Immunohistochemical examination of experimental Streptococcus iniae infection in Japanese flounder Paralichthys olivaceus

Abstract

To investigate the course of Streptococcus iniae infection in Japanese flounder Paralichthys olivaceus, fish (average weight 118±14 g) were experimentally infected by oral and bath methods, and the distribution and multiplication of S. iniae in the fish were monitored by bacteriological and immunohistochemical examinations. S. iniae was detected first in relatively high numbers in the kidney and spleen. Viable counts of S. iniae in the blood, brain, liver, stomach, intestine, gill, skin mucus and nares were high only when those in the kidney and spleen were high. S. iniae-laden phagocytic cells were observed in the lumen of the blood vessels distributing in the organs and tissues in the initial stage of infection. Many fish showed hemorrhagic lesions on the fins, and the extracellular multiplication of S. iniae in the hemorrhagic fins was observed in the initial stage of infection. These observations were common among the fish challenged by either method. There was no evidence of the entrance of S. iniae through the stomach, intestine, gill, eye or olfactory pouches of nares. It was presumed that S. iniae entered directly from the water through the abrasive sites of fins and was disseminated by the blood circulation to cause systemic infection.