Not all cows are epidemiologically equal: quantifying the risks of bovine viral diarrhoea virus (BVDV) transmission through cattle movements (original) (raw)
Related papers
Preventive Veterinary Medicine, 2016
The control of bovine viral diarrhoea virus (BVDV) mainly focuses on the identification and restriction of persistently infected (PI) animals. However, other transmission pathways can also result in new breakdowns, including the movement of animals pregnant with PI calves (Trojan animals) and the spread of infection between contiguous farms. Contiguous spread is likely an important problem in the BVD eradication programme in Ireland, given the spatial distribution of residual infection, and the highly fragmented nature of land holdings on many Irish farms. In this study, we seek to quantify the risk of BVD spread between contiguous herds in Ireland. Multivariable logistic models were used to estimate the risk of a herd having BVD positive calves in January to June 2014 (the study period) when contiguous to a herd that had at least one BVD positive calf born in 2013. The models included risk factors relating to the study herd and to neighbouring herds. Separate multivariable models were built for each of four "PI-neighbour" factors relating to the presence of BVD+ animals and/or the presence of offspring of PI breeding animals. In total, 58,483 study herds were enrolled. The final model contained the province, the log of the number of calf births born during the study period, the number of cattle purchased between January 2013 and January 2014, and with a two-way interaction between the number of animals of unknown BVD status in the study herd and the PI-neighbour risk factor. When the number of PI-neighbour herds was used as the PI-neighbour risk factor, the odds ratio (OR) associated with the number of PI-neighbour herds ranged from 1.07 to 3.02, depending on the number of unknown animals present. To further explore the risk associated with PI-neighbour factors, the models were repeated using a subset of the study herds (n = 7440) that contained no animals of unknown status. The best fitting model including "any PIneighbour" as the PI-neighbour factor and also contained the log of the number of calf births born during the study period and the number of cattle purchased. The OR associated with "any PI-neighbour" was 1.92 (95% C.I. 1.37-2.70). This study provides the first quantitative information on the risks posed by the presence of BVD+ animals in neighbouring herds and also highlights the importance of clarifying the BVD status of animals that have not yet been tested in the context of the Irish eradication programme.
Epidemiology and Infection, 2007
SUMMARYRelevance of epidemiological models depends on assumptions on the population structure and dynamics, on the biology of the host–parasite interaction, and on mathematical modelling. In this paper we reviewed published models of the bovine viral diarrhoea virus (BVDV) spread within a herd. Modelling options and assumptions on herd dynamics and BVDV transmission were discussed. A cattle herd is a population with a controlled size. Animals are separated into subgroups according to their age or their physiological status inducing heterogeneity of horizontal transmission. Complexity of models results from: (1) horizontal and vertical virus transmission, (2) birth of persistently infected animals, (3) excretion by transiently and persistently infected animals. Areas where there was a lack of knowledge were identified. Assumptions on the force of infection used to model the horizontal virus transmission were presented and discussed. We proposed possible ways of improving models (e.g....
The Veterinary Journal
A previously published model was re-employed to examine the potential impact of different epidemiological circumstances on output losses due to bovine viral diarrhoea virus (BVDV) infection in typical British hill cow-calf enterprises. The average annuity equivalent of unchecked losses from 100 simulated 10-year disease scenarios ranged from almost pound0/cow to approximately pound40/cow. Significant differences were found under certain circumstances, depending on the initial disease status of the herd, the initial source of virus, the probability and source of further infection, the probability of virus transmission within the herd and herd size. For naïve herds, losses depended only on the risk of incursion. In most other circumstances, the losses could be mitigated if the annual risk of incursion was <0.3 and risk of within herd transmission was extremely low. Greater understanding of the interaction between these risk factors and management actions are required so that total ...
A stochastic SIR model was developed to simulate the spread of bovine viral diarrhea virus (BVDV) through a cow-calf herd and estimate the effect of the virus on the herd, including abortions, calf morbidity, and calf mortality. The model was applied with three herd sizes (400, 100, and 50 head) and four control strategies (no intervention, vaccination of breeding stock, testing all calves pre-breeding and culling of persistently infected calves, and both vaccination of adults and testing and culling of calves). When no control strategy was implemented the BVDV reproductive rate (R E-PI ) of persistently infected calves (PI's), vertical transmission rate from cows to calves and the mortality rate of PI's were influential in the number of PI's produced in the herd. When a vaccination program alone was implemented the vaccine efficacy was influential in the number of PI's produced in the herd. All control strategies decreased the effects of BVDV on the herd at both 1 and 10 years compared to no control. In most cases the combination of adult vaccination and calf testing and culling resulted in the largest decrease in the both the median and 95% prediction interval for the range of effects from BVDV. The effect of control strategies was most apparent in the 400 head herds. All control strategies increased the probability of early clearance of PI's from the herd for all herd sizes. Fifty and 100 head herds cleared infection by 4 and 9 years respectively even without a control program but 400 head herds did not always clear infection after 10 years unless a testing program was implemented. The model presented is valuable in assessing the effect of control strategies and the effects of disease parameters on BVDV spread in beef herds.
The prevalence of bovine viral diarrhoea virus infection in beef suckler herds in Scotland
The Veterinary Journal, 2010
Bovine viral diarrhoea (BVD) is an endemic disease of cattle that causes substantial losses to both beef and dairy production worldwide. The goal of this study was to estimate the prevalence of active BVD virus (BVDV) infection in beef suckler herds in Scotland. Information was collected from 301 herds using a stratified random sampling design based on agricultural census data. Herds were classified as with and without active infection based on the within-herd BVDV seroprevalence in young stock using Bayesian finite mixture modelling. This method accounted for within-and between-herd variability and allowed for classification error by the diagnostic tests. The observed sample data enabled the discrimination of three distinct seroprevalence cohorts. The results provided evidence of active BVDV infection in 16% of herds and no evidence of recent exposure in approximately two thirds of herds. The epidemiological significance of the further 16% of herds containing young stock with a median BVDV seroprevalence of 26.3-38.5% remains unclear. The fact that a large percentage of herds did not show evidence of recent infection is encouraging from an animal health and welfare perspective and the study provides a model for the further exploration of strategies aimed at BVD control at national level.
A model of the spread of the bovine viral-diarrhoea virus within a dairy herd
Preventive Veterinary Medicine, 2004
Wet BVDSim (a stochastic simulation model) was developed to study the dynamics of the spread of the bovine viral-diarrhoea virus (BVDV) within a dairy herd. This model took into account herd-management factors (common in several countries), which influence BVDV spread. BVDSim was designed as a discrete-entity and discrete-event simulation model. It relied on two processes defined at the individual-animal level, with interactions. The first process was a semi-Markov process and modelled the herd structure and dynamics (demography, herd management). The second process was a Markov process and modelled horizontal and vertical virus transmission. Because the horizontal transmission occurs by contacts (nose-to-nose) and indirectly, transmission varied with the separation of animals into subgroups. Vertical transmission resulted in birth of persistently infected (PI) calves. Other possible consequences of a BVDV infection during the pregnancy period were considered (pregnancy loss, immunity of calves). The outcomes of infection were modelled according to the stage of pregnancy at time of infection. BVDV pregnancy loss was followed either by culling or by a new artificial insemination depending on the modelled farmer's decision. Consistency of the herd dynamics in the absence of any BVDV infection was verified. To explore the model behaviour, the virus spread was simulated over 10 years after the introduction of a near-calving PI heifer into a susceptible 38 cow herd. Different dynamics of the virus spread were simulated, from early clearance to persistence of the virus 10 years after its introduction. Sensitivity of the model to the uncertainty on transmission coefficient was analysed. Qualitative validation consisted in comparing the bulk-milk ELISA results over time in a sample of herds detected with a new infection with the ones derived from simulations.
Preventive Veterinary Medicine, 2017
Many countries have implemented control programmes aiming to eradicate Bovine Viral Diarrhoea Virus (BVDV). After obtaining the free status, a risk of re-introduction of the virus through import may remain. Therefore the risk of introduction of BVDV through cattle imports in the Netherlands was quantified and the effectiveness of subsequent intervention measures was assessed. Data, literature and expert opinion were used to estimate values for input parameters to feed a stochastic simulation model. The probability that BVDV was imported was differentiated into persistently infected (PI) cattle, trojan cows that transmitted the virus vertically resulting in a PI foetus (TR) and transient infected cattle (TI). The import risk was stratified to beef, dairy, small scale, suckler, trade, veal and young stock herds. The intervention scenarios that were evaluated consisted of virus testing, a combination of virus testing and antibody testing in pregnant cows, abolishment of imports from high risk countries (i.e. countries with a BVDV prevalence >15%) and a combination of import restrictions and testing prior to import. Each year, 334 (5 th and 95 th percentile: 65-902) Dutch cattle herds were estimated to be infected with BVDV through import. Veal herds account for most infections associated with import (87%), whereas in the other herd types, only 9 beef, 6 dairy, 2 small scale, 16 suckler, 10 trade and 2 young stock herds are infected through imports per year. Import of PI cattle is the most important risk for introduction in veal herds, while import of TR cows is the main source of BVDV introduction in dairy, small scale and suckler herds. With the intervention scenarios, the number of BVDV infected herds in the Netherlands could be reduced to 81 and 58 herds per year when respectively virus testing or a combination of virus and antibody testing was applied or to 108 herds when import from high risk countries was abolished. With the scenario in which both import from high risk countries was abolished combined with virus and antibody testing, the number of BVDV infected herds could be reduced to 17 herds per year. The risk assessment showed that BVDV is regularly imported in the Netherlands. The import risk can effectively be reduced by implementing diagnostic testing prior to import and only import cattle with a favourable result, eventually combined with certain trade restrictions.
Preventive Veterinary Medicine, 2020
A compulsory national BVD eradication programme commenced in Ireland in 2013. Since then considerable progress has been made, with the animal-level prevalence of calves born persistently infected (PI) falling from 0.67 % in 2013 to 0.06 % in 2018. The herd-level prevalence fell from 11.3 % in 2013 to 1.1 % in 2018. In the Irish programme, herds in which all animals have a known negative status and which have not contained any PI animals for 12 months or more are assigned a negative herd status (NHS). While considerable progress towards eradication has been made, PI calves have been identified in a small proportion of herds that had previously been assigned NHS. Given this context, a case-control study was conducted to investigate potential risk factors associated with loss of NHS in 2017. 546 herds which had NHS on 1 January 2017 and lost that status during 2017 (case herds) were matched with 2191 herds (control herds) that retained their NHS status throughout 2017. Previous history of BVD infection, herd size, herd expansion, the purchase of cattle including potential Trojan cattle and the density of BVD infection within 10 km of the herd emerged as significant factors in a multivariable logistic regression model. This work adds to the evidence base in support of the BVD eradication programme, particularly establishing why BVD re-emerged in herds which had been free of BVD for at least the previous 12 months prior to the identification of a BVD positive calf. This information will be especially important in the context of identifying herds which may be more likely to contain BVD positive animals once the programme moves to herd-based serology status for trading purposes in the post-eradication phase.
Veterinary Research, 2008
A herd is a population structured into groups not all equally in contact, which may influence within-herd spread of pathogens. Herd structure varies among cattle herds. However, published models of the spread of bovine viral diarrhoea virus (BVDV) assume no herd structure or a unique structure chosen as a representative. Our objective was to identify-for different index cases introduced into an initially BVDV-free dairy herd-risky (favourable) herd structures, which increased (decreased) BVDV spread and persistence compared to a reference structure. Classically, dairy herds are divided into calves, young heifers, bred heifers, lactating cows and dry cows. In the reference scenario, groups are all equally in contact. We evaluated the effect of isolating or merging groups. Three index cases were tested: an open persistently-infected (PI) heifer, an open transiently-infected heifer, an immune heifer carrying a PI foetus. Merging all groups and merging calves and lactating cows were risky scenarios. Isolating each group, isolating lactating cows from other groups, and merging calves and young heifers were favourable scenarios. In most structures, the most risky index cases were the following: first, the entry of a PI heifer; second, the birth of a PI calf; last, the entry of a transiently-infected heifer. Recommendations for dairy herds are to raise young animals together before breeding and to isolate lactating cows from others as much as possible. These recommendations will be less efficient if a PI adult enters into the herd. contact structure / epidemiological model / pestivirus / cattle