A Study of Mastitis in Dairy Herds with Particular Reference to Streptococcus uberis

Streptococcus uberis (S. uberis) is the infectious agent in a significant proportion of cases of subclinical mastitis in dairy cows in the British Isles, and in some cases of clinical mastitis. It is a pathogen which can exist outwith the bovine mammary gland parenchyma, having been isolated from bovine skin, ruminal fluid, faeces and bedding. Using DNA typing of individual strains of S. uberis, it was hoped that an epidemiological survey of a herd infected with S. uberis could resolve whether all or only some strains of S. uberis (and Streptococcus parauberis) found on cows' skin and in the environment could gain access to the glandular tissue and cause mastitis. In order to find a farm where S. uberis was endemic in the dairy herd, milk and other samples were examined from seven herds in south-west Scotland that were likely to have a subclinical S. uberis problem. However, bacterial analysis of the milk samples collected showed that Staphylococcus aureus (Staph. aureus) was the predominant pathogen causing mastitis in the herds. The infrequent isolation of S. uberis from only one of the herds suggested the possibility that the cows' immunological control of S. uberis infection was incidental to having to deal with the constant Staph, aureus infection and, were the latter resolved, infection by S. uberis may become a more noticeable and serious problem. A protocol was developed to screen a large number of milk samples for the presence of S. uberis employing as few biochemical tests as possible, consistent with accurate identification; the tests were fermentation of inulin followed by haemolysis of blood agar and hydrolysis of aesculin and hippurate. In addition, a new "enrichment" broth consisting of equal volumes of Todd Hewitt and tryptose broths was found to produce a greater harvest of S. uberis colony forming units than conventional enrichment broths: The use of litmus milk as an enrichment broth was also discussed.