Surveillance and Control of Enzootic Bovine Leucosis in Atlantic Canada PDF Download

Author: Emily Elizabeth John
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Languages : en
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Book Description
This research included four studies investigating bovine leukemia virus (BLV) in Atlantic Canada (New Brunswick, Newfoundland and Labrador, Nova Scotia, and Prince Edward Island). The current herd-level prevalence of BLV in dairy herds in Atlantic Canada was determined in the first study. The within-herd prevalence was estimated based on testing bulk tank milk for anti-BLV antibody levels. A risk assessment and management program workbook was designed and implemented using a subset of dairy herds in Atlantic Canada to investigate management factors associated with the BLV estimated within-herd prevalence. Herd-level prevalence for BLV was high with at least 88% of dairy herds in Atlantic Canada containing at least one BLV-seropositive cow. Farms with a history of diagnosing clinical disease associated with BLV infection, as well as farms where calves received colostrum from cows of unknown BLV status, were more likely to have an estimated within-herd BLV prevalence of at least 25% compared to herds without a history of clinical disease or herds where calves did not receive colostrum from BLV-unknown or BLV-infected cows. The second study investigated the variability between bulk tank milk samples from the same herd collected over different sampling intervals, as well as the variability between samples collected from different farms. Farms in the Canadian Maritimes (New Brunswick, Nova Scotia, and Prince Edward Island) participated. Samples from 2013 were collected at 1-month intervals and those from 2016 were collected at 3-month intervals. Samples were analyzed for the presence of anti-BLV antibodies and estimated within-herd prevalence was calculated for each herd. The most variability was seen in samples collected from different herds, and the least variability was seen in samples from the same herd collected at 3-month intervals. This information can help in designing sample collection intervals for disease monitoring or control programs. The third study investigated the age group of calves, heifers, and adult cows in which BLV was first identified. Most previous research has focused on adult cows, but calves and heifers are also able to become infected with BLV prior to entering the milking herd. Blood samples were collected from 6 pre-weaned calves, 6 weaned heifers, and 6 breeding-age heifers on 56 dairy farms in Atlantic Canada, and a questionnaire was administered to farmers asking about age-specific management factors for these three age groups as well as for adult cows. The majority of farms had BLV first identified in either the adult milking herd or in pre-weaned calves. Management factors associated with the age group where BLV was first identified included the number of times weaned heifers were regrouped after weaning, the type of fly control used in breeding-age heifers, the use of foot-trimming implements in breeding-age heifers, and whether the farm purchased bred heifers. The final study investigated the utility of commonly available, cost-effective diagnostic tests to predict proviral load (PVL) in BLV-seropositive cows. It is suspected that cows with a higher PVL are a higher infection risk to naïve cows than those with a low PVL. Currently there is no commercial test for PVL in cows, and so blood and milk were collected from 339 BLV-seropositive cows as well as basic demographic information. PVL was determined using qPCR, and blood was also used for determining anti-BLV antibody levels and for performing complete blood counts. Milk samples were analyzed for standard components by the regional dairy herd improvement agency, as well as having anti-BLV antibodies measured. The milk antibody level, as well as blood lymphocyte count, were significantly associated with increasing PVL.