Recently, researchers from Yellow Sea Fisheries Research Institute identified a new nodavirus, named covert mortality nodavirus (CMNV), to be associated with covert mortality disease (CMD) of shrimp which has been described in shrimp farming industry in China in the recent decade. The related paper was published online in Journal of General Virology in September, 2014.

High-density shrimp farming has been affected by an unknown disease insouthern China since 2002. Since most of the moribund shrimp hide on the bottom in deep water rather than swim to the surface or in shallow water, very much like the shrimp suffering from white spot disease, the emerging disease was generally called “covert mortality disease”. The shrimp suffering CMD represented cumulative mortality daily in farming ponds after 30 days post stocking and typical clinical signs as hepatopancreatic atrophy and necrosis, empty stomach and guts, soft shell, slow growth, and in many case, abdominal muscle whitening and necrosis. Additionally, environmental stresses, such as high concentration of nitrite and high temperature, could aggravate the CMD-related mortality. Mortality increases when the water temperature is over 28°C in CMNV-infected ponds. It is noted that the mortality got worse during 60~80 days post-stocking with a total mortality up to 80%. This disease was spread to northern China in 2009-2010 and wasprevalent in the main shrimp farming areas in China at present. Positive cases of the pathogen were also detected in the samples collected in different countries using the protocol developed by our group.

Fig. 1 Clinical signs of shrimps suffering from covert mortality disease (CMD). Black arrows show whitening muscle of abdominal segment. White arrows indicate hepatopancreas atrophy and colour fading. The framed triangles show the healthy shrimp, the black triangles indicate the shrimp suffering CMD and the white triangles indicate the hepatopancreas of healthy shrimp.

Fig. 2 Phylogenetic tree based on the deduced amino acid sequence RNA-dependent RNA polymerase from CMNV and other nodaviruses. The frame indicated the CMNV.

Fig. 3 Transmission electron micrographs of negative stained CMNV virions. (A and B) Virus-like particles collected from sucrose-gradient fraction. The size of the virus-like particles is not homogeneous. The larger virus-like particles are about 32 nm (black arrows) and the smaller virus-like particles (white arrows) are about 19 nm in diameter. (C and D) Magnified micrograph of the virus-like particles. Scale bars in A and B = 50 nm.