Department of Animal and Range Sciences, Montana State University, Bozeman, MT 59717, USA.
Department of Animal Science, University of Wyoming, Laramie, WY 82071, USA.
J Anim Sci. 2021 Apr 1;99(4). doi: 10.1093/jas/skab059.
Mastitis is an economically important disease and its subclinical state is difficult to diagnose, which makes mitigation more challenging. The objectives of this study were to screen clinically healthy ewes in order to 1) identify cultivable microbial species in milk, 2) evaluate somatic cell count (SCC) thresholds associated with intramammary infection, and 3) estimate relationships between udder and teat morphometric traits, SCC, and ewe productivity. Milk was collected from two flocks in early (<5 d) and peak (30 to 45 d) lactation to quantify SCC (n = 530) and numerate cultivable microbial species by culture-based isolation followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; n = 243) identification. Within flock and lactation stage, 11% to 74% (mean = 36%) of samples were culture positive. More than 50 unique identifications were classified by MALDI-TOF MS analysis, and Bacillus licheniformis (18% to 27%), Micrococcus flavus (25%), Bacillus amyloliquefaciens (7% to 18%), and Staphylococcus epidermidis (26%) were among the most common within flock and across lactation stage. Optimum SCC thresholds to identify culture-positive samples ranged from 175 × 103 to 1,675 × 103 cells/mL. Ewe productivity was assessed as total 120-d adjusted litter weight (LW120) and analyzed within flock with breed, parity, year, and the linear covariate of log10 SCC (LSCC) at early or peak lactation. Although dependent on lactation stage and year, each 1-unit increase in LSCC (e.g., an increase in SCC from 100 × 103 to 1,000 × 103 cells/mL) was predicted to decrease LW120 between 9.5 and 16.1 kg when significant. Udder and teat traits included udder circumference, teat length, teat placement, and degree of separation of the udder halves. Correlations between traits were generally low to moderate within and across lactation stage and most were not consistently predictive of ewe LSCC. Overall, the frequencies of bacteria-positive milk samples indicated that subclinical mastitis (SCM) is common in these flocks and can impact ewe productivity. Therefore, future research is warranted to investigate pathways and timing of microbial invasion, genomic regions associated with susceptibility, and husbandry to mitigate the impact of SCM in extensively managed ewes.
乳腺炎是一种具有重要经济意义的疾病，其亚临床状态难以诊断，这使得缓解病情更加具有挑战性。本研究的目的是对临床健康的母羊进行筛选，以 1）确定牛奶中可培养的微生物种类，2）评估与乳腺炎相关的体细胞计数（SCC）阈值，3）估计乳房和乳头形态特征、SCC 与母羊生产性能之间的关系。在泌乳早期（<5 d）和高峰期（30 至 45 d）采集两个羊群的牛奶，以定量 SCC（n = 530），并通过基于培养的分离 followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; n = 243) 鉴定来计数可培养的微生物种类。在每个羊群和泌乳阶段，11%至 74%（平均值为 36%）的样本为培养阳性。通过 MALDI-TOF MS 分析可对超过 50 种独特的鉴定进行分类，在每个羊群和整个泌乳阶段，最常见的微生物包括地衣芽孢杆菌（18%至 27%）、黄色微球菌（25%）、解淀粉芽孢杆菌（7%至 18%）和表皮葡萄球菌（26%）。根据 SCC 阈值的不同，将 SCC 范围为 175×103 至 1675×103 细胞/ml 的样本定义为培养阳性。母羊的生产性能评估为 120 天调整后的总产仔数（LW120），并在每个羊群内根据品种、胎次、年份以及泌乳早期或高峰期 SCC 的对数 10 转换值（LSCC）的线性协变量进行分析。尽管取决于泌乳阶段和年份，但当 SCC 每增加 1 个单位（例如，SCC 从 100×103 增加到 1000×103 细胞/ml）时，预测每个羊群的 LW120 将减少 9.5 至 16.1 kg。乳房和乳头特征包括乳房周长、乳头长度、乳头位置以及乳房两半之间的分离程度。在泌乳阶段内和泌乳阶段之间，特征之间的相关性通常较低，处于中度水平，并且大多数特征与母羊 LSCC 之间没有一致的相关性。总体而言，细菌阳性牛奶样本的频率表明，这些羊群中普遍存在亚临床乳腺炎（SCM），并会影响母羊的生产性能。因此，需要进行未来的研究，以调查微生物入侵的途径和时间、与易感性相关的基因组区域以及管理措施，以减轻集约化管理的母羊中 SCM 的影响。
