A preliminary study on the "zombie disease" of Exopalaemon carinicauda

In recent years, an epidemic disease called "zombie disease" by local farmers has appeared in the ridgetail white prawn (Exopalaemon carinicauda) breeding area of Jiangsu Province. The main symptoms of the disease include reddish body color, whitish muscles and milky liquid under the carapace. The diseased prawns usually show slow movement and reduce food intake and died quickly after leaving the water. In order to determine the pathogen and epidemiological characteristics of the "zombie disease"in farmed E. carinicauda in the coastal area of Jiangsu Province since the winter of 2018, LB medium and PDA medium were used to isolate colonies with a diameter of 1-3 mm, neat edges, and beige-colored elevation from the hemolymph of diseased shrimp, which was named MQ2101. This strain was identified as Metschnikowia bicuspidata by means of morphologic structure observation, 18S rRNA gene analysis and constructing phylogenetic tree. Furthermore, the pathogenicity of the strain MQ2101 was confirmed by the infection experiment in healthy E. carinicauda. The results showed the infected E. carinicauda died with the similar symptoms to naturally morbid E. carinicauda and the same bacterium could be isolated from the infected E. carinicauda. The LD₅₀ of strain MQ2101 to E. carinicauda was 1.39×10⁷ CFU/ind. Histopathology and transmission electron microscopic observation showed that there were different degrees of lesions in gills, muscle and hepatopancreas, of which hepatopancreas lesions were the most serious, with hepatic tubules vacuolated and lumen volume enlarged. Moreover, a large number of colonized bacteria were found in gill and hepatopancreas. Epidemiological findings showed the disease frequently occured in the cool season (February to May), mainly erupted in prawn and crab mixed ponds, with a incidence rate of 5%-30% and a mortality rate of 3%-10%, the morbidity was increasing year by year. In the present study, M. bicuspidata was identified as the pathogen of "zombie disease" in E. carinicauda in coastal areas of Jiangsu Province, which is highly pathogenic to E. carinicauda and mainly infects the hepatopancreas and gills. It was the first time that revealedM. bicuspidata infection in E. carinicauda and the pathological changes caused by the infection. The above findings provide a scientific basis for the prevention and control of "zombie disease" in the culture of prawns.