Identification and isolation of a spiroplasma pathogen from diseased freshwater prawns, Macrobrachium rosenbergii, in China: A new freshwater crustacean host

Disease epizootics in freshwater cultured crustaceans, including freshwater prawns (Macrobrachium rosenbergii), gained high attention recently in Jiangsu, China due to intensive development of freshwater aquaculture and their susceptibility to massive mortalities. Morphological observation indicated that the pathogen in diseased M. rosenbergii had a helical morphology and lacked a cell wall. The agent could infect hemocytes and all the connective tissues of the organs such as hepatopancreas, digestive tract, and cardiac muscle. The agent was able to pass through membrane filters with pores 220 nm in diameter and could be cultivated by R2 medium. The pathogenicity of the agent was evaluated by Koch's postulate and the mortalities produced by the pathogen were determined to be 80% through experimental infection. The phylogenetic position was investigated by analysis of the 16S rRNA complete sequence. The result revealed that the pathogen had a close relationship with other freshwater spiroplasmas and was situated in the Mirum clade together with Spiroplasma mirum in a phylogenetic tree. Taken in total, the findings demonstrated that the pathogen isolated from diseased M. rosenbergii was a spiroplasma microorganism. This is the fourth report of a spiroplasma pathogen in commercially exploited crustaceans and indicates that spiroplasma pathogens have spread rapidly in aquatic crustaceans; thus, requiring more substantial research attention.