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Background: BVD and IBR are contagious viral diseases highly prevalent in Irish cattle. Despite their significant reproductive and economic impact very little is known about the BVD and IBR status of stock bulls (a bull used for breeding purposes). There are still a high proportion of dairy farms in Ireland that rely on the use of a bull for breeding cattle and ensuring the fertility of the bulls is of paramount importance for the efficiency of the farms. The prevalence of BoHV-1 and BVD in stock bulls in Irish dairy herds has never been investigated. The objectives of this study therefore were: (i) to provide descriptive, observational data on the use of stock bulls on Irish dairy farms; (ii) to investigate the BVD and BoHV1 status of a sub-set of stock bulls; (iii) to investigate factors associated with BVD and BoHV1 status of stock bulls and (iv) to investigate factors associated with dairy herd status for BVD and BoHV1, including any associations with the use of stock bull. A total of 529 blood samples from bulls involved in the dairy breeding process were analysed for BVD virus using RT-PCR, and BoHV-1 antibodies by ELISA test. A total of 305 different dairy herds took part in the study and the overall BVD and BoHV-1 herd status was determined by ELISA using four bulk tank milk samples over the 2009 lactation. Logistic regression was used to investigate the associations between the stock bulls and BVD and BoHV-1 herd and individual status. Results: Of the 305 total participating farms, 235 farms (77 %) had at least one bull and 167 farms had purchased bulls. Two bulls (0.4 %) out of 529 tested were found positive for BVD virus and 87 (16.7 %) tested seropositive for BoHV-1. Some significant associations were identified between the purchase of bulls and both viral diseases. Purchased bulls were three times more likely to be seropositive for BoHV-1 than homebred bulls. In the same way, herds with purchased bulls were three times more likely to be classified as seropositive for BVD and four times more likely to have evidence of recent BoHV-1 circulation than farms where all the bulls were homebred. Conclusions: The prevalence of BoHV-1 and BVD in stock bulls in Irish dairy herds has never been investigated. This study highlights the widespread use of stock bulls in Irish dairy herds, as well as the high rate of exchange of bulls between farms. Significant associations were found between the origin of the bull and their serological BoHV-1 status. In keeping with these results, bulls with higher number movements between farms were more likely to be seropositive for BoHV-1. Keyword: Prevalence, BVD, BoHV-1, Bulls * Correspondence: [email protected] Animal and Biosciences Department, Animal & Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland Full list of author information is available at the end of the article © 2015 Martinez-Ibeas et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Martinez-Ibeas et al. Irish Veterinary Journal (2015) 68:28 DOI 10.1186/s13620-015-0059-9 Vet September 2017.indd 2 05/09/2017 16:56 Background Artificial insemination (AI) has become the breeding norm on the majority of dairy farms in the Republic of Ireland [1]. However, the stock bull (a bull used for breeding purposes) also remains an integral part of the breeding process on many farms, either being relied on as a sole means of impregnating cows or as an adjunct to AI later in the breeding season [2]. Milk production systems in Ireland are primarily pasture-based and involve seasonal calving [3]. Over 80 % of Irish dairy farms are spring-calving with the majority of cows calving down in the first 6 months of the year and 21 % of all calves are born in March alone [4]. Compact spring-calving is essential to the efficiency of farms which operate extensive grass-based systems as calving is timed to coincide with the period of maximal grass growth [3]. Calves born later in the year will reduce farm efficiency [5]. A key breeding goal in a spring-calving system is to achieve high 3 and 6-week pregnancy rates from breeding start date to achieve a concentrated calving pattern the next season [6]. Bulls can be an efficient means of getting cows in calf as the breeding season progresses and also reduce the need for continuing heat detection [2]. Use of 6 weeks of AI followed by release of bulls to complete the breeding season is, therefore, a common reproductive strategy on many Irish dairy farms [1]. For the purposes of genetic diversity, these bulls are often purchased and are only introduced to the female breeding herd during the breeding season. Ensuring the health of these bulls is of paramount importance, therefore, in terms of bull fertility and disease transmission between cows and bulls. Bovine Viral Diarrhoea (BVD) and Infectious Bovine Rhinotracheitis (IBR) caused by BVD virus (BVDv) and bovine herpesvirus 1 (BoHV-1), respectively, are highly contagious diseases of economic and trade importance. Both are listed as notifiable diseases by the World Organisation for Animal Health (OIE) and are prevalent in Ireland [7]. Several European countries have implemented programmes to eradicate both diseases to facilitate free trade of cattle, semen, and embryos within the European Union (EU). Ireland embarked on a compulsory national eradication programme for BVD in 2013 based on identification of BVD persistently infected (PI) calves. Movement restrictions are placed on individuals testing BVD virus positive [8]. As of yet, no regulation exists for control and eradication of BoHV-1. Cattle movement has been explored by many authors as a root cause of disease spread [9, 10]. Many Irish farmers operate what they perceive as closed herds i.e. no cattle introductions to the farm [7]. It has previously been highlighted, however, that discrepancies exist between what Irish dairy farmers perceive as a closed herd and a herd that does not engage in purchase of any cattle [7]. Bulls are often an ‘overlooked’ purchase in the maintenance of a closed herd. Breeding bulls, as with any purchased animal [9], are capable of introducing disease to a farm and may act as a vehicle through which diseases such as BVD and BoHV-1 can be transmitted. BVD and BoHV-1 can both adversely affect the fertility on a dairy farm with potentially devastating consequences for achieving a compact breeding season [11–14]. The levels of BVD and BoHV-1 infection amongst dairy stock bulls in Ireland have never been examined nor has the degree of bull movement between farms. Therefore, the objectives of this study were (i) to provide descriptive, observational data on the use of stock bulls on Irish dairy farms; (ii) to investigate the BVD and BoHV1 status of a sub-set of stock bulls; (iii) to investigate factors associated with BVD and BoHV1 status of stock bulls and (iv) to investigate factors associated with dairy herd status for BVD and BoHV1, including any associations with the use of stock bulls.