Thakur Krishna K, Revie Crawford W, Hurnik Daniel, Poljak Zvonimir, Sanchez Javier
Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PEI, Canada.
Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PEI, Canada.
Prev Vet Med. 2015 Mar 1;118(4):413-26. doi: 10.1016/j.prevetmed.2015.01.006. Epub 2015 Jan 16.
Porcine reproductive and respiratory syndrome (PRRS), a viral disease of swine, has major economic impacts on the swine industry. The North American Animal Disease Spread Model (NAADSM) is a spatial, stochastic, farm level state-transition modeling framework originally developed to simulate highly contagious and foreign livestock diseases. The objectives of this study were to develop a model to simulate between-farm spread of a homologous strain of PRRS virus in Ontario swine farms via direct (animal movement) and indirect (sharing of trucks between farms) contacts using the NAADSM and to compare the patterns and extent of outbreak under different simulated conditions. A total of 2552 swine farms in Ontario province were allocated to each census division of Ontario and geo-locations of the farms were randomly generated within the agriculture land of each Census Division. Contact rates among different production types were obtained using pig movement information from four regions in Canada. A total of 24 scenarios were developed involving various direct (movement of infected animals) and indirect (pig transportation trucks) contact parameters in combination with alternating the production type of the farm in which the infection was seeded. Outbreaks were simulated for one year with 1000 replications. The median number of farms infected, proportion of farms with multiple outbreaks and time to reach the peak epidemic were used to compare the size, progression and extent of outbreaks. Scenarios involving spread only by direct contact between farms resulted in outbreaks where the median percentage of infected farms ranged from 31.5 to 37% of all farms. In scenarios with both direct and indirect contact, the median percentage of infected farms increased to a range from 41.6 to 48.6%. Furthermore, scenarios with both direct and indirect contact resulted in a 44% increase in median epidemic size when compared to the direct contact scenarios. Incorporation of both animal movements and the sharing of trucks within the model indicated that the effect of direct and indirect contact may be nonlinear on outbreak progression. The increase of 44% in epidemic size when indirect contact, via sharing of trucks, was incorporated into the model highlights the importance of proper biosecurity measures in preventing transmission of the PRRS virus. Simulation of between-farm spread of the PRRS virus in swine farms has highlighted the relative importance of direct and indirect contact and provides important insights regarding the possible patterns and extent of spread of the PRRS virus in a completely susceptible population with herd demographics similar to those found in Ontario, Canada.
猪繁殖与呼吸综合征（PRRS）是一种猪的病毒性疾病，对养猪业有重大经济影响。北美动物疾病传播模型（NAADSM）是一个空间随机的农场层面状态转换建模框架，最初是为模拟高传染性外来家畜疾病而开发的。本研究的目的是开发一个模型，使用NAADSM模拟安大略省养猪场中PRRS病毒同源毒株通过直接（动物移动）和间接（农场间共用卡车）接触在农场间的传播，并比较不同模拟条件下疫情的模式和范围。安大略省的2552个养猪场被分配到安大略省的每个普查区，农场的地理位置在每个普查区的农业用地内随机生成。利用加拿大四个地区的生猪移动信息获得不同生产类型之间的接触率。共制定了24种情景，涉及各种直接（感染动物移动）和间接（生猪运输卡车）接触参数，并交替设定引入感染的农场的生产类型。对疫情进行了为期一年的模拟，重复1000次。使用感染农场的中位数数量、多次爆发疫情的农场比例以及达到疫情高峰的时间来比较疫情的规模、发展过程和范围。仅涉及农场间直接接触传播的情景导致疫情爆发，感染农场的中位数百分比占所有农场的31.5%至37%。在同时存在直接和间接接触的情景中，感染农场的中位数百分比增加到41.6%至48.6%。此外，与直接接触情景相比，同时存在直接和间接接触的情景导致疫情中位数规模增加了44%。在模型中纳入动物移动和卡车共用表明，直接和间接接触对疫情发展的影响可能是非线性的。当通过共用卡车的间接接触纳入模型时，疫情规模增加44%，这突出了适当生物安全措施在预防PRRS病毒传播中的重要性。对养猪场中PRRS病毒农场间传播的模拟突出了直接和间接接触的相对重要性，并为PRRS病毒在与加拿大安大略省类似的完全易感种群中具有相似畜群特征的情况下可能的传播模式和范围提供了重要见解。
