Southern Brown Shrimp Susceptible to IMNV

Infectious Myonecrosis Virus causes opacity in the abdominal muscles and necrosis of the tails in various shrimp species. To overcome IMNV-related mortalities in cultured white shrimp, some Brazilian shrimp farmers are testing the southern brown shrimp, Fenneropenaeus subtilis, in growout ponds. Testing found that the virus also affects the brown shrimp.

In September 2002,unusual mortality in farmed Pacific white shrimp, Litopenaeus vannamei, was reported at a shrimp farm in the state ofPiaui in northeastern Brazil.Affected animals were lethargic and presented opacity in the second or third abdominal segments.Persistent,low-level mortalities reduced the affected population by 50-70%. Some animals presented reddish coloration in the last abdominal segment and totally necrotic tails.Histopathological analyses showed numerous spheroids in their lymphoid organs and muscular necrosis with hemocytic infiltration.Some specimens presented necrotic lesions in the hepatopancreas.




Shrimp with IMN have reddish areas on their muscle and tail areas.





Testing of affected brown shrimp showed spheroid formation in the lymphoid organ and necrosis of the muscle tissue.




Infectious Myonecrosis

Virus Infectious Myonecrosis Virus (IMNV) has since been identified as the causative agent ofthe necrosis in L.vannamei. Until now,the disease was reported as naturally occurring in cultured L.vannamei.The nonenveloped virus was also experimentally induced in Pacific blue shrimp, L.stylirostris; and black tiger shrimp, Penaeus monodon.

Southern Brown Shrimp Infection

To overcome mortalities produced by IMNV in cultured L.vannamei and increase production,some Brazilian shrimp farmers are testing the southern brown shrimp, Fenneropenaeus subtilis, in growout ponds.

In a recent test at a commercial farm,two 2-ha ponds were stocked with southern brown shrimp at 10 postlarvae/m2.The animals were fed a 35%-crude protein commercial diet formulated for L.vannamei. Dissolved-oxygen levels remained 5-8 ppm during the whole cycle.After 65 days ofculture,the first symptoms ofIMN were noticed in animals in both ponds.At this time,the shrimp averaged 7 g in body weight.At harvest,body weights averaged 11 g,and the survival rates in the ponds were 23 and 26%.

Diseased shrimp presented the characteristic opacity in the abdominal muscles and necrosis ofthe tails. Histopathological sections stained with the salts hematoxylin and eosin (H & E) showed spheroids in the lymphoid organs and focal necrosis in muscle,with hemocytic infiltration.

To confirm the diagnosis,reverse transcription polymerase chain reaction (RT-PCR) testing was done using RNA extracted from the gills and pleopods ofaffected shrimp with a commercial kit for IMNV and primers designed at the University ofArizona.Results confirmed the presence ofIMNV in the cultured F.subtilis,and thus expanded the host range susceptibility ofIMNV.-