Department of Disease Control and Epidemiology, National Veterinary Institute (SVA), SE-751 89 Uppsala, Sweden.
Department of Biomedical Sciences and Veterinary Public Health, SLU, Swedish University of Agricultural Sciences, Box 7036, SE-750 07 Uppsala, Sweden.
J Dairy Sci. 2018 Nov;101(11):10177-10190. doi: 10.3168/jds.2018-14786. Epub 2018 Aug 23.
Based on Swedish legislation, all herds where Salmonella of any serotype is detected are put under restrictions, and measures aiming at eradication are required. Costs for sampling and control have increased in recent years and the aim of this study was to investigate the efficiency of different sampling strategies. We also compiled test results from recent surveillance activities and used these to complement and compare with calculated results. Sensitivities and specificities at group and herd level were calculated for different test strategies. A scenario-tree modeling approach was used to account for the hierarchy of animals within herds, and different relative risk of salmonella in different age groups. Negative and positive predictive values (NPV and PPV), and probability of freedom from Salmonella were calculated to compare the added value of different sampling strategies. Results showed that more fecal samples than serological samples per group are needed to reach a group sensitivity >0.50. This also means that serological testing leads to a higher NPV. For example, with 10 negative test-results from a group of 25 animals in a herd with a suspicion of Salmonella, the NPV based on serology was 0.75 and based on culture was 0.56. For the PPV, testing based on culture from fecal sampling was superior, as specificity of such testing was close to perfect. By changing the threshold for considering a group positive, from 1 test-positive animal to 2, the PPV of serological results could be increased without substantial loss in NPV. The herd sensitivity based on (1) bulk milk sampling, (2) fecal sampling of all animals, and (3) bulk milk sampling and individual sera from 20 animals within each age group was 0.53, 0.88, and 0.95, respectively. In low-prevalence regions, this sensitivity was enough to verify a high probability of freedom (>0.99), as the probability of infection in such Swedish regions has been shown to be 0.01. For herds with a higher prior probability of infection, repeated sampling (2-9 sampling occasions) was needed to reach the same level of confidence. Analysis of surveillance data indicated that boot swabs can be used to replace the standard fecal sampling presently used in Sweden. It was also confirmed that the individual specificity of the tests used for serological testing of Swedish calves is high (0.99). The results can form a basis for fit-for-purpose testing strategies (e.g., surveillance or prepurchase testing).
基于瑞典法律，所有检测出任何血清型沙门氏菌的畜群都将受到限制，并需要采取根除措施。近年来，抽样和控制成本有所增加，本研究的目的是调查不同抽样策略的效率。我们还汇编了最近监测活动的测试结果，并将其用于补充和比较计算结果。在群组和畜群层面计算了不同测试策略的敏感性和特异性。采用情景树建模方法考虑了畜群内动物的层次结构，以及不同年龄组中沙门氏菌的相对风险。计算了阴性和阳性预测值（NPV 和 PPV）以及无沙门氏菌的概率，以比较不同抽样策略的附加值。结果表明，与每群血清学样本相比，需要更多的粪便样本才能达到群体敏感性>0.50。这也意味着血清学检测可提高 NPV。例如，在畜群中怀疑有沙门氏菌的 25 只动物的一组中，有 10 个阴性测试结果，基于血清学的 NPV 为 0.75，基于培养的 NPV 为 0.56。对于 PPV，基于粪便采样的培养检测更优越，因为此类检测的特异性接近完美。通过将群体阳性的阈值从 1 个阳性动物改为 2 个，血清学结果的 PPV 可以提高，而 NPV 的损失不大。基于（1）批量奶采样、（2）所有动物的粪便采样和（3）批量奶采样和每个年龄组的 20 只动物的个体血清，畜群敏感性分别为 0.53、0.88 和 0.95。在低流行地区，这种敏感性足以验证高自由概率（>0.99），因为已经表明瑞典地区的感染概率为 0.01。对于感染可能性较高的畜群，需要重复采样（2-9 次采样）才能达到相同的置信水平。对监测数据的分析表明，可以使用靴拭子代替瑞典目前使用的标准粪便采样。还证实了用于瑞典犊牛血清学检测的测试的个体特异性很高（0.99）。结果可以为针对性测试策略（例如监测或购买前测试）提供依据。
