Interpretation of the Gamma Interferon Test for Diagnosis of Subclinical Paratuberculosis in Cattle

A group of 252 cattle without clinical signs of paratuberculosis (paraTB) in 10 herds infected with paraTB and a group of 117 cattle in 5 herds without paraTB were selected. Whole-blood samples were stimulated with bovine, avian, and johnin purified protein derivative (PPD) and examined for gamma interferon (IFN-γ) release. For diagnosis of paraTB, satisfactory estimated specificities (95 to 99%) could be obtained by johnin PPD stimulation irrespective of interpretation relative to bovine PPD or no-antigen stimulation alone, but numbers of test positives in the infected herds varied from 64 to 112 with different interpretation criteria. For a limited number of test-positive animals, no change in the test results could be observed with increasing antigen concentrations but IFN-γ responses were significantly reduced (P < 0.0001) and four out of seven reactors tested negative when stimulation was performed on day-old samples. Denmark is free of bovine tuberculosis, but cross-reactivity with paraTB could be documented for cattle more than 14 months old in paraTB-infected herds compared with those in non-paraTB-infected herds. In both paraTB-free and paraTB-infected herds, false positives were observed when the test was applied to calves less than 15 months of age. Until novel antigen formulations more specific for these diseases are available, interpretation of the IFN-γ test must be individually adjusted to fit specific needs and the context within which the test is applied and, for paraTB, the test seems most appropriate for use as a supportive tool for evaluation of disease-preventive measures in young stock.

本数据集选取了10个副结核分枝杆菌病（paratuberculosis, paraTB）感染牛群中的252头无副结核临床症状的牛只，以及5个未感染paraTB的牛群中的117头牛只作为研究对象。采集所有研究对象的全血样本，分别以牛型纯化蛋白衍生物（bovine PPD）、禽型纯化蛋白衍生物（avian PPD）及约翰菌素纯化蛋白衍生物（johnin PPD）进行刺激，随后检测样本中γ干扰素（gamma interferon, IFN-γ）的释放水平。针对paraTB的诊断，无论以牛型PPD作为参照，还是仅以无抗原刺激组作为参照，采用约翰菌素PPD刺激均可获得95%~99%的理想估计特异性；但不同判定标准下，感染牛群中的检测阳性数区间为64至112头。针对少量检测阳性的牛只，当抗原浓度升高时，其检测结果未出现明显变化，但IFN-γ应答水平显著降低（P < 0.0001）；而当对采集后放置1天的全血样本进行刺激实验时，7头阳性反应牛中有4头转为阴性。丹麦目前无牛结核病（bovine tuberculosis）疫情，但相较于未感染paraTB的牛群，感染paraTB的牛群中14月龄以上的牛只可检测出与paraTB的交叉反应。在未感染paraTB和感染paraTB的两类牛群中，对15月龄以下的犊牛开展该检测时均出现了假阳性结果。在开发出针对这两类疾病的更高特异性的新型抗原制剂之前，IFN-γ检测的判定标准需根据具体应用场景与需求进行个性化调整；而就paraTB而言，该检测最适合作为青年牛群疾病防控措施评估的辅助工具。