Estimation of the within-herd transmission rates of bovine viral diarrhoea virus in extensively grazed beef cattle herds

Abstract

Many research groups have developed mathematical models to simulate the dynamics of BVDV infections in cattle herds. However, most models use estimates for within-herd BVDV transmission rates that are either based on expert opinion or adapted from other dairy herd simulation models presented in the literature. There is currently little information on the transmission rates for BVDV in extensively grazed beef herds partly due to the logistical challenges in obtaining longitudinal data of individual animal's seroconversion, and it may not be appropriate to apply the same transmission rates from intensive dairy herds given the significant differences in herd demographics and management. To address this knowledge gap, we measured BVDV antibody levels in 15 replacement heifers in each of 75 New Zealand beef breeding farms after their first calving and again at pregnancy scanning or weaning to check for seroconversion. Among these, data from 9 farms were used to infer the within-herd BVDV transmission rate with an approximate Bayesian computation method. The most probable within-herd BVDV transmission rate was estimated as 0.11 per persistently infected (PI) animal per day with a 95% highest posterior density interval between 0.03 and 0.34. This suggests that BVDV transmission in extensively grazed beef herds is generally slower than in dairy herds where the transmission rate has been estimated at 0.50 per PI animal per day and therefore may not be sufficient to ensure that all susceptible breeding females gain adequate immunity to the virus before the risk period of early pregnancy for generating new PI calves.

Figure 1

Description of within-herd simulation for estimating within-herd BVDV transmission rate. Blue and red shaded areas indicate the breeding and calving periods, respectively. Red dotted arrow represents the contribution to the force of BVDV infection.

Figure 2

The posterior distribution of within-herd BVDV transmission rates for PI animals (${\mathit{\beta }}_{\mathit{P}}$). The mode was 0.11 with a 95% highest posterior density interval between 0.03 and 0.34.

Figure 3

The posterior distribution of the estimated parameters for each herd (from the second to the last row). Left: the initial proportion of BVDV seropositive animals ($\mu$), Middle: the proportion of introduced PI animals ($\rho$), Right: the day of PI animals being introduced ($\tau$). The distributions of the first row illustrate the prior distribution of each parameter. Green and red shaded areas indicate the first breeding and calving period, respectively. Day FS indicates the day of first sampling.