Establishment of optimal control strategies to eliminate bovine viral diarrhoea in New Zealand : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Epidemiology at Massey University, Manawatu, New Zealand

Abstract

Although there has been a noticeable reduction in the prevalence of bovine viral diarrhoea virus (BVDV) in New Zealand over the past decade, it is well recognised that New Zealand will need a systematic compulsory programme to eliminate BVDV. The aims of this thesis were to address the knowledge gaps around the epidemiology and economics of BVDV to explore the cost-effectiveness of different national BVDV control frameworks. First, the risk factors for BVDV infection were explored using data collected from cattle herds across the country. A Bayesian network analysis revealed that animal contacts between neighbouring farms significantly increased the risk of herds being seropositive for BVDV. The second study used data collected from New Zealand commercial beef farms to estimate the transmission rate of BVDV from extensively grazed persistently infected (PI) animals. Using an approximate Bayesian computation method, the BVDV transmission rate was estimated at 0.11 per PI animal per day, which was lower than previously derived estimates for dairy herds and intensively farmed beef herds. For the third study, BVDV simulation models were developed for New Zealand dairy and beef farms to estimate the economic impacts of BVDV outbreak and to identify the most cost-effective control strategies at an individual farm level. The direct losses due to BVDV outbreak were estimated as NZ$ 22.22 per dairy cow per year and NZ$ 41.19 per beef cow per year. Annual testing to cull identified PI calves and annual vaccination were economically beneficial to control a BVDV outbreak for a dairy and beef breeding farm, respectively. In the fourth study, BVDV transmission was simulated at a national scale with the models, predicting that BVDV could be successfully and economically controlled by requiring dairy farms to double fence boundaries and perform either annual calf testing or herd-level screening test and requiring beef farms to conduct annual vaccination. Overall, the findings from the thesis highlight that BVDV elimination is both technically feasible and cost-efficient in New Zealand. The outputs of this thesis can be used to facilitate discussion with farmers and stakeholders about the benefits and feasibility of national BVDV elimination in New Zealand.