Provided is a method for determining whether or not a ruminant has mastitis. The method includes testing a milk sample from the ruminant to determine Fusobacterium necrophorum as a percentage of bacteria in the milk sample. The ruminant is diagnosed as having mastitis if the percentage of Fusobacterium necrophorum is >3.3% of the bacteria in the milk sample, or the ruminant is diagnosed as not having mastitis if the percentage of Fusobacterium necrophorum is <3.3% of the total bacteria in the milk sample. In another aspect the disclosure provides a method for determining that a sample of milk was obtained from a ruminant which does not have clinical mastitis. The method involves testing a milk sample from the ruminant to determine Fusobacterium necrophorum as a percentage of bacteria in the milk sample, and identifying the sample of milk as having been obtained from a ruminant that does not have mastitis if the Fusobacterium necrophorum is less than 3.3% of the bacteria in the milk sample. The ruminants tested include bovine animals. Compositions and methods for prophylaxis against mastitis in ruminants are also provided and involve veterinary compositions which contain whole cells of Fusobacterium necrophorum, Fusobacterium necrophorum leukotoxin (LKT), or a combination thereof.Linvention concerne une méthode permettant de déterminer si un ruminant est atteint ou non de mastite. Cette méthode consiste à analyser un prélèvement de lait du ruminant afin de déterminer un pourcentage de bactéries Fusobacterium necrophorum dans le prélèvement. La mastite est diagnostiquée si le pourcentage de Fusobacterium necrophorum est >3,3% de bactéries dans le prélèvement de lait, ou le diagnostic de mastite est écarté si le pourcentage de Fusobacterium necrophorum est <3,3% de bactéries totales dans le prélèvement. Dans un autre aspect, linvention concerne une méthode permettant de déterminer quun prélèvement de lait provient dun ruminant ne présentant pas de mastite clinique. Cet
