Submitted to: Aquaculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2000
Publication Date: 2/13/2001
Citation: EVANS, J.J., SHOEMAKER, C.A., KLESIUS, P.H. DISTRIBUTION OF STREPTOCOCCUS INIAE IN HYBRID STRIPED BASS (MORONE CHRYSOPS X MORONE SAXATILIS) FOLLOWING NARE INOCULATION. AQUACULTURE. 194 (2001) 233-243 Interpretive Summary: Streptococcus iniae is recognized as one of the most problematic bacterial pathogens in intensively cultured hybrid striped bass and tilapia in the United States. Streptococcus iniae causes disease and mortality in these species which results in economic losses to the fish grower. The manuscript reports on experimentation conducted to examine the distribution of S. iniae in hybrid striped bass organs and blood over time. S. iniae was placed in the nares of 95 fish and fish were observed for 52 h for disease signs and/or mortality. To recover the S. iniae organisms, bacteriological samples were obtained from 9 tissues of 3-6 fish at 7 sample times. Our results indicated that the nares (olfactory organ) was the first organ from which we could recover S. iniae in as little as a half an hours time. The eye and the optic lobe of the brain were the last of the 9 tissues from which S. iniae was recovered. The spread of S. iniae was evident in all tissues by 18 hours although the numbers of bacteria remained low in the eye throughout the experiment. As the number of bacteria increased in all organs fish began to die or cease eating. Bacterial counts in the nares and kidney continued to increase throughout the experiment indicating a high affinity of S. iniae for these organs. The pattern of S. iniae distribution in the tissues suggest that nares serve as the route of infection and bacteria spread rapidly to the blood and brain prior to death. This may be an explanation for the rapid transmission of streptococcal disease in aquaculture facilities and indicates nares should be considered a routine organ for early microbiological detection of S. iniae.
Technical Abstract: Streptococcus iniae is an important fish pathogen worldwide. Previously, we showed that the olfactory system was an initial site of entry by which S. iniae infects the brain and then becomes septicemic. However, the pathogenesis of S. iniae, has not been examined. To determine the organ distribution of S. iniae over time, we inoculated posterior nares of 95 hybrid bass with 1.1 * 10**5 colony forming units (CFU) of S. iniae (5.23 * 10**4 CFU/nare). We sampled fish at 0.5, 4, 12, 18, 24, 48 and 52 h for S. iniae from 9 anatomical regions: the blood of the first two gill arches and blood of the third and fourth gill arches, nares, olfactory, optic and cerebellum regions of the brain, eye, heart and anterior kidney. The nare was the only organ colonized at 0.5 h. Colonization and growth of Streptococcus iniae were evident in the cerebellum, blood of the gills, heart and kidney at 4 h and in the olfactory lobe at 12 h. The optic lobe and eye became colonized at 18 h. S. iniae CFU in nare, blood of the third and fourth gill arches and kidney remained elevated through 52 h but declined in all other tissues at 52 h. S. iniae CFU in the eye remained low during all sample intervals suggesting poor tissue tropism. We have shown that S. iniae entered hybrid striped bass through the olfactory organ, readily colonizes the nares of these fish and disseminates to other tissues rapidly. The nares should be considered a routine organ for early microbiological detection of S. iniae from fish without disease signs to help to prevent the rapid transmission of streptococcal disease in aquaculture facilities.