Distinctive histopathology of Spiroplasma eriocheiris infection in the giant river prawn Macrobrachium rosenbergii

Abstract Spiroplasma eriocheiris was first named as the cause of tremor disease in the Chinese mitten crab Eriocheir sinensis in China in 2003. It was subsequently reported from China to infect also the crayfish Procambarus clarkii , the giant river prawn Macrobrachium rosenbergii , the oriental prawn Macrobrachium nipponensis and the whiteleg shrimp Penaeus vannamei . Unfortunately, none of these publications give clear pictures of the histopathology caused by the pathogen, even though this is one of the first diagnostic procedures often performed for newly reported diseases. Here we describe distinctive histological lesions caused in the hepatopancreatic tissue of M. rosenbergii infected with S. eriocheiris . In the hepatopancreas (HP), these consist of basophilic or mixed basophilic/eosinophilic lesions in cells confined to the interstitial spaces of the HP and not involving the hepatopancreatic tubule epithelial cells. Examination of the lesions by transmission electron microscopy revealed the presence of colonies of intracellular, un-walled bacterial cells of variable shape (but many crescent shaped) and approximately 150 nm in diameter. PCR using group-specific primers for the SSU rRNA followed by cloning and sequencing revealed 99.9% sequence identity to the matching sequence of S. eriocheiris . PCR tests for presence of S. eriocheiris in hepatopancreatic tissue samples gave positive test results, and in situ hybridization assays using a probe derived from the PCR product gave positive signals in the interstitial spaces of the HP, corresponding to the regions of the distinctive histological lesions. Less severe lesions containing cells with eosinophilic/basophilic inclusions were also found and confirmed by in situ hybridization in heart muscle, skeletal muscle, gills, connective tissue and the ventral nerve cord. Recognition of the distinctive histological lesions in interstitial spaces of the hepatopancreatic tissue of M. rosenbergii by histological analysis can be used as a strong presumptive indication of infection by S. eriocheiris and may help in the control of this rapidly-spreading, emerging pathogen.