Institute of Epidemiology, Friedrich-Loeffler-Institute , Greifswald, Insel Riems , Germany.
Institute for Veterinary Public Health, University of Veterinary Medicine Vienna , Vienna , Austria.
Front Vet Sci. 2016 Jun 21;3:48. doi: 10.3389/fvets.2016.00048. eCollection 2016.
The trade of livestock is an important and growing economic sector, but it is also a major factor in the spread of diseases. The spreading of diseases in a trade network is likely to be influenced by how often existing trade connections are active. The activity α is defined as the mean frequency of occurrences of existing trade links, thus 0 < α ≤ 1. The observed German pig trade network had an activity of α = 0.11, thus each existing trade connection between two farms was, on average, active at about 10% of the time during the observation period 2008-2009. The aim of this study is to analyze how changes in the activity level of the German pig trade network influence the probability of disease outbreaks, size, and duration of epidemics for different disease transmission probabilities. Thus, we want to investigate the question, whether it makes a difference for a hypothetical spread of an animal disease to transport many animals at the same time or few animals at many times. A SIR model was used to simulate the spread of a disease within the German pig trade network. Our results show that for transmission probabilities <1, the outbreak probability increases in the case of a decreased frequency of animal transports, peaking range of α from 0.05 to 0.1. However, for the final outbreak size, we find that a threshold exists such that finite outbreaks occur only above a critical value of α, which is ~0.1, and therefore in proximity of the observed activity level. Thus, although the outbreak probability increased when decreasing α, these outbreaks affect only a small number of farms. The duration of the epidemic peaks at an activity level in the range of α = 0.2-0.3. Additionally, the results of our simulations show that even small changes in the activity level of the German pig trade network would have dramatic effects on outbreak probability, outbreak size, and epidemic duration. Thus, we can conclude and recommend that the network activity is an important aspect, which should be taken into account when modeling the spread of diseases within trade networks.
牲畜贸易是一个重要且不断增长的经济领域，但也是疾病传播的主要因素。贸易网络中的疾病传播很可能受到现有贸易联系活跃程度的影响。活跃度α被定义为现有贸易联系发生的平均频率，因此 0<α≤1。观测到的德国猪贸易网络的活跃度为α=0.11，这意味着在 2008-2009 年的观测期间，两个农场之间的每个现有贸易联系平均有 10%的时间处于活跃状态。本研究的目的是分析德国猪贸易网络的活跃度水平变化如何影响不同疾病传播概率下疾病爆发的概率、规模和持续时间。因此，我们想调查一个假设的动物疾病传播是同时运输大量动物还是多次运输少量动物会有什么不同。我们使用 SIR 模型来模拟疾病在德国猪贸易网络中的传播。我们的研究结果表明，对于传播概率<1 的情况，随着动物运输频率的降低，爆发概率会增加，α的峰值范围在 0.05 到 0.1 之间。然而，对于最终的爆发规模，我们发现存在一个阈值，只有当α超过临界值（约为 0.1，接近观测到的活跃度水平）时才会发生有限的爆发。因此，尽管降低α会增加爆发的概率，但这些爆发只会影响少数农场。疫情的持续时间在α=0.2-0.3 的活动水平上达到峰值。此外，我们的模拟结果表明，即使德国猪贸易网络的活跃度水平发生微小变化，也会对爆发概率、爆发规模和疫情持续时间产生巨大影响。因此，我们可以得出结论并建议，网络活跃度是在贸易网络中模拟疾病传播时应考虑的一个重要方面。
