Ezanno Pauline, Fourichon Christine, Seegers Henri
INRA, UMR1300, Bio-agression, Epidémiologie et Analyse de Risques, BP 40706, 44307 Nantes, France.
Vet Res. 2008 Sep-Oct;39(5):39. doi: 10.1051/vetres:2008016. Epub 2008 Mar 18.
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.
畜群是一种由并非都有同等接触机会的群体构成的种群结构，这可能会影响病原体在畜群内的传播。不同牛群的畜群结构各不相同。然而，已发表的牛病毒性腹泻病毒（BVDV）传播模型要么假定不存在畜群结构，要么选择一种独特的结构作为代表。我们的目标是，对于引入到最初无BVDV的奶牛群中的不同指示病例，确定有风险（有利）的畜群结构，与参考结构相比，这些结构会增加（减少）BVDV的传播和持续存在。传统上，奶牛群分为犊牛、青年母牛、怀孕母牛、泌乳母牛和干奶期母牛。在参考情景中，各群体的接触机会均等。我们评估了隔离或合并群体的影响。测试了三种指示病例：一头开放型持续感染（PI）的青年母牛、一头开放型短暂感染的青年母牛、一头携带PI胎儿的免疫青年母牛。合并所有群体以及合并犊牛和泌乳母牛是有风险的情景。隔离每个群体、将泌乳母牛与其他群体隔离，以及合并犊牛和青年母牛是有利的情景。在大多数结构中，风险最大的指示病例如下：首先，PI青年母牛的引入；其次，PI犊牛的出生；最后，短暂感染青年母牛的引入。对于奶牛群的建议是，在配种前将幼畜集中饲养，并尽可能将泌乳母牛与其他牛只隔离。如果一头PI成年牛进入畜群，这些建议的效果将降低。
