Submitted to: European Association of Fish Pathologists
Publication Type: Abstract Only
Publication Acceptance Date: June 11, 1999
Publication Date: September 19, 1999
Citation: EVANS, J.J., SHOEMAKER, C.A., KLESIUS, P.H. ROUTE OF INFECTION, LETHAL DOSE AND ORGAN DISSEMINATION OF STREPTOCOCCUS INIAE IN HYBRID STRIPED BASS (MORONE CHRYSOPS X M. SAXATILIS) AND TILAPIA (OREOCHROMIS NILOTICUS). EUROPEAN ASSOCIATION OF FISH PATHOLOGISTS. NINTH INTERNATION CONFERENCE "DISEASE OF FISH AND SHELLFISH". 1999. Technical Abstract: Three experiments were conducted to collectively examine the major route of infection, lethal dose and dissemination of S. iniae in hybrid striped bass and tilapia. In the first experiment, we examined the effects of bilateral inoculation of S. iniae directly into the nares or eyes of 15 hybrid bass and tilapia with 10 'l of S. iniae for 5 min at 3 colony-forming unit (CFU) doses of 2.4, 24.0 and 240.0 ´ 103. The mortality rates of nare-inoculated hybrid bass and tilapia were 13.3, 53.3 and 26.7 % and 0, 0 and 20.0 %, respectively for the 3 CFU doses of S. iniae indicating hybrid bass were more susceptible to streptoccocal disease than tilapia. Classical streptococcal disease signs were observed in moribund fish within 12 days post-inoculation. Eye inoculation produced no mortality or streptococcal disease signs in either species. 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 3 CFU doses. The LD50 in hybrid bass and tilapia were 13.0 ´ 103 and 10.0 ´ 106 CFU, respectively at 14 days. In order to determine the organ dissemination of S. iniae over time, we inoculated nares of 50 hybrid bass with 53.0 ´ 103 CFU of S. iniae and sampled fish at 0.5, 4, 12, 18, 24, 48 and 52 h for S. iniae from 9 anatomical regions: the blood of the afferent and efferent branchial arteries, nares, olfactory, optic and cerebellum regions of the brain, eye, heart and kidney. The nares were the only organ colonized at 0.5 h. Streptococcus iniae was isolated from the nares, the blood of the afferent and efferent arteries and the cerebellum at 4 h. The nare, blood, olfactory, cerebellum, heart and kidney were colonized within 12 h and the eye became colonized at 18 h . Multiplication of S. iniae was evident in all these tissues by 24, 48 and 52 h. We have demonstrated that nare epithelium of hybrid bass and tilapia were readily colonized and that S. iniae entered these fish through the olfactory organ. The nares should be considered a routine organ for early microbiological detection of S. iniae.