Evaluation of diagnostic and therapeutic approaches to clinical and subclinical intramammary infection in lactating dairy cows

This thesis is an investigation into the use of on-farm diagnostic tests to aid in the identification of mastitis causing organisms, as well as an evaluation of risk factors and therapeutic approaches for the cure of clinical mastitis in lactating dairy cows.;The HyMast(TM) milk bacteriological test system was found to have moderate sensitivity and specificity compared to traditional milk culture for determining whether the causative organism in case of clinical mastitis was gram-positive or gram-negative. The test was found to be inadequate for determining the exact bacteriological cause of an intramammary infection. Use of the HyMast(TM) test system did not improve the time interval to clinical cure or the likelihood of bacteriological cure compared to intramammary antibiotic therapy or frequent milk-out of the infected gland with the aid of oxytocin.;The probability of a bacteriological or clinical cure was influenced by several factors. However, it would appear that the determinants of bacteriological cure are different for cases of clinical mastitis caused by contagious versus environmental pathogens.;The California Mastitis Test was found to be a practical screening tool for use on cows with newly elevated somatic cell count values. Composite samples were less likely to yield major mastitis pathogens than quarter milk samples. Test day milk production, linear score and number of quarters sampled were found to be predictive of the probability of isolating a major mastitis pathogen.;A milking and farm management survey was used to obtain baseline information regarding the approach Ontario dairy producers employ when dealing with clinical mastitis, and to determine associations between approaches to therapy and milk quality parameters. Milk production and region were associated with the approach to treating mild and moderate cases of clinical mastitis. Herd demographic data, milk production and the approach to treating cases of clinical mastitis were found to be associated with the number of elevated bulk tank somatic cell count (BTSCC) episodes. The probability of an inhibitor violation increased with the number of elevated BTSCC episodes and with the average number of cows in a herd.