探討高體細胞數乳牛對於大腸桿菌誘發乳房炎之抵抗能力

Abstract

本實驗由臨床上隨機取樣9頭健康且泌乳中的牛隻，其中5頭牛的體細胞數落於正常範圍，其餘4頭牛的體細胞數偏高（體細胞數大於500 × 103 細胞數/毫升），針對此9頭牛的單一乳房進行E. coli攻毒實驗。攻毒實驗後，連續監測168小時內急性反應蛋白 haptoglobin（Hp）的濃度及體細胞數等具指標性的乳房炎指標。經分析後發現，起始體細胞數偏高的牛隻相對於體細胞數正常的牛隻對於E. coli攻毒試驗有較好的抵抗能力。 起始體細胞數落於正常範圍的牛隻，牛奶中的 Hp濃度在經過攻毒實驗 20 小時後會快速上升，相對於另一個乳房炎指標-體細胞數而言，Hp的濃度變化要早 4 個小時。選取起始體細胞數高低兩組中各兩頭牛隻所分泌的生乳與E. coli 共同培養，並觀測各組生長曲線差異。從 in vitro 的 E. coli 生長曲線實驗的對數生長時期中發現，加入起始體細胞數偏高的生乳可以明顯抑制 E. coli 生長。接著，我們發現存在於生乳中的內生性抗 E. coli 抗體活性在施以攻毒實驗之前，其抗體效價在起始體細胞數偏高的牛隻當中已有較高的趨勢。 利用取自於商業牧場隨機採樣 48 頭牛隻的生乳，我們證明出兩種引起乳房炎的常見病原菌 E. coli 和 S. aureus 於生乳當中確實會因為體細胞數較高而有較高的抗體效價（p < 0.05）；S. aureus 的抗體效價高 E. coli 將近 15 倍之多。由上述實驗可以證明，起始體細胞數較高的牛隻對於 E. coli 所引起的乳房炎有高的抵抗力，意味著藉由提高體細胞數可以防範由 E. coli 所引起的乳房炎。根據 ELISA 的實驗結果，E. coli 和 S. aureus 或是其他病原菌抗體效價的高低可以提供另一種監測乳房炎的參考價值。

Nine healthy lactating Holstein cows, five with low milk somatic cell counts (SCC < 160 × 103) and four with high SCC (> 500 × 103/mL) were challenged by a single intramammary E. coli injection. Acute phase protein, haptoglobin (Hp), and SCC were monitored as a mastitis index during a period of 168 h following challenge. Remarkable significant interestingly, healthy cows with high SCC were highly resistant in developing experimental mastitis as compared to those with low SCC. In responsive group, Hp peaked at h 20 which was earlier than SCC peaked at h 24. Subsequently, we found endogenous antibody that reacted with E. coli was present in milk prior to the challenge with the titers significantly higher in cows with high SCC. Using 48 cows from a commercial farm, we further demonstrated that antibodies reacted with E. coli and S. aureus, two mostly common mastitic pathogens, were both present in milk and significantly correlated to the number of SCC (p < 0.05). The antibody recognized S. aureus was approximately 15-fold higher relative to that E. coli. Our study demonstrated that healthy cows with high SCC are resistant to E. coli induced mastitis, suggesting that they are potentially beneficial against acute E. coli mastitis. Whether the high reactive antibody found in high SCC group plays a key role in host immunity remains elusive. Owing to the simplicity of ELISA, knowing the titers of antibody reacted with E. coli, S. aureus or other pathogens may provide a reference value for the surveillance of mastitis.