Infectious myonecrosis (IMN): A new virus disease of Litopenaeus vannamei

 D.V. Lightner*, C.R. Pantoja, B.T. Poulos, K.F.J. Tang, R.M. Redman, T.

Andreas and J.R. Bonami

dvl@u.arizona.edu

 Infectious myonecrosis (IMN) is a recently identified disease in cultured Litopenaeus vannamei  in northeast Brazil. It causes significant disease and mortalities in juvenile and subadult pond-reared stocks of L.vannamei. Outbreaks of the disease seem to be associated with certain types of environment and physical stresses (i.e. extremes in salinity and temperature, collection by cast net, etc.), and possibly with the use of low quality feeds.

  IMN presents as a disease with an acute onset of gross signs and elevated mortalities, but it progresses with a more chronic course accompanied by persistent low level mortalities.  To date, IMN appears to be limited to northeast Brazil, but shrimp with similar gross signs have been also reported from other countries where L. vannamei are cultured.  Affected shrimp present focal to extensive white necrotic areas in the striated muscle, especially of the distal abdominal segments and tail fan.  These may become necrotic and reddened in some individual shrimp.  By histopathology, shrimp with acute phase disease present lesions with coagulative muscle necrosis, often with edema. In shrimp recovering from acute disease or those in the more chronic phase of the disease, the myonecrosis appears to progress from coagulative to liquefactive necrosis.  This progression of myonecrosis is accompanied with hemocytic infiltration and fibrosis.  Significant lymphoid organ spheroid formation is typically present, and ectopic lymphoid organ spheroids are often found in the hemocoel and loose connective tissues, especially in the heart lumen and adjacent to antennal gland tubules. 

The infectious nature of the disease has been demonstrated by transmission of the disease into SPF indicator shrimp by injection and per os challenge studies using cell-free filtrates prepared from diseased shrimp or chopped diseased shrimp carcasses, respectively.  A 40 nm diameter spherical virus has been isolated from naturally infected shrimp with the disease.  The virus has been partially characterized and portions of its nucleic acid (RNA) genome have been cloned and sequenced. Molecular probes and RT-PCR methods for diagnosis of the disease and detection of IMN have been developed and are being commercialized since October, 2004, by Farming IntelliGene. (You can find more information here)

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