Recently, the swollen siphon disease in cultured otter clams (Lutraria philippinarom) causes severe lost for the farmers and still there is no sign of disease recovering. The study was conducted from December 2012 t9 June 2014 with 1,150 samples collected from otter clams with and without swollen siphon signs in Ha Long Bay and Cam Ranh Bay, Vietnam. The samples were fixed in 10 percent buffer formalin adding 2 percent salt and processed for hitopathological sections following with H&E staining. The study results showed that both of siphon and mantle have characteristic structural changes such as relaxation of muscle, dilation of haemolymph vessels. In addition, other changes in the outer membrane of the siphon as lifting and separation of cuticle from underlying epithelium, anomalous development of cuticle with marked thickening and exuberant disorganised production of eosinophilic membranous material entrapping detritusfmlso oedematous lifting and separation of the cuticular epithelium from underlying stroma, moderate to marked vacuolation and necrosis of the epithelial cells of cuticular epithelium. The changes of relaxed muscle, dilated haemolymph vessel of siphon and mantle in diseased animals corresponding to 52.3 percent and 31.8 percent. The epithelial changes on diseased otter clam is 32.7 percent. Abnormal changes in the outer membrane of diseased animals is 27.5 percent. These changes are also observed in otter clams without signs of disease, but light level and prevalence lower. Specifically, for otter clams without disease signs, the ratio of siphon and mantle sample with relaxed muscle, dilated haemolymph vessel corresponding to 20.5 percent and 7.3 percent
the epithelial changes is 32.7 percent
abnormal changes in the outer membrane is 27.5 percent. The results in gill tissue and digestive system have a few changes. However, presenting low numbers of Rickettsial-like cysts in gill tissue with a high prevalence of 49.2 percent and 47.9 percent in both with and without disease sign otter clams, respectively. But there is not any associated host responses. The study results have identified characteristic changes in the siphon and the mantle tissue of swollen siphon diseased otter clams in the disease outbreaks in recent years.