The present invention pertains to a substance or method for preventing or treating mastitis in ruminant animals, particularly in dairy cows. Specifically provided are: a ruminant-animal-mastitis preventive or therapeutic agent characterized by containing, as an active ingredient, Bacillus genus bacteria having the property of making it possible to increase or maintain CD11c+ CD172a+ blood dendritic cell gate 2 in cows； a feed additive； feed； or a drug composition.本發明係關於一種用於預防或治療反芻動物、尤其是乳牛之乳房炎之物質或方法，具體而言，本發明提供一種反芻動物之乳房炎之預防或治療劑、飼料添加物、飼料、或醫藥組合物，該預防或治療劑、飼料添加物、飼料、或醫藥組合物之特徵在於：包含具有以下性質之桿菌屬細菌作為有效成分，即，可於牛體內增加或維持CD11c+CD172a+血中樹狀細胞閘2。
