Pathology of experimentally induced mouthrot caused by <i>Tenacibaculum maritimum</i> in Atlantic salmon smolts

Mouthrot, caused by Tenacibaculum maritimum is a significant disease of farmed Atlantic salmon, Salmo salar on the West Coast of North America. Smolts recently transferred into saltwater are the most susceptible and affected fish die with little internal or external clinical signs other than the characteristic small (usually < 5 mm) yellow plaques that are present inside the mouth. The mechanism by which these smolts die is unknown. This study investigated the microscopic pathology (histology and scanning electron microscopy) of bath infected smolts with Western Canadian T. maritimum isolates TmarCan15-1, TmarCan16-1 and TmarCan16-5 and compared the findings to what is seen in a natural outbreak of mouthrot. A real-time RT-PCR assay based on the outer membrane protein A specific for T. maritimum was designed and used to investigate the tissue tropism of the bacteria. The results from this showed that T. maritimum is detectable internally by real-time RT-PCR. This combined with the fact that the bacteria can be isolated from the kidney suggests that T. maritimum becomes systemic. The pathology in the infected smolts is primarily mouth lesions, including damaged tissues surrounding the teeth; the disease is similar to periodontal disease in mammals. The pathological changes are focal, severe, and occur very rapidly with little associated inflammation. Skin lesions are more common in experimentally infected smolts than in natural outbreaks, but this could be an artefact of the challenge dose, handling and tank used during the experiments.由海洋噬纤维杆菌（Tenacibaculum maritimum）引发的口腐病，是北美西海岸养殖大西洋鲑（Salmo salar）的一种重大病害。近期刚转入海水的降海幼鲑（smolts）易感性最强，患病幼鲑死亡时，除口腔内部出现特征性小型（通常直径小于5 mm）黄色斑块外，几乎无其他内部或外部临床症状。此类幼鲑的致死机制目前尚不明确。本研究针对加拿大西部分离得到的3株海洋噬纤维杆菌TmarCan15-1、TmarCan16-1及TmarCan16-5，通过浸泡感染的方式处理降海幼鲑，对其显微病理特征（组织学及扫描电子显微镜（scanning electron microscopy））展开分析，并将实验结果与自然暴发的口腐病病例进行比对。本研究设计了针对该菌外膜蛋白A（outer membrane protein A）的实时逆转录聚合酶链式反应（real-time RT-PCR）检测体系，用于探究该菌的组织嗜性。检测结果显示，通过实时逆转录聚合酶链式反应可在宿主体内检出海洋噬纤维杆菌。结合可从肾脏组织中分离得到该菌的实验现象，表明海洋噬纤维杆菌可造成全身性感染。受感染幼鲑的病理损伤主要集中于口腔部位，包括牙齿周围组织的破坏，该病症与哺乳动物的牙周病（periodontal disease）表现相似。其病理变化呈局灶性、严重性特征，且进展极为迅速，伴随的炎症反应却极为轻微。实验感染幼鲑的皮肤损伤发生率高于自然暴发病例，但这可能是实验过程中攻毒剂量、操作手法及养殖水槽所致的人为偏差。