Herd-level factors associated with the presence of bovine viral diarrhoea virus in herds participating in the voluntary phase of the Irish national eradication programme

Abstract

A risk factor study was conducted to identify variables associated with initial positive or inconclusive results for bovine viral diarrhoea virus (BVDV) in ear punch samples collected from calves between 1st January and 15th July 2012 (the study period) as part of the voluntary phase of the Irish national BVD eradication programme based on testing of ear tag tissue samples from calves born in participating herds. Univariable analysis indicated significant associations with the following factors: herd type; the number of cows in the herd; the number of calves born in the study period; the number of calves tested in the study period; the number of cattle purchased in 2011, between 2009 and 2011 and between 2007 and 2011; the number of tested calves whose dams had been purchased within 9 months of their calving date; and the percentage of calf mortality within 28 days of birth. The percentage of the cows in each herd that was homebred, location (province) the number of separate land parcels used by each herd, the presence of an associated sheep enterprise and the purchase of cattle through marts were not found to be significant. An initial logistic regression model was developed to model the probability of a herd having one or more BVD virus-positive or inconclusive calves. When vaccination status was initially excluded, province, log of the numbers of cows in the herd, the number of cows purchased between 2009 and 2011, the number of tested calves whose dams had been purchased within 9 months of their calving date and calf mortality were significant. When vaccination status was included, using a subset of the data based on farmers responding to a survey on vaccination status, it was retained as a significant variable along with the same variables already listed, showing a significant 2-way interaction with the log of the number of cows. There was not a significant association between an initial positive or inconclusive result and the length of time for which herds had been vaccinating. The study provides a series of key communication messages relating to both the delivery of, and benefits from, the national eradication programme.