Vaccination of Sheep with Bovine Viral Diarrhea Vaccines Does Not Protect against Fetal Infection after Challenge of Pregnant Ewes with Border Disease Virus

Abstract

Border Disease (BD) is a major sheep disease characterized by immunosuppression, congenital disorders, abortion, and birth of lambs persistently infected (PI) by Border Disease Virus (BDV). Control measures are based on the elimination of PI lambs, biosecurity, and frequent vaccination which aims to prevent fetal infection and birth of PI. As there are no vaccines against BDV, farmers use vaccines directed against the related Bovine Viral Diarrhea Virus (BVDV). To date, there is no published evidence of cross-effectiveness of BVDV vaccination against BDV infection in sheep. We tested three commonly used BVDV vaccines, at half the dose used in cattle, for their efficacy of protection against a BDV challenge of ewes at 52 days of gestation. Vaccination limits the duration of virus-induced leukopenia after challenge, suggesting partial protection in transient infection. Despite the presence of BDV neutralizing antibodies in vaccinated ewes on the day of the challenge, fetuses of vaccinated and unvaccinated sheep were, two months after, highly positive for BDV RNA loads and seronegative for antibodies. Therefore, BVDV vaccination at half dose was not sufficient to prevent ovine fetal infection by BDV in a severe challenge model and can only be reconsidered as a complementary mean in BD control.

Keywords: Border Disease Virus; Bovine Viral Diarrhea Virus; fetus; protection; sheep; vaccination.

Figure 1

Experiment timeline of vaccination, artificial insemination, and BDV challenge and sampling.

Figure 2

Mean ELISA antibody competitive percentages calculated according to the manufacturer’s recommendations (OD sample/mean OD of negative controls, validation of the assay if the mean OD of negative controls > 0.7 and the OD of the positive control is less than 30% of the mean OD of negative controls: OD positive control/mean OD of negative controls < 0.3). Interpretation: positive: ≤40%; doubtful: >40% and ≤50%; negative: >50%.

Figure 3

Mean neutralizing antibody titers (VNT) calculated by the Spearman-Karber method and expressed in log₂. Sera from D0 and D+66 of vaccinated (Mucosiffa in red, Bovela in blue, Bovilis in green) and unvaccinated (in dark) ewes were tested against the BVDV-1a NADL and the BDV-6 6390 strains for neutralization. Standard errors of the mean are indicated.

Figure 4

Percentage of BDV-6 positive animals, detected by real-time RT-qPCR in the blood of vaccinated (Mucosiffa in red, Bovela in blue, Bovilis in green) and unvaccinated (in dark) ewes after challenge.

Figure 5

Mean leukocytes (white blood cell) and lymphocyte counts (with standard error of the mean) of the vaccinated (red line: Mucosiffa; blue line: Bovela; green line: Bovilis BVD) and non-vaccinated (black line) groups after BDV-6 challenge. For each animal and each cell population, a baseline was calculated as the average of the data obtained over the 3 measurements preceding viral inoculation (D-3, D-1, and D0). The individual data were then transformed into the percentage of each cell population back to the baseline ((X/baseline)/100). Significant differences in the mean number of leukocytes or lymphocytes between the group of the line and the other groups corresponding to a letter are indicated. Significant differences in the mean number of leukocytes or lymphocytes between the group corresponding to the line and the other groups designated by their respective letters are indicated.

Figure 6

BDV-6 RNA loads (Log₁₀/100 mg of tissue) in the thymus and the brain of fetuses sampled at D+66 of euthanasia from vaccinated (Mucosiffa in red, Bovela in blue, Bovilis in green) and unvaccinated (in dark) ewes.