Risk factors for introduction of BoHV1 into Dutch dairy herds in a national control programme

Frederik Waldeck1, Irene Bisschop1, Jet Mars1, Katrien van den Brink1, Mattheus Hekman1, Emma Strous1, Paul Wever1, Gerdien van Schaik1,2 1Royal GD, Deventer, the Netherlands 2Department of population Health Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands

Objectives

After many years of voluntary control and decreasing prevalence, a national control programme for Infectious Bovine Rhinotracheitis (IBR) is in place for Dutch dairy herds since 2018. Introduction of the causative agent Bovine herpesvirus-1 (BoHV1) is minimized by mandatory vaccination with gE-deleted markervaccines in infected herds and by regulations with respect to purchase of cattle from herds with a non-IBR-free status. However, BoHV1 was introduced in several tens of herds in 2018-2021. The aim of the study was to determine risk factors for introduction of BoHV1 in the context of the national control programme.

Materials and methods

In a case-control design, herds with a confirmed BoHV1 introduction (cases) were compared with control herds that were located near the case herd, which remained free of IBR. Both case and control herds were visited by a veterinarian of Royal GD and an extensive questionnaire was applied about the possible risk factors for BoHV1 introduction in the previous period. In total, 92 cases and 84 controls were visited. Logistic regression analysis was carried out to determine the significant risk factors (P<0.05). The questionnaire consisted of 368 questions, which were divided into sixteen different categories: general farm data, status development of IBR, animal numbers on date of status loss, housing, total acreage of land, purchase of cattle, neighbourhood farms, external labour, secondary activities, animal contacts, grazing, contacts between farms through people and materials, animal health management, vaccinations, contacts with small ruminants and warning of other cattle farmers in the direct area. Multivariable logistic regression analyses were carried out to quantify these risks in odds ratios (OR).

Results

In outbreak herds, the final multivariable model contained the following three risk factors:

1 raising of young stock on other locations (OR=1.37)

2 purchase of cattle from non-IBR-free herds (OR=1.33)

3 escaped cattle from other farms mingling with own cattle (OR=1.22)

Farmers did not always have correct information on the IBRstatus of farms from which they purchased cattle. More than 20% of the farmers who said they were purchasing from an IBRfree farm, actually purchased cattle from a non-IBR-free farm.

Conclusions

The risk factors for introduction of BoHV1 in free herds were all related to direct animal contact. However, the risk per factor is quite low, thereby suggesting BoHV1 can be introduced by several factors. Farmers were often unaware of the risks they take and were often inclined to make adjustments to their management routines after the farm visit. Continuous communication on biosecurity measures to farmers, veterinarians and other professional visitors is important to avoid loss of the IBR-free status.