Seroprevalence, production impacts, economics and risk factors of Mycobacterium avium subspecies paratuberculosis in Canadian dairy cattle

The purpose of this thesis was to determine the seroprevalence, production impacts, economics and risk factors associated with Mycobacterium avium subspecies paratuberculosis (MAP) in Canadian dairy herds. Seroprevalence of bovine leukemia virus (BLV) and bovine viral diarrhoea virus (BVDV), and production impacts of BLV, BVDV and Neospora caninum (NC) were also estimated. Within 373 randomly selected Canadian herds (from New Brunswick, Nova Scotia, Prince Edward Island, Quebec, Ontario, Manitoba, Saskatchewan and Alberta), a serum sample was obtained from approximately 30 randomly selected lactating animals for ELISA testing for exposure to BLV, MAP and NC, while 5 unvaccinated cattle over 6 months of age were selected for virus neutralization testing for exposure to BVDV.; Seroprevalence at the cow level was 30.3% and 3.1% for BLV and MAP, respectively. At the herd level, 76.6% had at least one seropositive cow for BLV, 31.2% had at least one animal with a titer >1:64 for BVDV, and 18.9% had at least two seropositive cows for MAP. The Global Moran's I statistic for BLV and MAP indicated significant positive spatial autocorrelation of 0.16 and 0.15. The K-function for BVDV did not indicate any significant spatial autocorrelation. Using the spatial scan statistic, 4 and 1 significant clusters were detected for BLV and MAP-seroprevalence, respectively. No significant clusters were detected for BVD.; Regarding 305 day milk production (305 d milk), MAP-seropositivity was significantly associated with a lower 305 d milk of 212 kg in 4-plus lactation cows. NC-seropositivity was associated with a lower 305 d milk of 158 kg in primiparous cows compared to NC-seronegative primiparous cows. Cows in BVDV-seropositive herds (at least one unvaccinated animal with a titer ≥ 1:64) had significantly lower 305 d milk (by 368 kg) compared to cows in BVDV-seronegative herds. Among 1st lactation animals, NC-seropositivity was associated with a significant reduction of 5.5 kg and 3.3 kg of 305 d fat and 305 d protein yields, respectively. However, there were no statistically significant effects of MAP-seropositivity on 305 d fat or 305 d protein yields. BVDV-seropositivity (at the herd level) was associated with reductions in 305 d fat and 305 d protein yields of 10.20 kg/cow and 9.46 kg/cow, respectively. Regarding somatic cell counts, NC- (4+ lactation) and MAP-seropositivity were associated with a 0.094 decrease and 0.098 increase in the lactational mean of the log 10 somatic cell count. The cows in BVDV-seropositive herds had a significant increase in the mean log10 somatic cell count of 0.096. BLV-seropositivity was not associated with 305 d milk, 305 d fat, 305 d protein or mean log 10 somatic cell count. There was no significant effect modification of any of the outcomes examined from interaction between seropositivity for any of the pathogens.; For all reasons of culling, MAP-seropositive cows had a 1.38 (95% C.I., 1.05--1.81) times increased hazard of culling compared to MAP-seronegative cows. Seropositivity for the other pathogens was not associated with an increased risk of overall culling. Among cows that were culled because of either decreased reproductive efficiency or decreased milk production or mastitis, MAP-seropositive cows were associated with 1.55 (95% C.I., 1.12--2.15) times increased hazard compared to MAP-seronegative cows. Among cows that were culled because of reproductive inefficiency, NC-seropositive cows had a 1.43 (95% C.I., 1.15--1.79) times greater hazard than NC-seronegative cows. Among cows that were culled because of decreased milk production, cows in BVDV-seropositive herds had a 1.86 (95% C.I., 1.28--2.70) times increased hazard compared to cows in BVDV-seronegative herds. BLV-seropositive cows did not have an increased risk of reason-specific culling as compared to BLV-seronegative cows. No significant interaction on culling among seropositivity for the pathogens was detected, but only a limited number of...