Biosecurity, bovine viral diarrhoea virus (BVDv), and bovine herpesvirus-1 (BoHV-1): epidemiological investigations in Irish dairy herds

The fundamental basis of herd health planning and disease control is a science-based risk analysis. A disease risk analysis involves examining the probability of a disease occurring and the impact of that disease should it occur. Epidemiological investigations are required to provide the necessary data on biosecurity, disease prevalence, vaccination, and production losses associated with a particular infectious agent for the purposes of a comprehensive risk analysis. BVDv and BoHV-1 are highly contagious cattle viruses, exhibit a worldwide distribution and are listed as notifiable diseases by the OIE (Office International des Epizooties). A national eradication scheme is currently underway in Ireland for BVDv but no co-ordinated programme exists for the control or eradication of BoHV-1. Relatively little published data are available on the prevalence or production losses associated with BVDv and BoHV-1 infection in Ireland. Neither are data available relating to current biosecurity and vaccination practices on Irish dairy farms. The research programme outlined in this thesis aimed to provide relevant Irish stakeholders, such as AHI (Animal Health Ireland) and DAFM (Department of Agriculture, Food and the Marine), the necessary data to conduct an informed risk analysis on BVDv and BoHV-1. Results show that biosecurity practices, in general, were poorly implemented on dairy farms with lack of information cited as the primary cause. The concept of biosecurity was not well understood, a 20% discrepancy highlighted between self-declared and truly ‘closed’ herds. Additionally, inconsistencies in biosecurity practices and opinions across veterinarians, dairy advisors, and farmers were highlighted emphasising the need for improved communication amongst these stakeholders. Poor implementation of biosecurity is reflected in the prevalence of exposure to BVDv and BoHV-1 recorded in the current study. From a geographically representative population of 312 dairy herds, an apparent prevalence of 88% and 80% was recorded for BVDv and BoHV-1, respectively. Additionally, approximately one third of participating herds were spot test (youngstock screen used to indicate recent viral circulation) positive for BVDv antibodies indicating extensive BVDv circulation in 2009. Over 60% of study herds vaccinated against BVDv and 12% against BoHV-1. Such extensive use of BVDv vaccine necessitated examination of the impact of vaccine administration on BVDv bulk milk testing as bulk milk analysis is likely to be an important tool in on-going national BVD surveillance. One commercially available inactivated BVD vaccine was shown not to interfere with BVD p80 bulk milk antibody interpretation making it a useful tool in BVD control during eradication. BVDv and BoHV-1 antibody positive herds were found to record inferior production performance than antibody negative herds. Herds with evidence of recent BVDv circulation and herds bulk milk antibody positive for BoHV-1 recorded the most significant production losses. These losses will act as important motivators in engaging farmers in national eradication and control programmes for both BVDv and BoHV-1. The baseline data documented in this thesis will form the basis for more detailed sociological, economic, and epidemiological research to facilitate targeting of future training of, and risk communication with, the dairy farming community in Ireland.