Lactococosis en Pseudoplatystoma reticulatum

Abstract

Lactococcosis is reported worldwide in numerous fish species causing heavy financial losses. Pseudoplatystoma reticulatum is a native species of Paraná River, edible and highly appreciated in the region, so the consideration of diseases in intensive crops is important. In this work we present a high mortality case of P. reticulatum juveniles from a farm located in Northeastern Argentina. For this purpose, animals with symptoms were euthanized and the head was removed. Liver was collected and sections of the organ were used for histopathology. Small pieces were placed in tubes containing Reinforced Clostridial Medium (Difco) and then incubated. Cultures with microorganisms were incubated on plates containing Man Rogosa and Sharpe agar (MRS Merck) during 24 to 28 h. The colonies obtained were restrocked and kept refrigerated at -20°C in media containing glycerol at 20%. Cultured microorganisms were submitted to various biochemical tests to determine their taxonomy. Samples of gills, liver, spleen, gastrointestinal tract and kidney were fixed in Bouin's solution and then dehydrated and embedded in paraffin. Histological sections were stained with Hematoxylin-Eosin, PAS and Giemsa. Hemorrhagic lesions were found in different parts of the tegument, and presence of ascites and numerous punctuate lesions in the liver were observed. In the imprints and biochemical tests of isolates Gram positive cocci were isolated, in diplos or short chains, that were negative catalase test and unable to reduce nitrate and synthesize tryptophan from indol. In the liver it was observed predominance of vascular alterations, mainly hemorrhagic of high intensity. Bacterial invasion stood out with predominantly peripheral location. In the spleen intense vascular alterations were observed with congestive and hemorrhagic dominance. It can be concluded according to the clinical characteristics of lesions, biochemical tests and histopathological diagnosis that mortality of fishes was due to Lactococcus garvieae.