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Title: EXPERIMENTAL STREPTOCOCCUS INIAE INFECTION OF HYBRID STRIPED BASS (MORONE CHRYSOPS X MORONE SAXATILIS) AND TILAPIA (OREOCHROMIS NILOTICUS) BY NARES INOCULATION

Author
item Evans, Joyce
item Shoemaker, Craig
item Klesius, Phillip

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2000
Publication Date: 8/8/2000
Citation: EVANS, J.J., SHOEMAKER, C.A., KLESIUS, P.H. EXPERIMENTAL STREPTOCOCCUS INIAE INFECTION OF HYBRID STRIPED BASS (MORONE CHRYSOPS X MORONE SAXATILIS) AND TILAPIA (OREOCHROMIS NILOTICUS) BY NARES INOCULATION. AQUACULTURE. 189 (2000) 197-210.

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 two experiments conducted to examine the major route of infection and lethal dose of S. iniae in hybrid striped bass and tilapia by eye and nare (nose) inoculation. S. iniae was placed in either the eyes or nares of the fish and observed for 14 days for disase signs and/or mortality. Bacteriological samples were obtained from dead fish to recover the S. iniae organisms. Our results indicated that hybrid striped bass were most susceptible to streptococcal disease than tilapia. Striped bass began to show disease signs earlier and died sooner following exposure to lower numbers of organisms than tilapia. We also demonstrated that fish became infected through the nares but not the eyes. This mode of infection 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: Experiments were conducted to examine the route of infection and lethal dose of S. iniae in hybrid striped bass and tilapia. We examined the effects of bilateral inoculation of S. iniae directly into the nares or eyes of 15 hybrid bass and tilapia with 10ul of S. iniae for 5 min at 3 colony-forming unit (CFU) doses of 2.4, 24.0 and 240.0*10**3. The mortality rates of nare-incolulated hybrid bass and tilapia were 26.7, 66.7, and 13.3% and 0,0 and 20.0%, respectively for the 3 CFU doses of S. iniae. Eye inoculation produced no mortality or streptococcal disease signs in either species. Recovery of S. iniae from moribund striped bass and tilapia were 88.3 and 66.7%, respectively. The frequency of recovery of S. iniae from nares were greater than any of the other organs. Bacteriological examination of fish 14 days post nare or eye inoculation showed that S. iniae was absent from all striped bass and present in a small percentage of tilapia (3.6) organs. To determine the LD50 for hybrid bass and tilapia, we examined the effects of bilateral inoculation of S. iniae directly into the nares of 3 replicates of 20 fish of each species to S. iniae at additional CFU doses. The doses for striped bass were 1.31, 2.62 and 5.23 * 10**4 and for tilapia 7.05, 70.5 and 705.0 *10**6. The LD50 in hybrid bass was 6.0*10**6. Our results indicated hybrid bass were more susceptible to streptococcal disease than tilapia. We have further demonstrated that the mode of infection for S. iniae was the olfactory organ of these fish. The nare mode of infection may be an explanation for the rapid transmission of streptococcal disease in aquaculture facilities and nares should be considered a routine organ for early microbiological detection of S. iniae.