Infectious myonecrosis (IMN), caused by infectious myonecrosis virus (IMNV), is the disease of greatest impact on shrimp farming in the northeast region of Brazil. The occurrence of IMN remained restricted to northeastern Brazil until 2006, when its presence was also confirmed in Indonesia. To determine the occurrence and evolution of IMN in Litopenaeus vannamei farmed along the coast of the state of Pernambuco, Brazil, histopathological examinations were performed on 60 samples collected from four farms in both predominant seasons in the northeastern region: dry and wet seasons. Samples made up of ten specimens were collected monthly from each pond. Histopathological results were associated to wet-mount exams and rearing performance data. Lesions suggestive of IMN (coagulative necrosis, hemocytic infiltration in the musculature, ectopic spheroids in the lymphoid organ) were identified in all the farms, with a higher occurrence during the dry season. Longer rearing periods and higher stocking densities were the variables with the most significant influence (p<0.05) in the occurrence of IMN.
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