Histological analysis of an outbreak of red gill disease in cultured oriental river prawn Macrobrachium nipponense

An epidemic of red gill disease with a high mortality rate recently occurred in the cultured oriental river prawn Macrobrachium nipponense in China. The prawns had flaccid bodies and gills were red in color with numerous black spots. By wet smear microscopy and negative staining, numerous membrane-bound plasmodia or degenerated spores were observed. Histological sections indicated the infection was limited to the gills. The gill filaments were filled with masses of mononuclear parasites, multinucleate and multicellular plasmodia and became enormously hypertrophic. Host encapsulation response and inflammatory foci with phagocytized parasite spores were evident. Transmission electron microscopy (TEM) analysis confirmed the gross plasmodia almost filling the entire space of gill epithelial cell. The intracellular parasite was a multinucleate plasmodium enclosed in a vacuole within the cytoplasm of the host cell. Large numbers of trophonts were visible in each plasmodium. High degree of vesiculation of early sporonts was evident. Scanning electron microscopy (SEM) and TEM observations all revealed characteristic tubular extension from the surface of a late sporont. We recognized the risk of associating the presence of a parasite with a particular pathology in the absence of total evidence that demonstrates an unequivocal link between the parasite, the perceived pathology and a clear disease manifestation that would be demonstrated by e.g. experimental studies. However, given the widespread importance of the host in aquaculture, it is necessary to provide this short report to highlight to other researchers and to the industry the potential for a new and emerging disease. It is hoped that this will lead to additional sightings of the condition and additional research to confirm the link between parasite and the pathology seen. It is our intention to complete life-cycle studies with to satisfy Koch's postulate.