A retrospective case-control study was undertaken to investigate the temporal pattern of, and factors associated with, the survival of BVD virus-positive calves, identified between January and July 2012 during the voluntary phase of the Irish national eradication programme. Potential statuses for case and control calves consisted of: alive in birth herd; slaughtered; sold; dead (due to culling or death from natural causes and termed 'involuntary removal'). An initial comparison of cases and controls found significant differences between the outcomes for cases and controls and also between cases in relation to herd type (beef, dairy, dual purpose), sex, age and test status (BVDPOS - no confirmatory test; BVDPI - positive on confirmatory test). Key differences included a higher level of case animals still alive in, or slaughtered from, beef herds, a greater proportion of BVDPI animals being retained relative to those with a BVDPOS status and a significantly lower slaughter weight (89kg) for case animals relative to controls. Separate multivariable models were constructed for dairy and beef cases. In the final dairy model breed (Jersey or non-Jersey), county and BVD status were retained, with the last two found to be time-varying covariates with significant changes in hazard ratios (HR) over time. In the beef model, herd size, county and BVD status were retained, with the HR for the last two factors again varying significantly over time. With the exception of the addition of the number of BVD positive calves in the herd to the dairy model, the same factors were identified when models were restricted to the first 90 days following the birth of case animals. A greater knowledge and understanding of all of these factors will allow refinement of programme communications and incentives to encourage prompt removal of PI calves from all sectors of the Irish breeding herd during the compulsory phase of the national eradication programme.
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