Theses and Dissertations

Title: Streptococcus miae: Effects on Hematology in Tilapia (Oreochromis niloticus) and Routes of Infection in Hybrid Striped Bass (Morone chrysops x Morone saxatilis)

Name: McNulty, Shawn Thomas

Degree: MS

Chair: Dr. Phillip H. Klesius and Dr. Craig A. Shoemaker


University: Auburn University

Location: Auburn, Alabama

Date: 2002

Pages: 56

Keywords: Tilapia,hybrid striped bass,Streptococcus iniae,hematology,route of infection


This study characterized the effect of Streptococcus iniae infection on two species of cultured fish. There is little or no information on the effects of Streptococcus iniae infection on the hematology in Nile tilapia. The present study characterized the hematological changes in juvenile and adult tilapia infected with S. iniae by nare inoculation. The hematological values measured included mean hematocrit (%), mean red blood cell count (RBC)(106/µL), mean total hemoglobin (g/dL), mean corpuscular volume (MCV)(fL), mean corpuscular hemoglobin (MCH)(pg), and mean corpuscular hemoglobin concentration (MCHC)(g/dL). The most pronounced change caused by S. iniae infection was the reduction in red blood cell counts. In the juvenile tilapia the RBC count was significantly (p< 0.05) different and had a range of 3.9±1.6 to 1.4± 0.6 x 106/µL. The RBC counts in the adult tilapia were significantly (p< 0.05) different and had a range of 4.64 ± 2.7 to 1.19± 0.3 x l06/µL. The decrease in mean RBC counts may be due to the hemolytic nature S. iniae. Thus, the data indicated that S. iniae infection alters the hematology of both juvenile and adult Nile tilapia. Several studies have been conducted to determine the route through which Streptococcus iniae enters the body of hybrid striped bass. This study was designed to test the possibility that S. iniae enters through the gills. Two aquaria were used to determine the dissemination to different tissues at different time points following inoculation. We sampled fish at 0.5, 4, 8, 12, 24, 48, 72, and 96 hours for S. iniae from 13 anatomical regions: external gill swab, blood of the 1st and 2nd gill arches, blood of the 3rd and 4th gill arches, nares, olfactory, optic and cerebellum regions of the brain, eye, head kidney, caudal kidney, spleen, liver and intestine. Aquaria one and two were inoculated with a dose of 2.6 x 106 CFU/ fish and 5.0 x 106 CFU/ fish, respectively. S. iniae was isolated within 12 h with the 5.0 x 106 CFU/ fish dose and within 48 h with the 2.6 x 106 CFU/ fish dose. Behavior and mortality were also monitored for 14 days in hybrid striped bass following gill inoculation of S. iniae. Four aquaria with 15 fish were inoculated with S. iniae via the gills with a dose of 5.0 x 105 CFU/ fish, 2.6 x 106 CFU/ fish, 5.0 x 106 CFU/ fish, or 1.0 x 108 CFU/ fish. Control fish were inoculated with tryptic soy broth (TSB) via the gills. The cumulative mortality was 13.33%, 26.67%, and 100%, respectively. The results indicated that S. iniae can enter hybrid striped bass through the gills.

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