| The incidence of new intramammary infection (NIMI ) is a key determinant of the prevalence of subclinical intramammary infection (IMI) in dairy herds. Determining the incidence of NIMIs, however, and studying factors affecting this incidence requires extensive longitudinal data, which are difficult and expensive to collect. The publication of the operating characteristics of traditional diagnostics and single quarter milk samples, along with a large, multilevel data collection platform from the Canadian Bovine Mastitis Research Network presented an opportunity to address a number of factors affecting the prevalence of IMIs and the incidence of NIMIs in Canadian dairy cows.;Firstly, the organization and overview of the data collected through the National Cohort of Dairy Farms was described and documented for use by scientists worldwide. These data were used to estimate effects of freezing on the outcome of milk culture in order to further inform researchers performing bacteriological analyses on frozen samples. Recovery of most of the major pathogens was reduced, but recovery of coagulase-negative staphylococci ( CNS) was not affected by long-term freezing. Secondly, the use of composite samples for detection of IMI and NIMI for all types of mastitis-causing bacteria was investigated, and the operating characteristics of these samples relative to the results of a set of quarter samples were described. Although composite samples showed lower sensitivity than quarter samples, these samples did demonstrate high to moderate sensitivity for most mastitis pathogens, and high specificity for almost all pathogens.;Controversy surrounds the effect of the presence of minor mastitis pathogens -- Corynebacterium spp. and CNS -- on the risk of quarters acquiring infections with major pathogens, and an in-depth investigation of this question incorporating susceptibility factors and somatic cell measures was undertaken. Few statistically significant effects of minor pathogens were seen, although CNS appeared to be a risk factor for acquiring a NIMI with S. aureus, and increased quarter-level susceptibility was associated with increased risk of major pathogen NIMI. To complement these analyses, a systematic review and meta-analysis was conducted to critically assess the extant literature and investigate factors contributing to heterogeneity among studies. Overall, observational studies showed no effect of the minor pathogens, while challenge studies showed strong and significant protective effects, specifically when major pathogens were introduced into the mammary gland using methods which by-passed the teat end.;Finally, the clustering of NIMI within dairy operations was addressed. Quarters from cows with an infection in the ipsilateral quarter were always more likely to acquire a NIMI, and having a number of quarters infected with a specific pathogen was a significant risk factor for acquisition of a NIMI with that pathogen. Substantial clustering was found at the herd, cow and quarter levels for all pathogens investigated.;The availability of a large amount of quality data makes the analyses detailed in this thesis powerful and definitive. These analyses help inform the dairy industry regarding factors affecting the incidence of NIMI and provide more information aimed at decreasing new infection in dairy cows, thereby aiding in control of mastitis. |
