Department of Production Animal Clinical Sciences, Faculty of Veterinary Medicine, Norwegian University of Life Sciences, Oslo, Norway.
Department of Production Animal Clinical Sciences, Faculty of Veterinary Medicine, Norwegian University of Life Sciences, Oslo, Norway.
J Dairy Sci. 2019 Sep;102(9):8385-8399. doi: 10.3168/jds.2018-16155. Epub 2019 Jul 10.
The primary objective of the present study was to estimate the effect of Streptococcus agalactiae intramammary infection on milk production and somatic cell count (SCC) in Norwegian dairy cows. A secondary objective was to assess differences in the effect of common Strep. agalactiae sequence types (ST) found in Norwegian dairy herds. We performed a cohort study combining registry data with sequence-type data from Strep. agalactiae isolates. Herds in which Strep. agalactiae had been detected in individual animals (bacteriological culture or quantitative PCR) between 2012 and 2015 were included. We accessed monthly test-day milk yield records for the entire period to compare milk yield and SCC between cows that were Strep. agalactiae positive and all other cows, within each herd. The study sample consisted of 150 herds, 15,757 cows, 30,850 lactations, and 204,126 test days. We evaluated the effects of Strep. agalactiae on test-day milk yield and SCC using mixed linear regression models, controlling for clustering by herd, cow, and lactation. Multilocus sequence typing of Strep. agalactiae was available for isolates from 86 herds. Additional models were fit to a subset of herds (n = 59) in which ST1, ST23, ST103, and ST196 had been found, to compare the effects of ST on milk production and SCC. In the period 3 to 2 mo before diagnosis, Strep. agalactiae-positive cows produced an average of 1.3 kg more DIM-adjusted milk/d than their negative herd mates. At the time of diagnosis, production was on average 0.13 kg less DIM-adjusted milk/d in Strep. agalactiae-positive cows than in negative cows; 2 to 3 mo after diagnosis, they produced 1.24 kg less DIM-adjusted milk/d than negative cows. Losses persisted for the rest of the investigated period. Cows with ST23, ST103, and ST196 followed a similar pattern as the overall analysis with respect to milk production, whereas ST1-affected cows produced similar amounts of milk before diagnosis as the negative cows. Cows with ST1 experienced the largest milk loss 1 to 2 mo after diagnosis but then recovered to some extent; for cows with ST103, the severe milk loss persisted for the rest of the investigation period. The cow-associated ST103 elicited a lower response in peak SCC compared with ST23, ST103, and ST196. The results indicate an effect of Strep. agalactiae on milk production and SCC. Production was lowest 2 to 3 mo after a positive sample. Peak SCC was reached the month before diagnosis, with notable differences between sequence types.
本研究的主要目的是估计无乳链球菌（Streptococcus agalactiae）乳房内感染对挪威奶牛产奶量和体细胞计数（Somatic Cell Count，SCC）的影响。次要目的是评估在挪威奶牛群中常见的无乳链球菌序列型（Streptococcus agalactiae sequence types，ST）的影响差异。我们进行了一项队列研究，将登记数据与无乳链球菌分离株的序列型数据相结合。在 2012 年至 2015 年期间，对个体动物（细菌培养或定量 PCR）中检测到无乳链球菌的畜群进行了研究。我们获取了整个时期的每月测试日产奶记录，以比较每个畜群中无乳链球菌阳性牛和所有其他牛的产奶量和 SCC。研究样本包括 150 个畜群、15757 头奶牛、30850 个泌乳期和 204126 个测试日。我们使用混合线性回归模型评估了无乳链球菌对测试日产奶量和 SCC 的影响，通过畜群、奶牛和泌乳期进行聚类控制。无乳链球菌的多位点序列分型可用于 86 个畜群的分离株。在发现 ST1、ST23、ST103 和 ST196 的畜群（n=59）中拟合了额外的模型，以比较 ST 对产奶量和 SCC 的影响。在诊断前 3 至 2 个月期间，无乳链球菌阳性奶牛的 DIM 调整后产奶量平均比阴性畜群中的奶牛多 1.3 公斤。在无乳链球菌阳性奶牛的诊断时，其 DIM 调整后的产奶量平均比阴性奶牛少 0.13 公斤；在诊断后 2 至 3 个月，其 DIM 调整后的产奶量比阴性奶牛少 1.24 公斤。这种损失一直持续到调查期间的剩余时间。ST23、ST103 和 ST196 的奶牛在产奶量方面呈现出与整体分析类似的模式，而 ST1 感染的奶牛在诊断前与阴性奶牛的产奶量相似。ST1 感染的奶牛在诊断后 1 至 2 个月出现最大的产奶量损失，但随后有所恢复；对于 ST103 感染的奶牛，严重的产奶量损失持续到调查期间的剩余时间。与 ST23、ST103 和 ST196 相比，与奶牛相关的 ST103 在 SCC 峰值方面的反应较低。结果表明无乳链球菌对产奶量和 SCC 有影响。阳性样本后 2 至 3 个月产奶量最低。SCC 峰值在诊断前一个月达到，不同序列型之间存在显著差异。
