A single cohort time delay differential equation model of Lepeophtheirus salmonis population dynamics was developed. The model was parameterised using data from tank infection trials, which followed a single experimental cohort of post-settlement lice through all the developmental stages. This simple mathematical model successfully predicted the timing and numbers of parasites present on the host. However, the death rates of parasitic louse stages within the tank trials were found to be highly variable despite the simplified conditions prevailing within such a system. Embryonic development, free-living nauplius stages and infection parameters were added to this model to allow the complete parasite life-cycle to be described. In addition simulations were carried out to determine the effect upon short-term salmon louse population dynamics of employing a notional chemotherapeutant having an arbitrary instantaneous efficacy of 80%. These results were used to determine the optimal timing of treatment relative to initial lice counts. This was found to be different for male and female lice stages, reflecting their respective developmental rates.
Mortalities with an unknown etiology occurred in three-line grunt Parapristipoma trilineatum, which had been imported from China or produced from parental fish imported from China, in fish aquaculture facilities around the Bungo Channel, Japan. The affected fish characteristically showed small white spots (0.21 mm in diameter) in the kidney and spleen. No known pathogen for three-line grunt was detected by conventional histological, microbiological and immunological diagnoses. The observations of histological specimens and tissue imprints suggested that the spots were granulomas caused by the intracellular infection with a coccoid gram-negative bacterium (0.51.0μm in diameter). When healthy three-line grunt were injected intraperitoneally with the spleen homogenate of affected fish, they died from 5th day post-inoculation showing granulomas with the intracellular bacterium similar to that in naturally affected fish. These results indicate that the disease was caused by the intracellular bacterium.
To clarify the causative agent of the anemia that has been recently prevailing in wild and cultured Japanese flounder Paralichthys olivaceus, anemic flounder were generated by repeated bleedings and examined hematologically. When flounder were bled once a week at 0.5 or 1.0% (volume/body weight) for 12 weeks without feeding, severe anemia developed in the fish.The anemia showed significant reductions in the hemoglobin concentration, mean corpuscular cell volume, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration, and structural anomalies of erythrocytes occurred such as low hemoglobin accumulation in the cytoplasm and a deformed outline. The hematological characteristics of the anemia caused by the repeated bleedings was almost identical to the anemia observed in wild Japanese flounder, supporting the previous conclusion that the blood-feeding activity of the monogenean Neoheterobothrium hirame is the cause of the anemia. Even after the last bleeding, it required 8 weeks for the anemic fish to recover in a feeding condition. The long period needed for the recovery also supports the previous conclusion that the anemic wild fish without N. hirame can be recognized as fish in the recovering phase from the anemia.
Investigations were made on the infection dynamics of Neoheterobothrium hirame among wild 0-year-old Japanese flounder Paralichthys olivaceus caught in the western Sea of Japan off Tottori Prefecture from spring to autumn in 1999 and 2000. The infection shows the same tendency in both years; flounder became infected in June, a second parasite generation started to appear in early August, and both the prevalence of infection and mean parasite abundance increased thereafter, reaching 100% prevalence in October. One- to two-year-old infected flounder probably acted as the source of infection to the 0-year-old flounder. The parasite distribution showed an increasing overdispersion among 0-year-old fish population from July to August 1999, followed by a continuous decrease in overdispersion in the following months. Furthermore, only a few numbers of fish could be collected in late August to October. These suggest that parasite-induced host mortality occurred among the 0-year-old flounder in this period. In 2000, the parasite showed a similar overdispersed distribution among the 0-year-old flounder populations in late August, but the overdispersion was not clearly decreased afterwards. This is probably because very small-sized parasites were dominant in mid-October. The fish density was extremely reduced from October to November as in 1999. In a survey in August 2000, there was a tendency that severity of anemia was correlated with the intensity of infection. As a conclusion, it is highly probable that N. hirame contributed to the sharp decline in the 0-year-old flounder population in Tottori Prefecture in the autumn of 1999 and 2000.
This study was conducted to verify the previous contrasting results on the motility of Pseudomonas plecoglossicida and clinical conditions (bloody ascites) in affected ayu Plecoglossus altivelis. In the present field surveys at Tokushima Prefecture in 1999 and 2001, all P.plecoglossicida isolates from dead ayu, except one isolate from a fish with ascites, were non-motile irrespective of the presence of bloody ascites. When ayu were injected intramuscularly with nonmotile strains, either motile or non-motile isolates were obtained from the kidney of dead fish, while only motile ones were isolated after injection of motile strains. Bloody ascites manifested in most of the affected fish after injection of either type of the bacterium. In in vitro broth culture conditions, the non-motile strains altered into motile ones during two-week incubation. These results indicate that there are no relationship between in vitro motility of P. plecoglossicida and the presence of bloody ascites in affected ayu, and that the phenotypic alteration in the motility of P. plecoglossicida occurs under both in vitro and in vivo conditions.
In this study, we conducted a vaccination trial using live attenuated cells of Edwardsiella tarda in tilapia Oreochromis niloticus. A mutant strain (SPM31) with lowered siderophore production was constructed from a wild-type E. tarda FPC498 using transposon Tn5. FPC498 had a calculated LD50 of 1.8 ×107CFU/100 g body weight, while SPM31 had a 2.9×108CFU/100 g body weight. Formalin-killed cells or live bacteria of SPM31 were intraperitoneally injected to tilapia at a dose of 0.1 mg/100 g (about one third of the LD50 value) for vaccination. Both the formalin-killed and the live cells activated the antibody production. The vaccinated fish were challenged with the parent strain FPC498 at the 2nd, 3rd and 4th weeks of postvaccination. There were no deaths due to the challenge with the bacterium in the fish immunized with the live cells. On the contrary, the vaccination with formalin-killed cells resulted in 80100% mortality. These results indicate that the live cells with lowered siderophore-production are capable of conferring protective immunity against edwardsiellosis.
To elucidate the causative agent of the mass mortality of cultured pearl oyster Pinctada fucata martensii, with a reddish brown adductor muscle and histopathological changes in the loose connective tissue, experimental challenges on healthy oysters were undertaken using hemolymph from diseased oysters as an inoculum under a feeding or a non-feeding condition. The oysters inoculated with the supernatant, the precipitated hemocytes or the filtrate of the diseased oyster hemolymph died within 2 to 3 months, showing signs similar to those of spontaneously affected oysters. These results indicate that a filterable agent in both the supernatant and the pellet of hemolymph of diseased oysters causes the disease.
A study was undertaken to profile antibiotic sensitivity of Nocardia seriolae isolated from diseased fish cultured in Ehime and Kochi Prefectures from 1999 to 2001. In vitro antibacterial activities of 10 chemotherapeutic agents against N. seriolae isolates were determined as minimum inhibitory concentrations (MICs). All the 60 isolates examined showed susceptibility to kanamycin. Resistance to oxolinic acid and fosfomycin was found in all the isolates and some of the isolates showed resistance to erythromycin, spiramycin and kitasamycin. Multi-drug resistance patterns were also recorded in some of the isolates.