Monti G E, Frankena K, De Jong M C M
Wageningen University and Research Department of Animal Sciences, Quantitative Veterinary Epidemiology Group, Marijkeweg, Wageningen, The Netherlands.
Epidemiol Infect. 2007 Jul;135(5):722-32. doi: 10.1017/S0950268806007357. Epub 2006 Nov 1.
In Argentina, bovine leukaemia virus (BLV) infection is common in dairy herds. The country currently has a National Voluntary Control Programme but relatively few farms have enrolled. However, there is increased interest from authorities and farmers to implement regional compulsory programmes but there is scarce quantitative information of the transmission of BLV in cattle herds. This information is a prerequisite to develop effective BLV control strategies. Mathematical modelling offers ways of integrating population-level knowledge and epidemiological data to predict the outcomes of intervention scenarios. The purpose of the current paper is to gain understanding about the dynamics of the transmission of BLV in dairy herds from Argentina by simulation and to compare various BLV transmission models and select the one that is most appropriate. The hypothetical herd is conceptually described in terms of BLV status as a population of individuals that are protected by maternal antibodies (M), that are susceptible (S), that are in the latent period (E) or that are infectious (I). BLV is spread by horizontal and vertical transmission. We used an age-structured population model and within-herd transmission was simulated by Monte Carlo techniques. The next-generation approach has been used for the systematic computation of the basic reproduction ratio (R0). Parameter values for disease transmission were derived from previously published data; rates of entry, exit or transition between age groups were calculated based on our previous study, observational data, expert opinions and literature. With these parameter values the probability of a minor outbreak was estimated to be 10%, the probability of extinction was estimated as <0.001% and the expected time to extinction as more than 80 years. The probability of a minor outbreak and changes in prevalence were different when the index case was an adult cow compared to introduction by a heifer. Prediction of prevalences from MSI models fit the data satisfactorily. R0 was estimated as 9.5. The sensitivity analysis on R0 showed that all measures directed to reduce the transmission rate are potentially effective given operational control measures. An important prediction of these models is that, even in a relatively small, closed dairy herd, the time-scale for a BLV outbreak may be as long as several years and within-herd control of BLV requires intensive efforts.
在阿根廷，牛白血病病毒（BLV）感染在奶牛群中很常见。该国目前有一项国家自愿控制计划，但参与的农场相对较少。然而，当局和农民对实施区域强制计划的兴趣日益增加，但关于BLV在牛群中传播的定量信息却很少。这些信息是制定有效的BLV控制策略的先决条件。数学建模提供了整合种群水平知识和流行病学数据以预测干预方案结果的方法。本文的目的是通过模拟了解阿根廷奶牛群中BLV传播的动态，并比较各种BLV传播模型，选择最合适的模型。假设的牛群在概念上根据BLV状态被描述为一个由母体抗体保护的个体群体（M）、易感个体群体（S）、处于潜伏期的个体群体（E）或具有传染性的个体群体（I）。BLV通过水平和垂直传播。我们使用了一个年龄结构种群模型，并通过蒙特卡洛技术模拟了牛群内的传播。下一代方法已用于基本繁殖数（R0）的系统计算。疾病传播的参数值来自先前发表的数据；年龄组之间的进入、退出或转变率是根据我们之前的研究、观测数据、专家意见和文献计算得出的。根据这些参数值，估计小规模爆发的概率为10%，灭绝的概率估计为<0.001%，灭绝的预期时间超过80年。与小母牛引入相比，当指示病例为成年母牛时，小规模爆发的概率和患病率的变化有所不同。MSI模型对患病率的预测与数据拟合良好。估计R0为9.5。对R0的敏感性分析表明，在实施操作控制措施的情况下，所有旨在降低传播率的措施都可能有效。这些模型的一个重要预测是，即使在相对较小的封闭奶牛群中，BLV爆发的时间尺度可能长达数年，并且在牛群内控制BLV需要付出巨大努力。
