Studies on the epidemiology of Johne's disease and their impact on control programs in Minnesota dairy herds

This dissertation describes aspects of the epidemiology of Johne's' disease (JD) and their impact on control programs in Minnesota dairies. The first study evaluated the distribution of Mycobacterium avium subsp paratuberculosis (Map) in the environment of 108 Minnesota dairies (including 80 and 28 herds that are likely to be infected and noninfected based on previous testing), the estimation of within-herd prevalence of fecal shedding using fecal pool culture on the same farms, and the prevalence of Map in deer and rabbit pellets on 114 farms. The second study evaluated risk factors (clinical and subclinical diseases) for the onset of clinical JD or fecal shedding by the end of the lactation on two Minnesota dairies. The impact of fecal shedding and clinical JD on lactation performance was also evaluated. The third study analyzed 1,200 Risk Assessments (RA) from 714 herds participating in the Minnesota JD control program (JDCP), characterized these herds and evaluated the RA tool.; Of the known infected herds, 80% had at least one culture-positive pool. The farm environment was detected as contaminated on 95% of the herds with positive pools, mainly in cow alleyways (77% of the herds) and manure storage (68%). Rabbit and deer samples were positive on 2% (rabbit) and 4% (deer) of farms.; In multivariable analysis, clinical JD was associated with pneumonia (OR = 2.6) and level of fecal shedding (light: OR = 13.0, moderate: OR = 33.0, heavy: OR = 63.0), and onset of fecal shedding was associate only with pneumonia (OR = 2.2). Fecal culture positive cows produced 700 kg less and stayed in the herd on average 81 d less than their negative herdmates. Clinical JD cows produced on average 3,300 kg milk less and left the herd on average 161 d earlier than all the cows that completed the study.; Mean herd size and annual milk production per cow was higher in JDCP herds than for all Minnesota dairies.