Martinez Brayan Alexander Fonseca, Leotti Vanessa Bielefeldt, Borba Mauro Riegert, Silva Gustavo de Sousa E, Corbellini Luís Gustavo
Laboratory of Veterinary Epidemiology, Department of Preventive Veterinary Medicine, Faculty of Veterinary, Federal University of Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 9090, CEP 91540-000 Porto Alegre, RS, Brazil.
Department of Statistics, Institute of Mathematics and Statistics, Federal University of Rio Grande do Sul, Av. Bento Gonçalves 9500, CEP 91509-900 Porto Alegre, RS, Brazil.
Vet Parasitol. 2017 Apr 15;237:77-82. doi: 10.1016/j.vetpar.2017.02.016. Epub 2017 Feb 24.
The purpose of this study was to use mixed logistic regression to evaluate the association between Neospora caninum serostatus and abortion, accounting for the effects of farms. The main objective was to explore the interpretation of this model and discuss the contribution of this analysis to our understanding of the disease's epidemiology. A mixed-effects logistic model using farms as a random effect and the serostatus for N. caninum, age of cattle and farm location as fixed effects was performed. The data from 1256 cows over 15 months of age from 60 farms were used, and the abortion information was obtained from farm records. A significant association between N. caninum serostatus and abortion was found (p<0.0001). Seropositive cows had 6.63 times greater odds of having a history of previous abortion (95% CI: 4.35-13.37). There was remarkable variability across farms in the probability of a cow having a history of an abortion. Including the effects of the farms in the regression, it was possible to estimate an intraclass correlation coefficient (ICC) of 16%. That means that 16% of the variation in abortion occurrence that was not explained by the fixed effects was due to farms. In practical terms, this variation means that while there are farms with several seropositive cows and no/few abortion cases, the opposite is also true, with a high number of abortions in farms with low/medium seroprevalence. The remaining variability (84%) was not explained by the effect of N. caninum, age, region, and the effect of farms, i.e., it is due to unknown factors that are causing abortions. The results of this study reinforce the importance of N. caninum as a cause of abortions and demonstrate the significant heterogeneity in the probability of a cow having a history of an abortion that cannot be explained completely by N. caninum serostatus, age or location. Including the hierarchical structure of the population along with correct interpretation of the model estimates helps us understand the effect of the farms, i.e., the probability of a cow abortion conditional to the farms, which also contributes to our understanding of the epidemiology of abortions caused by neosporosis. The use of hierarchical models and reporting the ICC should be encouraged.
本研究的目的是使用混合逻辑回归来评估犬新孢子虫血清状态与流产之间的关联，并考虑养殖场的影响。主要目标是探讨该模型的解释，并讨论该分析对我们理解该疾病流行病学的贡献。我们构建了一个混合效应逻辑模型，将养殖场作为随机效应，犬新孢子虫血清状态、牛的年龄和养殖场位置作为固定效应。使用了来自60个养殖场的1256头15个月以上奶牛的数据，流产信息来自养殖场记录。研究发现犬新孢子虫血清状态与流产之间存在显著关联（p<0.0001）。血清阳性的奶牛有过流产史的几率比血清阴性的奶牛高6.63倍（95%可信区间：4.35-13.37）。不同养殖场的奶牛有流产史的概率存在显著差异。在回归分析中纳入养殖场的影响后，可以估计组内相关系数（ICC）为16%。这意味着，固定效应未解释的流产发生率变异中有16%是由养殖场引起的。实际上，这种变异意味着，虽然有些养殖场有几头血清阳性奶牛但没有/只有很少的流产病例，但情况相反的情况也存在，即在血清阳性率低/中等的养殖场有大量流产病例。其余的变异（84%）无法用犬新孢子虫、年龄、地区以及养殖场的影响来解释，即它是由导致流产的未知因素引起的。本研究结果强化了犬新孢子虫作为流产病因的重要性，并表明奶牛有流产史的概率存在显著异质性，无法完全用犬新孢子虫血清状态、年龄或位置来解释。纳入群体的层次结构并正确解释模型估计值有助于我们理解养殖场的影响，即奶牛在各养殖场发生流产的概率，这也有助于我们理解新孢子虫病引起的流产的流行病学。应鼓励使用层次模型并报告ICC。
