Abstract
Prawns naturally infected with vibriosis collected from prawn farms and those experimentally infected with Vibrio sp. by placing them for 2h in bacterial suspension were studied histopathologically. No essential differences in histopathological findings were noted between the naturally and the experimentally infected prawns. Most characteristic pathological changes were extensive necrosis caused by severe bacterial invasion and multiple formation of melanized nodules in the lymphoid organ. Nodules were mostly composed of a bacterial colony in the center, a melanized zone around the bacterial colony and multiple layers of hemocytes encapsulating the melanized zone. Most of the hemocytes had pyknotic nuclei. No extensive necrotic lesions were found in other organs such as heart, gills, hepatopancreas, gonads, abdominal musculature, whereas small melanized nodules were frequently observed in these organs. The nodule formation is thought to be due to protective responses to invaded bacteria. It is conceivable that the lymphoid organ is highly susceptible to Vibrio sp. and in this organ successively invading bacteria overwhelm the protective responses, resulting in extensive necrosis which may possibly lead to the death of infected prawns.
