Center for Animal Disease Modeling and Surveillance, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Prev Vet Med. 2011 Dec 1;102(3):175-84. doi: 10.1016/j.prevetmed.2011.07.005. Epub 2011 Aug 15.
An epidemic of infectious salmon anemia virus (ISAV) has greatly impacted salmon production in Chile with devastating social and economic consequences. The epidemic is analyzed here and is likely the largest ISAV outbreak reported affecting one of the most productive regions for salmon farming activities in the world. After re-emerging in 2007, ISAV rapidly expanded the following two years, both in magnitude and geographic range, affecting about 65% and 50% of salmon farms located at the 10th and 11th regions of Chile, respectively. A useful metric for the control of infectious diseases that quantifies the progression of an epidemic is the reproduction number at the farm level (R(f)), which describes the mean number of secondary cases generated by an infectious farm. The parameter in this study was estimated for individual farms (R(fi)), specific phases (R(tf)), and for the entire epidemic (R(f)) by using several analytical approaches based on the characterization of the epidemic curves for the two regions. For the initial spread and the epidemic growth phase, initial and intrinsic growth rates were used to estimate R(tf). In addition, two approaches (epidemic final size and nearest neighbor analyses) were used to obtain an individual (R(fi)) and overall estimate of R(f) for the complete epidemic. In general, two distinct regional patterns of spread were identified. In the 10th region, after an explosive initial spread of ISAV in which R(tf) reached 12.0-16.9, a smaller epidemic growth of 1.6≤R(tf)≥2.5 and a final burnout with R(tf)<1 were observed. For the 11th region, R(tf) only reached 2.4 during the initial spread phase, ranged from 1.6≤R(tf)≥4.4 during the epidemic growth phases and ended when R(tf) was <1.0. The epidemic was characterized by clustering of ISAV 'superspreaders' farms i.e., farms with statistically significantly (P<0.047) higher R(fi) values. Distances between pairs of infected farms were statistically significantly (P=0.003) shorter in the 10th compared to the 11th region. Overall, R(f) ranged from 1.6 to 2.5 and 1.3 to 1.7 in the 10th and 11th regions, respectively. Our findings suggest that control efforts were able to protect 38-60% and 23-41% of the farms in the 10th and 11th regions, respectively, and may have resulted in the epidemic not spreading further. In addition, control strategies in highly populated areas using a control zone of at least 10km radius may be more effective than the 5km zone recommended by the World Animal Health Organization.
传染性鲑鱼贫血病毒 (ISAV) 的流行对智利的鲑鱼生产造成了巨大影响，带来了毁灭性的社会和经济后果。本文对此次疫情进行了分析，这很可能是有记录以来规模最大的 ISAV 爆发事件，影响了世界上鲑鱼养殖活动最活跃的地区之一。2007 年再次出现后，ISAV 在随后两年内迅速蔓延，无论是在规模还是地理范围上，都分别影响了智利第 10 和第 11 大区约 65%和 50%的鲑鱼养殖场。一个用于量化传染病进展的有用指标是农场层面的繁殖数（R(f)），它描述了一个传染性农场产生的继发性病例的平均数量。本研究使用了几种基于对两个地区的疫情曲线特征的分析方法，分别对个别农场（R(fi)）、特定阶段（R(tf)）和整个疫情（R(f)）的参数进行了估计。对于初始传播和疫情增长阶段，使用初始和内在增长率来估计 R(tf)。此外，还使用了两种方法（疫情最终规模和最近邻分析）来获得整个疫情中个体（R(fi)）和总体 R(f)的估计值。一般来说，确定了两种不同的区域传播模式。在第 10 大区，ISAV 最初呈爆炸式传播，R(tf)达到 12.0-16.9，随后是较小的疫情增长，1.6≤R(tf)≥2.5，最终R(tf)<1。对于第 11 大区，R(tf)仅在初始传播阶段达到 2.4，在疫情增长阶段从 1.6≤R(tf)≥4.4 不等，当 R(tf)低于 1.0 时结束。疫情的特点是传染性鲑鱼贫血病毒的“超级传播者”农场聚集，即农场的 R(fi)值具有统计学意义（P<0.047）更高。受感染农场之间的距离在第 10 大区与第 11 大区相比具有统计学意义（P=0.003）较短。总体而言，R(f)在第 10 和第 11 大区分别为 1.6 至 2.5 和 1.3 至 1.7。我们的研究结果表明，控制措施能够保护第 10 和第 11 大区 38-60%和 23-41%的农场，这可能导致疫情没有进一步蔓延。此外，在人口稠密地区使用至少 10 公里半径的控制区的控制策略可能比世界动物卫生组织建议的 5 公里半径控制区更有效。
