的多个相对光单位 (RLU) 阈值，我们使用实验室培养的≥100 cfu/mL、Petrifilm 的≥50 cfu/mL 和 Tri-Plate 的≥1 cfu 的阈值。我们拟合了贝叶斯潜在类别分析模型来估计每种测试识别 IMI 的敏感性 (Se) 和特异性 (Sp)，置信区间为 95%（BCI）。使用不同的流行率测量值（0.30、0.50 和 0.70），我们计算了不同假阴性与假阳性比（FN：FP）的预测值 (PV) 和误分类成本项 (MCT)。从一个商业荷斯坦牛群中总共纳入了 333 头奶牛进行研究。对于所有阈值，发光计的有效性都很差，当使用≥150 RLU 的阈值时，Se 为 0.51（95% BCI = 0.43-0.59），Sp 为 0.38（95% BCI = 0.26-0.50）。实验室培养的 Se 为 0.93（95% BCI = 0.85-0.98），Sp 为 0.69（95% BCI = 0.49-0.89）；Petrifilm 的 Se 为 0.91（95% BCI = 0.80-0.98），Sp 为 0.71（95% BCI = 0.51-0.90）；Tri-Plate 的 Se 为 0.65（95% BCI = 0.53-0.82），Sp 为 0.85（95% BCI = 0.66-0.97）。细菌学测试具有良好的 PV，所有 3 种测试的阳性 PV 相当，但与实验室培养和 Petrifilm 相比，Tri-Plate 的阴性 PV 较低。对于 IMI 流行率为 0.30，所有 3 种测试的 MCT 相似，但对于流行率为 0.50 和 0.70，Tri-Plate 在不治疗有 IMI 的奶牛比治疗健康奶牛的不利影响更大的情况下（即 FN：FP 为 3：1）具有更高的 MCT。我们的结果表明，细菌学测试具有足够的有效性来诊断奶牛干奶期的 IMI，但发光计则不然。我们得出的结论是，尽管发光计在识别奶牛干奶期的 IMI 方面没有用处，但 Petrifilm 和 Tri-Plate 测试与实验室培养的效果相似，具体取决于 FP 和 FN 结果的流行率和重要性。
