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

A Metapopulation Model for Preventing the Reintroduction of Bovine Viral Diarrhea Virus to Naïve Herds: Scotland Case Study

Background

Bovine viral diarrhea (BVD) virus is one of the most problematic infectious pathogens for cattle. Since 2013, a mandatory BVD eradication program has successfully reduced the number of infected cattle living on Scottish farms; however, England remains at high prevalence and presents a risk to Scotland through animal movement.

Methods

We analyze cattle movements in the UK from 2008 to 2017 and recorded incidence of BVD in Scotland from 2017 to 2020. To simulate BVD reintroduction into Scotland, we developed an epidemiological model that combines transmission between cattle and animal movements between farms. A total of four control strategies were implemented in the model: no intervention, import restriction, targeted vaccination, and combined strategy.

Results

During the course of the eradication scheme, movements into Scotland became increasingly distributed in regions close to the England–Scotland border. The prevalence of BVD in this region decreased at a slower rate than the rest of Scotland during the eradication scheme. Our model showed that the change in the prevalence is expected, given that the change in the patterns of movement and if vaccination is targeted to the border areas that decrease in the prevalence will be seen throughout the whole of Scotland.

Conclusion

Scottish farms are susceptible to BVD virus reintroduction through animal imports from non-BVD-free nations with farms in border areas being the most vulnerable. Protecting the border regions provides direct and indirect protection to the rest of Scottish farms by interrupting chains of transmission.

Modeling the Effect of Bovine Viral Diarrhea Virus in Australian Beef Herds

Bovine viral diarrhea virus (BVDV) is an economically important disease in Australian beef farming. The disease typically results in low-level production losses that can be difficult to detect for several years. Simulation modeling can be used to support the decision to control BVDV; however, current BVDV simulation models do not adequately reflect the extensive farming environment of Australian beef production. Therefore, the objective of this study was to develop a disease simulation model to explore the impact of BVDV on beef cattle production in south-east Australia. A dynamic, individual-based, stochastic, discrete-time simulation model was created to simulate within-herd transmission of BVDV in a seasonal, self-replacing beef herd. We used the model to simulate the effect of herd size and BVDV introduction time on disease transmission and assessed the short- and long-term impact of BVDV on production outputs that influence the economic performance of beef farms. We found that BVDV can become established in a herd after a single PI introduction in 60% of cases, most frequently associated with the breeding period. The initial impact of BVDV will be more severe in smaller herds, although self-elimination is more likely in small herds than in larger herds, in which there is a 23% chance that the virus can persist for >15 years following a single incursion in a herd with 800 breeders. The number and weight of steers sold was reduced in the presence of BVDV and the results demonstrated that repeat incursions exacerbate long-term production losses, even when annual losses appear marginal. This model reflects the short- and long-term production losses attributed to BVDV in beef herds in southeast Australia and provides a foundation from which the influence and economic utility of BVDV prevention in Australian beef herds can be assessed.

Modelling the economics of bovine viral diarrhoea virus control in pastoral dairy and beef cattle herds

Bovine viral diarrhoea virus (BVDV) is a prevalent pathogen in the New Zealand cattle industries, yet few studies to date have evaluated the economics of BVDV in pastoral dairy and beef herds to help inform management decisions. To address this knowledge gap, we developed stochastic individual-based simulation models to represent the transmission dynamics of BVDV in typical spring-calving dairy and beef farms in New Zealand. The models conservatively estimated the direct losses due to a BVDV outbreak at NZ$ 22.22 and NZ$ 41.19 per mixed-age cow per year for a naïve dairy and beef farm, respectively, over a 5-year period. The greatest economic impacts for the dairy farm occurred when persistently infected replacement heifers joined the lactating cow group and caused transient infection of cows to drop in milk production, whereas the greatest impacts for the beef farm was through the loss of fattening stock for sale due to lowered pregnancy rates. Various combinations of diagnostic testing, vaccination, and biosecurity measures were then explored to evaluate the cost-efficiency of different strategies for controlling BVDV at the farm-level. Providing farmers with the estimates of economic impacts of BVDV in their herds may further encourage the uptake of control measures, but close collaboration with a veterinarian to determine the optimal strategy for their unique farm circumstances is still required.