Knopf Lea, Schwermer Heinzpeter, Stärk Katharina D C
Swiss Federal Veterinary Office, Bern, Switzerland.
BMC Vet Res. 2007 May 18;3:10. doi: 10.1186/1746-6148-3-10.
International trade regulations require that countries document their livestock's sanitary status in general and freedom from specific infective agents in detail provided that import restrictions should be applied. The latter is generally achieved by large national serological surveys and risk assessments. The paper describes the basic structure and application of a generic stochastic model for risk-based sample size calculation of consecutive national surveys to document freedom from contagious disease agents in livestock.
In the model, disease spread during the time period between two consecutive surveys was considered, either from undetected infections within the domestic population or from imported infected animals. The @Risk model consists of the domestic spread in-between two national surveys; the infection of domestic herds from animals imported from countries with a sanitary status comparable to Switzerland or lower sanitary status and the summary sheet which summed up the numbers of resulting infected herds of all infection pathways to derive the pre-survey prevalence in the domestic population. Thereof the pre-survey probability of freedom from infection and required survey sample sizes were calculated. A scenario for detection of infected herds by general surveillance was included optionally.
The model highlights the importance of residual domestic infection spread and characteristics of different import pathways. The sensitivity analysis revealed that number of infected, but undetected domestic herds and the multiplicative between-survey-spread factor were most correlated with the pre-survey probability of freedom from infection and the resulting sample size, respectively. Compared to the deterministic pre-cursor model, the stochastic model was therefore more sensitive to the previous survey's results. Undetected spread of infection in the domestic population between two surveys gained more importance than infection through animals of either import pathway.
The model estimated the pre-survey probability of freedom from infection accurately as was shown in the case of infectious bovine rhinotracheitis (IBR). With this model, a generic tool becomes available which can be adapted to changing conditions related to either importing or exporting countries.
国际贸易法规要求各国记录其家畜的总体卫生状况，并详细记录无特定感染因子的情况，前提是应实施进口限制。后者通常通过大规模的国家血清学调查和风险评估来实现。本文描述了一个通用随机模型的基本结构和应用，该模型用于基于风险的连续国家调查样本量计算，以证明家畜无传染病原体。
在该模型中，考虑了两次连续调查期间疾病的传播情况，传播途径既可能是国内未被检测出的感染，也可能是进口的感染动物。@Risk模型包括两次国家调查之间的国内传播情况；来自卫生状况与瑞士相当或低于瑞士的国家的进口动物对国内畜群的感染情况，以及汇总表，该表汇总了所有感染途径导致的感染畜群数量，以得出国内畜群的调查前患病率。据此计算调查前无感染的概率和所需的调查样本量。该模型还可选择纳入通过常规监测检测感染畜群的情景。
该模型突出了国内残留感染传播的重要性以及不同进口途径的特点。敏感性分析表明，感染但未被检测出的国内畜群数量和调查期间的传播倍增因子分别与调查前无感染的概率和最终样本量最相关。因此，与确定性前身模型相比，随机模型对先前调查结果更敏感。两次调查之间国内未被检测出的感染传播比通过任何一种进口途径的动物感染更为重要。
如传染性牛鼻气管炎（IBR）案例所示，该模型准确估计了调查前无感染的概率。借助此模型，可获得一个通用工具，该工具可根据进出口国家的变化情况进行调整。
