Evolution of Bovine Herpesvirus-1 infection prevalence and infection dynamics in Irish dairy herds following an IBR...

Bovine Herpesvirus-1 (BoHV-1) is endemic in the Irish cattle population, but there is no current formal eradication programme for this disease in the Republic of Ireland. This observational, prospective field study is aimed to assess the evolution of the BoHV-1 seroprevalence on fifteen Irish dairy farms (85-250 cows) between 2020 and 2023 following the implementation of a hyperimmunisation IBR vaccination programme. Eligible farms had positive bulk tank results and estimated within-herd seroprevalence ≥ 25%. Animals from three months of age received a live, monovalent, double-deleted (gE-/tk-) marker vaccine (six-monthly boosters). Annually, biosecurity was evaluated using a 44-item survey. Each year, approximately 20 randomly selected animals per farm (4 calves, 4 heifers, 12 cows) were tested by gE-ELISA; in addition, animals with a negative result were retested in subsequent years (N = 818 animals). Seroprevalence was analysed using a logistic regression model with year and the epidemiological unit (calf, heifer or cow) as response variables, and farm as a random effect. A Pearson correlation between within-herd seroprevalences and biosecurity scores was also performed.

The overall estimated animal level seroprevalence decreased from 55.7% to 37.2% after one year and was maintained at 37.5% after two years. Among the eleven herds that followed the hyperimmunisation IBR vaccination protocol, the estimated animal level seroprevalence was reduced from 57.3% to 33.6%, and the estimated within-herd seroprevalence from 54.5% to 22.2% within the study period. Reductions were most evident in calves (72.0%) and heifers (76.6%), compared with adult cows (33.1%). A significant negative correlation was observed between within-herd seroprevalences and biocontainment scores (R = -0.51, p < 0.001). Most retested animals maintained a seronegative status across the study period. However, data on culling and replacement rates were not available on the studied farms.

The control and eradication of BoHV-1 from the Republic of Ireland present many challenges to the Irish dairy industry. The results suggest that the maintenance of a hyperimmunisation protocol with an IBR gE-/tk- marker vaccine contribute to effectively and efficiently reducing BoHV-1 seroprevalence within the Irish dairy sector at farm level. Further large-scale controlled studies are required to confirm its role in national eradication strategies.