Francisella orientalis is an important bacterial pathogen of marine and freshwater fish with worldwide distribution. Fish francisellosis is a severe subacute to chronic granulomatous disease, with high mortalities and high infectivity rates in cultured and wild fish. To date, there is no approved vaccine for this disease. In this study, we evaluated the efficacy of a defined F. orientalis pathogenicity determinant protein A (pdpA) mutant (ΔpdpA) as a live attenuated immersion vaccine against subsequent immersion challenge with the wild-type organism. Immunized Nile tilapia Oreochromis niloticus were protected (45% relative percent survival) from the lethal challenges and presented significantly lower mortality than nonvaccinated and challenged treatments. Although serum IgM was significantly higher in immunized fish, similar bacterial loads were detected in vaccinated and nonvaccinated survivors. In conclusion, although the F. orientalis ΔpdpA is attenuated and effectively stimulated an adaptive immune response, the low relative percent survival and high bacterial persistence in survivors of immunized and challenged treatments indicates low suitability of ΔpdpA as a mucosal vaccine for tilapia under conditions used in this study.

中文翻译：

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东方弗朗西斯菌 ΔpdpA 作为尼罗罗非鱼弗朗西斯氏菌病减毒活疫苗的评价

Francisella orientalis是一种重要的海洋和淡水鱼类病原菌，在世界范围内均有分布。鱼肉芽肿病是一种严重的亚急性至慢性肉芽肿病，在养殖和野生鱼类中具有高死亡率和高传染率。迄今为止，还没有针对这种疾病的获批疫苗。在这项研究中，我们评估了确定的东方镰孢致病性决定蛋白 A ( pdpA ) 突变体 (Δ pdpA ) 作为减毒浸没活疫苗对抗野生型生物体随后浸没攻击的功效。免疫尼罗罗非鱼Oreochromis niloticus受到保护（45% 的相对存活率）免受致命挑战，并且死亡率明显低于未接种疫苗和挑战的治疗。尽管免疫鱼的血清 IgM 明显更高，但在接种疫苗和未接种疫苗的幸存者中检测到相似的细菌载量。总之，尽管F. orientalis Δ pdpA被减弱并有效地刺激了适应性免疫反应，但在免疫和攻击治疗的幸存者中较低的相对存活率和较高的细菌持久性表明 Δ pdpA在条件下作为罗非鱼的粘膜疫苗的适用性较低在本研究中使用。