Therapeutic effect of fosfomycin (FOM) was studied in red sea bream experimentally and naturally infected with Edwardsiella tarda. Minimum inhibitory concentration of FOM against 67 strains of E. tarda isolated from red sea bream cultured in Ehime Prefecture ranged from 1 to 4 μg/mL, and no strains resistant to FOM were observed. In experimental infections, the mortality decreased with the increase in FOM dosage, and a remarkable therapeutic effect of FOM was demonstrated by oral administration at a dose of 40-80 mg/kg body weight/day for 6 days. In field trials, FOM was administered to high and low mortality cases of red sea bream naturally infected with E. tarda orally for consecutive 6 days at a dosage of 40 mg/kg/day. Cumulative mortalities of the FOM-medicated groups were significantly lower than those of the non-medicated groups in both cases. In medicated groups, the administration of FOM was more efficient in the lower mortality case. These results indicate that FOM administration is effective against edwardsiellosis in red sea bream and the treatment in early stages of infection is important to obtain a favorable outcome.
In 2008, several episodes of mortality were recorded in cultured populations of juvenile greater amberjack reared in the southwest region of Japan. Diseased fish had asymmetrical abdominal distention and pale gills. The head kidney, trunk kidney, and spleen of every fish that was examined was enlarged and discolored. The results of all microbiological and molecular biological assays of tissues taken from diseased fish were negative for major known pathogens. Histopathologically, the disease was characterized by proliferative interstitial nephritis and proliferative splenitis associated with minute, round structures within the cytoplasm of proliferating mono-nucleated cells. Transmission trial using the enlarged trunk kidney from a naturally infected fish successfully reproduced the disease. The results indicate that this disease is caused by an infectious microorganism, and the most likely etiological agent is the minute, round structures which are probably a hitherto unknown eukaryotic microorganism.
Tenacibaculum soleae is a recently described pathogen that has been reported as the causative agent of considerable losses in sole cultures in Spain. This report documents the first case of T. soleae as an etiological agent of tenacibaculosis in farmed sea bass Dicentrarchus labrax. Its identification was performed employing phenotypical, serological and molecular methods. Although the sea bass isolates were homogeneous from a biochemical, chemotaxonomic and molecular point of view, they belonged to a serological group different from the type strain CECT7292, which can be of great importance in the development of future vaccines and other methods of disease prevention in sea bass industry. Virulence assays with a representative isolate confirmed the pathogenic potential for sea bass. The disease was experimentally reproduced by prolonged bath, while no mortalities could be recorded by intraperitoneal injection. The results presented here show that T. soleae must be taken into account as an important pathogen in the marine aquaculture industry.
Anti-bacterial activity of six approved drugs for ayu Plecoglossus altivelis, i.e. florfenicol (FFC), oxolinic acid (OXA), sulfisozole sodium (SIZ), sulfamonomethoxine, ormetoprim (OMP) and sulfamonomethoxine-ormetoprim combination, was examined against four Edwardsiella ictaluri strains. Of these drugs, FFC, OXA and OMP inhibited bacterial growth with MIC lower than 1 μg/mL. When chemotherapeutic efficacy of FFC, OXA and SIZ was examined against experimentally E. ictaluri-infected ayu, FFC and OXA strongly reduced mortality of fish. Although SIZ was slightly effective, survivors from the SIZ-treated group showed high carrier rates for E. ictaluri, suggesting that FFC and OXA are candidates in treating E. ictaluri infection of ayu.
To develop a rapid diagnosis method for Lancefield group C Streptococcus dysgalactiae (GCSD) infection, latex beads were coated with recombinant S. dysgalactiae surface immunogenic protein (rSd-Sip). The usefulness of the beads for slide agglutination tests with fish sera was examined. The beads were agglutinated with GCSD-infected sera collected from farmed amberjack Seriola dumerili. However, the beads were not agglutinated with non-infected sera or immunized sera collected from fish vaccinated with Lactococcus garvieae and Streptococcus iniae. The results demonstrate that the slide agglutination test using rSd-Sip-coated latex beads is an easy, rapid, and effective method for agglutinating antibody-detection in GCSD-infected fish.