Many whiteleg shrimp farms in Vietnam have been attacked by an emerging disease which is recognized with focal white muscle on abdominal segments of dead or moribund shrimp along with significant mortality. The occurrence of the disease is commonly related to the poor management of water quality in shrimp ponds such as low dissolved oxygen, low alkalinity, high toxic gas, and algal bloom. The gross signs are similar to some infectious diseases, including both bacterial and viral agents that cause damage to the shrimp muscle. However, the histopathology of this disease was different from the diseases caused by viruses but similar to the cases caused by bacteria. Furthermore, the PCR results showed that both virus factors such as Infectious Myonecrosis Virus (IMNV), Penaeus Nodavirus (PvNV), and Covert Mortality Nodavirus (CMNV) and bacterial factors (Vibrio harveyi) were not detected. Therefore, the study hypothesized that this disease is caused by new bacterial agents.
Bacterial isolation, subculture, reinfection, and reisolation were carried out according to Koch’s Postulates. Three consecutive challenge tests along with some microbiological, histopathological, and molecular analyses were conducted to confirm the causative agents of the disease. Based on the biochemical test, PCR, and 16s sequencing results, strains of Vibrio spp. were identified as the causative agents. Notably, these strains produced green colonies on the Thiosulfate-citrate-bile salts-sucrose (TCBS) media. The present study proved that these strains were able to induce symptoms of the disease via immersion route with high mortality and signs of damage exhibiting skeletal muscle necrosis as an opaque, whitish, discoloration of several abdominal segments in infected shrimps. In addition, the histopathology of infected shrimps in laboratory conditions was the same as the samples from the farm with extensive coagulative necrosis, hemocytic infiltration in skeletal muscle, and the presence of rod-shaped bacteria. Thus, the present study revealed that the muscle necrosis disease was caused by strains of Vibrio spp.